mud pump flow loop schematic quotation

During drilling in Oil and Gas exploration, drilling mud or Bentonite is pumped into boreholes for multiple reasons. Pumping drill mud into boreholes cools the drill bit as well as bringing drill cuttings to the surface as the way in which mud is pumped into boreholes forms a closed loop system. The use of drilling mud also provides hydrostatic pressure to prevent liquids such as oil and gas rising to the surface, as drilling mud is thixotropic meaning when it is not agitated it stiffens forming a mud which is an effective liquid and gas barrier.

When two (or more) pumps are arranged in serial their resulting pump performance curve is obtained by adding theirheads at the same flow rate as indicated in the figure below.

Centrifugal pumps in series are used to overcome larger system head loss than one pump can handle alone. for two identical pumps in series the head will be twice the head of a single pump at the same flow rate - as indicated with point 2.

With a constant flowrate the combined head moves from 1 to 2 - BUTin practice the combined head and flow rate moves along the system curve to point 3. point 3 is where the system operates with both pumps running

When two or more pumps are arranged in parallel their resulting performance curve is obtained by adding the pumps flow rates at the same head as indicated in the figure below.

Centrifugal pumps in parallel are used to overcome larger volume flows than one pump can handle alone. for two identical pumps in parallel and the head kept constant - the flow rate doubles compared to a single pump as indicated with point 2

Note! In practice the combined head and volume flow moves along the system curve as indicated from 1 to 3. point 3 is where the system operates with both pumps running

In practice, if one of the pumps in parallel or series stops, the operation point moves along the system resistance curve from point 3 to point 1 - the head and flow rate are decreased.

According to this study, increasing the plastic viscosity of the mud results in a remarkable increase in the amount of recovered cuttings. Surprisingly enough, the surplus amount of viscosity inverses the result. These phenomena are clearly seen in Fig. 3a–d.

As far as cuttings transport in highly deviated wells is concerned, researchers offer various ideas about the effect of viscosity on hole cleaning. Some researchers such as Zeidler (1972), Okrajni and Azar (1986), Pilehvari et al. (1999), Jawad (2002), Kelessidis et al. (2007) and Mohammadsalehi and Malekzadeh (2011) believe that raising viscosity of the drilling fluid deteriorates hole cleaning, because type of flow regime changes from turbulent flow to laminar flow; and it has been proved that cuttings can be better displaced in turbulent flow than laminar flow. On the other hand, there are also some investigators, for instance, Ford et al. (1990), Iyoho and Takahashi (1993), Belavadi and Chukwu (1994), Shou (1999), Li et al. (2004) who claim that improvement in hole cleaning occurs as viscosity increases.

Figure 3a–d are for 1.74 mm cuttings size and each figure is a representative of one specific angle (i.e., 60°, 70°, 80°, or 90°). In all figures, cuttings transport performance (CTP) has been plotted versus viscosity. A line in each individual graph depicts a constant flow rate for the three distinctive types of drilling fluid with three different viscosities (1, 2.5, and 6 cp).

Cuttings transport performance versus viscosity for four distinct hole inclinations from vertical, i.e. a θ = 60°, b 70°, c 80° and d 90°, with different flow velocities , i.e. 1.84, 2.21, 2.58, 3.31 ft/s

It is visible from Fig. 3a–d that primary increasing viscosity from 1 to 2.5 cp results in better CTP for all four different flow rates regardless of hole inclination. The effect of viscosity was, nevertheless, negative when the viscosity of the mud was intensified to 6 cp.

The best explanation for such a behavior of mud can be extracted from rheological laws defined for distinguishing various types of flow, namely laminar flow, transient flow, and turbulent flow. The type of flow of a fluid is characterized by the Reynolds number Re.

It is true to say that by increasing viscosity of the fluid at the same flow rate, the current flow regime tends to convert to laminar flow from turbulent flow. However, it should be noticed that converting a turbulent flow to laminar flow requires passing through a wide range of Reynolds number including transition zone. Thus, the author prepared a general graph which indicates how viscosity affects CTP by changing flow regime (Fig. 4).

It can be interpreted from Fig. 4 graph that CTP is improved by increasing viscosity while other factors such as velocity and hole inclination are kept constant. This happens until the flow is still turbulent, but once reaching the transition zone CTP gradually decreases till the end of this region. Subsequently, laminar flow becomes transition flow by further increase of viscosity at the same condition. Among those three types of flow regimes, turbulent flow is the most desirable one followed by transition and laminar flows. The effect is prevailing at lower and higher velocities and for all hole inclinations. All these phenomena can be explained by the definition of each flow regime. As the flow is laminar, any laminar layer of the fluid is displaced, with respect to other laminar layers, in parallel to the direction of flow, and is moving at its individual speed. For flow through a cylindrical tube, the flow rate is highest along the axis of the tube. At the tube wall it is zero throughout the volume of the fluid. However, turbulent flow consists of small eddies throughout the volume of the fluid, and this character of this type of flow produces more momentum force which gives better movement of cuttings of course.

The resistance of drilling fluid to flow is defined by the term called plastic viscosity which indicates the extent of physical and chemical interactions between fluids and solid particles applied into the mud. In general, any increment of solid content in drilling mud, such as barite and fluid loss materials, will result in higher plastic viscosity. The presence of shale particles in drilling fluid system when drilling through a shale zone will also increase the plastic viscosity. Strictly speaking, the higher plastic viscosity generates higher resistance in mud which in turns will affect cutting lifting performance. The situation may be worsened by the increase of ultra-fine drill solids in the drilling fluid which causes incremental trend of plastic viscosity at constant mud weight. This phenomenon explains how geology variations impacts on the cutting transport. Therefore, the size of drill cuttings as well as formation shale content should be put into consideration before designing the drilling fluid. In this research work, clean sand particles of 1.70 mm were used as the drill cuttings. Therefore, their effect on mud viscosity was negligible.

Lines in these curves indicate types of chosen mud in every run of the experiment. Rhombic markers belong to the mud with viscosity of 1 cp, square markers have a viscosity of 2.5 cp and triangle markers have the highest viscosity by the value of 6 cp. The existing distance between lines also shows different types of mud with different rheological properties. Four angles had been selected for this study namely 60°, 70°, 80°, and 90° from vertical. All lines give an upward trend by diverging from vertical, indicating that CTP has improved. Some researchers such as Okrajni and Azar (1986), Ford et al. (1990), and Bilgesu and Chukwu (2007), believed that if hole angle increases, their negative influences on cutting transports will increase as well. However, the author believes that it also has some positive effects especially when the flow is laminar. It is obvious that at higher degrees of inclination the tendency of downward cuttings bed sliding is more likely to occur which increases the hydraulic requirement for adequate hole cleaning. These beds moved at a lower speed along the annular space so that at the end of the test period, there may be some beds about to approach the outlet of the annulus section and if the test period has lasted for a few extra minutes there is a great chance for the nearby moving beds to leave the annular section. Thus, it is believed that a slight increase in hole inclination causes these large moving bed to reach the outlet before the end of the test period; thus a considerable increase in WRP and CTP.

Most portions of the injected cuttings were recovered by highest velocity of the drilling fluid and the most viscous mud. This fact also supports previous discussion about viscosity of mud in which increasing the viscosity can improve CTP if the flow regimes is still turbulent.

All in all, these results coincide well with vast majority of previous experimental work. Many researchers have already reported the effect of flow rate on hole cleaning, such that as the flow rate is increased, a reduction on cuttings bed area will be experienced.

Mud Pump Pulsation Dampener is usually installed on the discharge line to reduce the fluctuation of pressure and displacement of the drilling mud pump.

Mud Pump Pulsation Dampener is a pneumatic device built into the outflow line of each UUD pump to dampen the pressure fluctuations resulting from the action of the pump. Although presented as a surge tank, this device is really a device that can be tuned to greatly diminish the output pulsations transmitted downstream from the mud pump. Unfortunately, the effectiveness of the pulsation dampener is a function of both output pump pressure and frequency of the pump pulsations.

The piston is one of the parts that most easily become worn out and experience failure in mud pumps for well drilling. By imitating the body surface morphology of the dung beetle, this paper proposed a new type (BW-160) of mud pump piston that had a dimpled shape in the regular layout on the piston leather cup surface and carried out a performance test on the self-built test rig. Firstly, the influence of different dimple diameters on the service life of the piston was analyzed. Secondly, the analysis of the influence of the dimple central included angle on the service life of the piston under the same dimple area density was obtained. Thirdly, the wear of the new type of piston under the same wear time was analyzed. The experimental results indicated that the service life of the piston with dimples on the surface was longer than that of L-Standard pistons, and the maximum increase in the value of service life was 92.06%. Finally, the Workbench module of the software ANSYS was used to discuss the wear-resisting mechanism of the new type of piston.

The mud pump is the “heart” of the drilling system [1]. It has been found that about 80% of mud pump failures are caused by piston wear. Wear is the primary cause of mud pump piston failure, and improving the wear-resisting performance of the piston-cylinder friction pair has become the key factor to improve the service life of piston.

Most of the researchers mainly improve the service life of piston through structural design, shape selection, and material usage [1, 2]. However, the structure of mud pump piston has been essentially fixed. The service life of piston is improved by increasing piston parts and changing the structures of the pistons. However, the methods have many disadvantages, for example, complicating the entire structure, making piston installation and change difficult, increasing production and processing costs, and so on. All piston leather cup lips use rubber materials, and the material of the root part of the piston leather cup is nylon or fabric; many factors restrict piston service life by changing piston materials [3]. Improving the component wear resistance through surface texturing has been extensively applied in engineering. Under multiple lubricating conditions, Etsion has studied the wear performance of the laser surface texturing of end face seal and reciprocating automotive components [4–6]. Ren et al. have researched the surface functional structure from the biomimetic perspective for many years and pointed out that a nonsmooth surface structure could improve the wear resistance property of a friction pair [7, 8]. Our group has investigated the service life and wear resistance of the striped mud pump piston, and the optimal structure parameters of the bionic strip piston have improved piston service life by 81.5% [9]. Wu et al. have exploited an internal combustion engine piston skirt with a dimpled surface, and the bionic piston has showed a 90% decrease in the average wear mass loss in contrast with the standard piston [10]. Gao et al. have developed bionic drills using bionic nonsmooth theory. Compared with the ordinary drills, the bionic drills have showed a 44% increase in drilling rate and a 74% improvement in service life [11]. The present researches indicate that microstructures, like superficial dimples and stripes, contribute to constituting dynamic pressure to improve the surface load-carrying capacity and the wear resistance of the friction pair [12–21].

In nature, insects have developed the excellent wear-resistant property in the span of billions of years. For instance, the partial body surface of the dung beetle shows an irregularly dimpled textured surface with the excellent wear-resistant property that is conducive to its living environment [7, 8, 22]. The dung beetle, which is constantly active in the soil, shows a body surface dimple structure that offers superior drag reduction. These dimples effectively reduce the contact area between the body surface and the soil. Moreover, the friction force is reduced. Therefore, the dung beetle with the nonsmooth structure provides the inspiration to design the bionic mud pump piston. This paper proposed a new type of piston with dimpled morphology on its surface and conducted a comparative and experimental study of different surface dimpled shapes, thus opening up a new potential to improve the service life of the mud pump piston.

A closed-loop circulatory system was used in the test rig, which was built according to the national standard with specific test requirements. The test rig consisted of triplex single-acting mud pump, mud tank, in-and-out pipeline, reducer valve, flow meter, pressure gauge, and its principle, as shown in Figure 1. Both the pressure and working stroke of the BW-160 mud pump are smaller than those of the large-scale mud pump, but their operating principles, structures, and working processes are identical. Therefore, the test selected a relatively small BW-160 triplex single-acting mud pump piston as a research object, and the test results and conclusion were applicable to large-scale mud pump pistons. The cylinder diameter, working stroke, reciprocating motion velocity of piston, maximum flow quantity, and working pressure of the BW-160 triplex single-acting mud pump were 70 mm, 70 mm, 130 times/min, 160 L/min, and 0.8–1.2 MPa, respectively.

The mud pump used in the test consisted of water, bentonite (meeting the API standard), and quartz sand with a diameter of 0.3–0.5 mm according to actual working conditions. The specific gravity of the prepared mud was 1.306, and its sediment concentration was 2.13%. Whether mud leakage existed at the venthole in the tail of the cylinder liner of the mud pump was taken as the standard of piston failure. Observation was made every other half an hour during the test process. It was judged that the piston in the cylinder failed when mud leaked continuously; its service life was recorded, and then it was replaced with the new test piston and cylinder liner. The BW-160 mud pump is a triplex single-acting mud pump. The wear test of three pistons could be simultaneously conducted.

The mud pump piston used in the test consisted of a steel core, leather cup, pressing plate, and clamp spring. The leather cup consisted of the lip part of polyurethane rubber and the root part of nylon; the outer diameter on the front end of the piston was 73 mm, and the outer diameter of the piston tail was 70 mm, as shown in Figure 2. We proceeded in two parts during the design of the dimpled layout pattern because the piston leather cup consisted of two parts whose materials were different. The dimples at the lip part of the leather cup adopted an isosceles triangle layout pattern, and the dimples at the root part were arranged at the central part of its axial length, as shown in Figure 3(a). Dimple diameter (D, D′), distance (L), depth (h), and central included angle (α) are shown in Figure 3. The dimples on the piston surface were processed by the CNC machining center. Since then, the residual debris inside the dimples was cleaned.

Schematic of dimpled piston: (a) dimpled layout of piston, (b) dimpled array form diagram, (c) cross section view of the piston leather cup, and (d) original picture of dimpled piston.

Table 1 shows that average service lives of L-Standard, L-D1, L-D2, and L-D3 were 54.67 h, 57.17 h, 76.83 h, and 87.83 h, respectively. Therefore, the mud pump pistons with dimples provide longer service life than the L-Standard piston. As the dimple diameter increases, the piston service life was improved, and the largest percentage increase of the service life was 60.65%. The service life of the L-D4 piston was about 81.17 h, which increased by 7.94% compared with that of the L-D2 piston, indicating that the piston with dimples at the leather cup root could improve piston service life.

Figure 4 illustrates the surface wear patterns of pistons with different dimple diameters in the service life test. Figures 4(a) and 4(a′) show wear patterns on the surface of the L-Standard piston. This figure shows that intensive scratches existed in parallel arrangement on the piston leather cup surface, enabling high-pressure mud to move along the scratches from one end of the piston to the other easily, which accelerated the abrasive wear failure with the abrasive particles of the piston. Figures 4(b), 4(b′), 4(c), 4(c′), 4(d), and 4(d′) show the wear patterns of the leather cup surfaces of L-D1, L-D2, and L-D3 pistons, respectively. Figures 4(b), 4(b′), 4(c), 4(c′), 4(d), and 4(d′) show that the scratches on the leather cup surface became shallower and sparser and the surface wear patterns improved more obviously as the dimple diameter increased. If the piston leather cup surface strength was not affected to an extent as the dimple diameter increased, the reduced wear zone near the dimple would become greater and greater, indicating that the existence of dimples changed the lubricating status of the leather cup surface, their influence on nearby dimpled parts was more obvious, and they played active roles in improving the service life of the piston.

Figure 5 displays the wear patterns of the leather cup root parts of the L-D4 and L-D2 test pistons. The wear patterns of the nylon root parts of the L-D4 pistons are fewer than those of the L-D2 pistons, as shown in Figure 5. When the leather cup squeezed out high-pressure mud as driven by the piston steel core, it experienced radial squeezing while experiencing axial wear. Therefore, the area with the most serious wear was the piston leather cup root part, and the friction force at the leather cup root was much greater than that at the other areas. The rapid wear at the root decreased the piston load-carrying capacity and then affected the service life of piston. The dimples at the piston leather cup root could reduce the wear of the piston leather cup root and improve the service life of piston.

Figure 6 shows the surface wear patterns of the L-S1 and L-S2 test pistons. In Figures 6(a) and 6(a′), the scratches on the piston leather cup surface became sparse and shallow in the dimpled area. Figures 6(b) and 6(b′) show that the wear was slight in the area close to the dimples. The farther the scratches were from the dimpled area, the denser and deeper the scratches would be. The L-S1 piston had a small dimple central included angle, which was arranged more closely on the piston surface. The lubricating effects of oil storage in each row of dimples were overlaid very well, which was equivalent to amplifying the effect of each row of dimples in Figure 6(b), making the wear on the whole piston leather cup surface slight, preventing the entry of high-pressure mud into the frictional interface, and lengthening the service life of piston.

During the operation of the mud pump piston, the outside surface of the piston leather cup comes in contact with the inner wall of the cylinder liner and simultaneously moves to push the mud. The lip part of the piston leather cup mainly participated in the piston wear and exerted a sealing effect, while the piston root part mainly exerted centralizing and transitional effects. In the mud discharge stroke, the lip part of the piston experienced a “centripetal effect,” and a gap was generated between the lip part and the cylinder liner. The greater the contact pressure between the lip part and cylinder liner of the piston was, the smaller the gap was, and the entry of high-pressure mud into the contact surface between the piston and cylinder liner was more difficult. The piston root easily experienced squeezing under high pressure, and the smaller the equivalent stress caused by the piston root was, the more difficult the squeezing to occur. Hence, the contact pressure at the lip part of the piston and the equivalent stress at the root were analyzed, and they would provide a theoretical basis for the piston wear-resisting mechanism. The ANSYS Workbench module was used to perform a comparative analysis between the contact pressure at the lip part and the equivalent stress at the root of the three kinds of pistons (i.e., L-Standard piston, L-S1 piston, and L-D1 piston). The service life of the L-S1 piston exhibited the best improvement effect, whereas that of the L-D1 piston demonstrated the worst improvement effect. The piston adopted a 1 mm hexahedral grid, and the grid nodes and elements are as shown in Table 4.

The lubricating oil on the mud pump piston surface could reduce the wear of piston and cylinder liner and improve the service life of pistons with the reciprocating movement. The lubricating oil would eventually run off and lose lubricating effect, which would result in piston wear. The finite element fluid dynamics software CFX was used to establish the fluid domain model of the dimpled and L-Standard pistons and analyze the lubricating state on the piston surface. The piston surface streamlines are shown in Figure 10. This figure shows that the lubricating fluid did not experience truncation or backflow phenomenon when passing the surface of the L-Standard piston. When the lubricating fluid flowed through the surface of the dimpled piston, it presented a noncontinuous process. Its flow velocity at the dimpled structure slowed down obviously because it was blocked by the dimpled structure.

Figure 11 shows the piston cross section streamline. This figure shows that the existence of dimples changed the distribution status of the lubricating flow fields on the contact surface between the piston and cylinder liner. The lubricating oil entered the dimpled structure in a large quantity, and the flow velocity slowed down. The dimpled structure on the piston surface enlarged the storage space of the lubricating oil and made it difficult for the lubricating oil inside the dimpled structure to be taken away by the cylinder liner to improve the lubricating conditions of the friction pair interface, reduce the frictional resistance between the piston and cylinder liner, reduce wear, and improve the piston service life.

When the piston moved in the cylinder liner, a small quantity of solid particles in mud entered gap of piston and cylinder liner and participated in abrasion. The dimpled structure on the piston surface could store some abrasive particles (as shown in Figure 6(a′)) during the piston wear process to prevent these particles from scratching the piston and cylinder liner and generating gullies, thus avoiding secondary damage to the piston and cylinder liner and improving the piston service life.

This paper presented a dimpled-shape mud pump piston; that is, the piston leather cup surface had a dimpled array morphology in regular arrangement. The experimental results can provide the basic data for design engineering of the mud pump piston with a long service life. The comparative analyses of service life and wear patterns for dimpled mud pump pistons and L-Standard pistons were conducted. The main results and conclusions were summarized as follows:(1)The service life of the mud pump piston with dimpled morphology on the surface improved in comparison with that of the L-Standard piston, and the service life increase percentages were from 4.57% to 92.06%.(2)The piston service life would increase with the dimple diameter under the same dimpled arrangement pattern, and the maximum increase in the value of service life was 60.65%.(3)The service life of the piston with dimples increased by 7.94% in comparison with that with none.(4)Under the same dimpled arrangement patterns and area densities, the tighter and closer the dimples were arranged on the piston surface, the longer the service life of piston was, and the maximum increase in the value of service life was 92.06%.(5)Under the same wear time, the wear of the dimpled piston slightly decreased in comparison with that of the L-Standard piston, and the minimum value of wear mass percentage was 3.83%.(6)The dimpled shape could not only change the stress state of the piston structure, improve piston wear resistance, and reduce root squeezing, but also increase oil storage space, improve lubricating conditions, and enable the accommodation of some abrasive particles. Furthermore, the dimpled shape was the key factor for the service life improvement of the mud pump piston.

This invention relates to the preparation and maintenance of drilling mud properties, especially rheology, viscosity, shear, density, solids, and water/oil ratio, either at a mud plant where drilling fluids are mixed and stored for delivery or during the ongoing drilling process in the recovery of hydrocarbons from the earth. The invention utilizes a cavitation mixer placed in a loop capable of isolating an aliquot of drilling mud from either the conduit leading to the well or a tank used to make up new mud or to store mud. The cavitation mixer imitates the shear and heating generated when pumping the drilling mud through the drilling bit at various temperatures. The rheology and other properties of the drilling mud are maintained at desired values by regulating, in the loop, the addition of viscosity-adjusting agents, other additives, the flow rate, and the speed of the mixer to obtain a desired shearing effect in the mixer in real time without the need for lab tests. The cavitation mixer not only heats and shear mixes, but is able to function as a viscometer, reinforcing optional separate viscometer readings. Other properties can be monitored and regulated in the loop. BACKGROUND OF THE INVENTION

Drilling muds are complex, typically non-Newtonian fluids that serve multiple, critical functions in drilling wells for oil and gas extraction. The fluid is used to remove formation drill cuttings from the wellbore, and the fluid adds hydrostatic mass to help prevent uncontrolled flow of hydrocarbons from the well. The fluid also enables buoyancy to counteract the weight of the drill pipe so that one can drill deeper wells. The fluid also lubricates the bit and stabilizes the wellbore as drilling continues deeper. Limiting the loss of fluid to the recently drilled formation is another important function, and limiting fluid loss typically means the use of bridging agents that are sized particles. It is essential to know the properties of the fluids so they can perform their many functions efficiently.

Some fluid properties are relatively easy to determine in line as the fluid is being used. For example, a Coriolis Meter can accurately determine the flow rate and density of the fluid, but determining rheology of a complex drilling fluid is more complicated; it is commonly done by a manual mud check according to API 13B. Accurate knowledge of a fluid"s rheology is required to calculate a Yield Point and plastic viscosity. If the mud is too thick, the mud pump cannot pump it. If the fluid is too thin, it may not suspend the solids that have to be removed from the wellbore as one continues drilling deeper. To continue drilling deeper, drill pipe has to be added to the string. During additions to the string, the mud is no longer being pumped, and Yield Point, which is part of the rheology, determines the pressure needed to move the fluid again after it has been static in the wellbore. If Yield Point (YP) is too high, the pump cannot begin to move the flow of mud.

Prior to the present invention, it has been common not to attempt to shear mix a drilling mud before it is sent down the well to the drill bit, but rather to utilize the drill bit itself to shear mix the drilling mud. This means the rheological properties of the drilling mud are not the most desirable when the mud arrives at the point of drilling, and often can be far from optimum. Moreover, the drilling process adds drill cuttings and other solids and fluids to the drilling fluid, which continuously change significantly the physical properties of the fluid. The prior method, relying on the drill bit for shear mixing, injects considerable uncertainty into the overall process.

One reason the art has relied on the drill bit for shear mixing is that there had not been available a practical way to shear mix the ponderous drilling mud components in a continuous recycling mode.

It takes time to run the “mud checks” specified by the American Petroleum Institute (API) 13B. Mud checks require a skilled operator to successfully run and to report the mud properties. Without shearing the mud, however, the chemistry is not fully activated and the desired rheology is not achieved. In the laboratory either a Hamilton Beach blender or a Silverson mixer is used to imitate the shear developed by a trip through the drilling bit. There is disagreement about which device to use and the amount of time required to mix the mud before running a mud check. Both the Hamilton Beach blender and the Silverson have commercial units that replicate their laboratory units, but they are not typically used for large batches at a drill site for a number of reasons. One problem is simply time. Typically in the laboratory it is common to make 350 ml portions to represent one barrel of fluid. If you shear a one barrel equivalent drilling fluid sample in the laboratory for 5 minutes, then to “scale up” the shear process at the wellsite, it takes the same 5 minutes per actual barrel. Unfortunately it takes too much time. If a rig has 1,000 bbls of drilling fluid, it would take 5,000 minutes or 3.5 days of processing to equal 5 minutes of shear used in the laboratory for the 1 bbl equivalent volume.

Volumes of drilling mud can range from 500 bbls to over 10,000 bbls on location that is stored in pits or tanks, and the mud can stratify based on the density of the additives. When relying on samples for API 13B, it is critical that they are representative of the drilling fluid to be used in the well, but all too often, imperfect sampling practice introduces errors into the API 13B procedure.

Rheology of drilling muds is measured using a Fann 35 or equivalent rotational viscometer that directly reads viscosity on a dial at different rpm. The dial reading is based on the deflection of a bob inside of a rotating cylinder, and the instrument must be calibrated regularly to be accurate. Temperature changes mud rheology, and to determine an accurate downhole rheology means the mud must be heated before measuring its viscosity. By definition, a mud check is done “offline” which takes valuable time and can delay critical decisions about well control. Rig time is often lost while the fluid is circulated in the hole to adjust drilling fluid for the proper rheology after a time delay and before continuing to drill.

A better way to conduct the shear mixing and the rheology measurement process is needed. Ideally a realtime, inline measurement of the mud properties is desired, but there are several challenges to its achievement. One challenge is simply the shear that happens at the bit needs to be replicated at the surface. There are high-pressure mixing devices that accomplish this shear, but they are expensive to build and operate; moreover, high-pressure is an HSE (health, safety and environmental) issue. The rheology measuring device is another challenge. Rheology measurement is used in numerous industries and there are a number of devices adapted for oil field use that include, but are not limited to, the Brookfield TT-100, Grace M3900, and Chandler 3300. The advantage of these types of devices is that they can be calibrated to replicate the Fann 35, and Fann 35 readings have become a de facto standard and it is not uncommon for mud engineers to quote viscosity at various Fann 35 speeds, or add chemistry based on a particular Fann 35 reading. A Fann 35 is a Couette style viscometer as are these three devices, and while they can be correlated to Fann 35 readings, they have intricate internal parts and small flow lines that can easily plug when fluid loss additives are in the drilling fluid. There are numerous other viscometers used in other industries that presumably would also work; however, a viscosity measurement is taken at a single shear rate or at shear rates that are harder to relate to a Fann 35 viscosity reading.

Rheology requires a shear rate vs shear stress curve to accurately calculate plastic viscosity and yield point. A pipe rheometer can be used to measure viscosity by accurately measuring the pressure drop across a known length of pipe of a known internal dimension while measuring an accurate flow rate. Pipe rheometers are commercially available from Chandler Engineering, Stim-Lab, Inc, or Khrone but they are relatively simple devices that can easily be built assuming an understanding of flow and viscosity calculations that are widely published. An example of the calculation required has been published by Petroleum Department of The New Mexico Institute of Mining and Technology as a class exercise available on the Internet as “L5_PipeViscometer.pdf”.

An ideal device to measure flow in a pipe rheometer is a Coriolis meter which has a full opening pipe internal diameter such that it is not easily plugged. A Coriolis meter also gives an accurate mass flow, not just a volume flow rate. Coriolis meters such as the E+H Promass 83I can also measure viscosity. Given the critical performance required of drilling muds to ultimately prevent uncontrolled well events, using a combination of rheology measurement devices based on different principles would make sense. For example, a pipe rheometer requires accurate flow rate. Using the E+H Promass 83I for accurate flow rate could also validate the viscosity being reported by pressure drop. Whereas the pipe rheometer calculations are based on flow and pressure drop, the Promass 83I viscosity is a function of a vibration frequency.

Even with otherwise proper rheology measurement techniques, heat is an additional challenge. The fluid rheology should be measured at more than one temperature. Therefore the ideal device would shear the mud to replicate the shear imparted by the drill bit, heat the fluid to the proper temperature and report rheology at different predetermined temperatures.

Drilling mud is monitored and adjusted with immediate response to requirements by placing a cavitation mixer in a loop on the conduit leading from the source of mud ingredients to the well. The loop can isolate an aliquot of the mud to be used so that its rheology, viscosity, density and other properties can be determined at known flow rates and at temperatures present around the drill bit, and adjusted accordingly. Lab tests are not needed.

A cavitation mixer is a cavitation device used for mixing and heating fluids; in the present invention, it is also used to determine rheology of the drilling mud.

The phenomenon of cavitation, as it sometimes happens in pumps, is generally undesirable, as it can cause choking of the pump and sometimes considerable damage not only to the pump but also auxiliary equipment. However, cavitation, more narrowly defined because it is deliberately created, has been put to use as a source of energy that can be imparted to liquids. Certain devices employ cavities machined into a rotor turning within a cylindrical housing leaving a restricted space for fluid to pass. A motor or other source of turning power is required. The phenomenon of cavitation is caused by the passage of the fluid over the rapidly turning cavities, which creates a vacuum in them, tending to vaporize the liquid; the vacuum is immediately filled again by the fluid and very soon recreated by the centrifugal movement of the liquid, causing extreme turbulence in the cavities, further causing heat energy to be imparted into the liquid. Liquids can be simultaneously heated and mixed efficiently with such a device. Also, although the cavitation technique is locally violent, the process is low-impact compared to centrifugal pumps and pumps employing impellers, and therefore as a mixing technique is far less likely to cause damage to sensitive polymers used in oilfield fluids. Good mixing is especially important in mixing drilling muds.

The basic design of the cavitation devices described in the HDI patents comprises a cylindrical rotor having a plurality of cavities bored or otherwise placed on its cylindrical surface. The rotor turns within a closely proximate cylindrical housing, permitting a specified, relatively small, space or gap between the rotor and the housing. Fluid usually enters at the face or end of the rotor, flows toward the outer surface, and enters the space between the concentric cylindrical surfaces of the rotor and the housing. While the rotor is turning, the fluid continues to flow within its confined space toward the exit at the other side of the rotor, but it encounters the cavities as it goes. Flowing fluid tends to fill the cavities, but is immediately expelled from them by the centrifugal force of the spinning rotor. This creates a small volume of very low pressure within the cavities, again drawing the fluid into them, to implode or cavitate. This controlled, semi-violent action of micro cavitation brings about a desired conversion of kinetic and mechanical energy to thermal energy, elevating the temperature of the fluid without the use of a conventional heat transfer surface.

I refer to the cavitation device I use as a cavitation mixer because it is sometimes, in my invention, used as a shearing device instead of heating by cavitation, as will be explained below. The loop in which the cavitation mixer is placed will also be described and explained below.

The ingredients for a drilling mud are placed in a mud tank or other container and may be roughly mixed together in any conventional manner. As they are withdrawn to be sent down the well, they encounter a cavitation mixer, preferably a flow-controlled cavitation mixer, referred to herein as a FCCM. The preferred FCCM is a TrueMud™ mixer. The FCCM has variable mixing rates based on the speed of the disc (rotor) and the rate that fluid is pumped through the device, is able to take in additives at controlled rates or dosages, assures a uniform and turbulent entry, preheats the fluid before beginning the cavitation process, and includes means for setting the gap in the entryway as a function of the viscosity of the fluid. The drilling mud passes continuously through the FCCM to the well, where it is destined for the drilling bit. On its way to the well, a rheology meter or viscosity meter reads its rheology or viscosity directly in the conduit, or from samples taken from it, or on a bypass line.

A cavitation device comprises a cavitation rotor within a housing. The cylindrical surface of cavitation rotor has a large number of cavities in it. Its housing has a cylindrical internal surface substantially concentric with the cavitation rotor. The cavitation rotor is mounted on a shaft turned by a motor. Fluid entering through an inlet spreads to the space between the cavities and the conforming cylindrical internal surface of the housing and is subjected to cavitation—that is, it tends to fall into the cavities but is immediately ejected from them by centrifugal force, which causes a partial vacuum in the cavities; the vacuum is immediately filled, accompanied by the generation of heat and violent motion in and around the cavities. This highly turbulent action in the cavitation zone between the two cylindrical surfaces of the cavitation device thoroughly mixes and heats the materials before the mixture passes through an outlet. While any cavitation device as just described may perform satisfactorily in my invention, I prefer to use a “flow-controlled” cavitation device, which has a generally conical surface positioned centrally on the rotor to face the incoming fluid and to assist the flow of the fluid to the perimeter of the rotor.

The rheology or viscosity meter combined with the FCCM mixer eliminates the need for routine mud checks by continuously reporting rheology using a process control loop. Mud checks primarily report rheology by measuring shear rate and shear stress at predetermined temperatures. In the prior art, typically a sample of the fluid is heated to 100° F. where viscosity measurements are taken with a Fann 35 or equivalent rotor and bob viscometer and then the measurements are repeated at 150° F. This is a time-consuming procedure which delays reports, often resulting in their misapplication. The TrueMud™ mixer used in the present invention is a heating device that not only shears the mud, but heats it by converting shaft horsepower into heat. By adding temperature controls to the device, the heat can be adjusted with the speed (RPM, or angular velocity) of the disk and the flow through the device. By adding a pump and a bypass line, a process control loop with a known volume can be circulated in the present invention to shear the mud at a given flow rate and to heat the mud to report accurate, temperature dependent rheology of the fluid actually headed to the well.

There are several devices that can measure viscosity of drilling mud including, but not limited to a Brookfield TT-100 that measures viscosity at different reciprocal seconds of shear to provide real time rheology. Other devices such as an Endress+Hauser Promass 83i measures viscosity based on vibration feedback correlated to different reciprocal seconds. Since drilling muds contain solids, a pipe rheometer also works well, also enabling the calculation of a friction factor at different flow rates. A pipe rheometer calculates rheology based on flow rate and pressure drop across a known length of pipe. Once steady-state is reached in the process control loop, fluid can be added and removed from the same loop by controlling the valves that allow fluid into and out of the process control loop. The flow into and out of the loop can be automatically adjusted by simple temperature, desired process flowrate or by viscosity/rheology measurement. Using all digital sensors, the process control loop can be automated by a process logic controller, and/or a computer and then easily reported remotely using the Internet. Such a device including the process control loop is scalable. A 1 inch device may be used simply as a method to do automated mud checks or as a laboratory device, whereas a 2 inch or 3 inch device can fully process the fluid being used at the well.

The combined shearing device and rheology process control loop acts on a realtime aliquot of the drilling fluid in the well or in a tank. The realtime aliquot solves the problem of sampling in a stratified tank, or running mud checks in a dynamic system where the mud is changing for any number of reasons including water influx. The known volume of fluid in the loop is actually used in the well, unlike a sample tested in a laboratory. The aliquot may be isolated and the method of the invention performed while drilling is stopped (for example when a pipe is added to the drill string) and the mud is not flowing to the well, or while drilling is progressing.

The process control loop can contain (contains) mixing equipment such that the realtime aliquot of the larger volume of drilling mud can be adjusted and rheology, density, pH, electrical stability, and other properties measured immediately to check potential mud treatments before mixing the full volume of drilling fluid. In the laboratory such adjustments are done on a “barrel equivalent” of mud which consists of 350 ml of fluid and 1 gram equals 1 pound per barrel. In the field you have to adjust the full mud volume and wait to run mud checks after the mud has circulated through the drill bit. That process often takes more than one circulation and more than one mud check to get the desired mud properties. There is a known volume of mud in the process control loop of the present invention and a known flow rate. Small volumes of chemistry can be mixed into the mud either by isolating the process control loop or by proportioning the chemical concentration based on flow in the process control loop to immediately know the mud properties before adding the chemistry (chemicals) to the full volume of mud.

Furthermore, the meter can send readings continuously or intermittently to a controller which controls the addition of mud thinner, polymers to add viscosity, and other additives. The controller also controls the speed of the mixer and/or flow through the mixer. The thus prepared drilling mud proceeds to the drill bit where it already has the desired attributes.

In the vicinity of the drill bit, the drilling mud picks up drill cuttings and other solids; the drilling mud is designed to do so efficiently and carry the solids back to the surface where the bulk cuttings are removed with surface separation equipment such as hydrocyclones, screens and centrifuges. The separation process is designed to be efficient, but some of the mud is lost in this process and low gravity solids below 10 micron in size are generally not removed. New mud is mixed in a mud tank where the used mud including low gravity solids that could not be removed is mixed with new drilling mud ingredients, thus changing the properties of the material in the tank. The invention continues to adjust the properties of the drilling mud by monitoring and maintaining viscosity or rheology by regulating the energy input to the FCCD and the amount of additives replenished or added to the drilling mud. Furthermore information generated in the present invention can be used to remotely monitor the drilling mud such that a skilled mud engineer is not required to do continual mud checks at the rig site and can therefore manage more rigs.

The TrueMud™ (FCCD) mixer configuration allows for viscosity measurements since it is essentially a Couette style device with a rotor. The calculations are widely reported in the literature and can be found on Page 21 of “More Solutions to Sticky Problems” published by Brookfield Engineering Labs, Inc.

The FCCD is a spinning disk inside of a cylinder and can be set up to measure rheology. Rheology is shear stress measured at different shear rates. Shear rate is represented by the speed of the spinning disk. Shear stress is the torque required to spin the disk. Both can be accurately measured and the calculations are known to convert the speed of the disk and torque into a viscosity. There are some unknowns such as critical velocity. To measure rheology, the fluid should be in laminar flow below the critical velocity. To overcome any unknowns, the FCCD can be calibrated either by using a calibration fluid such as 100 centistoke silicone oil that is also used to calibrate laboratory rheometers, or the viscosity can be normalized to one known viscosity point using pressure drop across a known length of pipe or a device that measures viscosity at one shear rate. Furthermore, the viscosity can be compared to a manual Fann 35 reading done in the field and in all cases software can be used to adjust the viscosity to match the Fann 35 viscosity. BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates diagrammatically the prior art method of relying on the drill bit to shear mix the mud ingredients. The parts are not shown in relative proportion. Mud tank 1 contains the ingredients for a drilling mud. It may have a rough mixing capability, not shown. As drilling commences and proceeds, the mud in tank 1 is sent in conduit 2 to the well 3 below rig 6, following the path indicated by the downwardly oriented arrows to the bottom of the well 3 and the drill bit 4. The fluid may be directed through nozzles or ports on the drill bit, causing shearing. As the drill bit 4 does its work, drill cuttings are created, and these are picked up by the drilling mud and removed as indicated by the upwardly oriented arrows. From the top of the well, the solids-laden used drilling fluid is returned through conduit 5 to the tank 1 where it mingles with the mud ingredients already there. The effects of shearing through or around the drill bit are difficult to relate to the properties of the fluid in the tank. Moreover, the fluid is not sheared prior to entering the well, as is desirable. In addition, the prior art method, and modifications of it, rely on time-consuming and error-prone sampling and laboratory tests.

Referring to the simplified diagram of the invention in FIG. 2, mud tank 11 contains the ingredients for a drilling mud. It normally will have a rough mixing capability, not shown. As drilling commences and proceeds, the mud in tank 11 is sent in conduit 12 to the flow-controlled cavitation mixer 17, where it is shear mixed, and then through conduit 18 to viscometer 19, which measures its viscosity. It then continues in conduit 18 to well 13 associated with rig 16, following the path indicated by the downwardly oriented arrows to the bottom of the well 13 and the drill bit 14. As the drill bit 14 does its work, drill cuttings are created, and these are picked up by the drilling mud and removed as indicated by the upwardly oriented arrows. From the top of the well 13, the solids-laden used drilling fluid is returned to the tank 11 where it mingles with the mud ingredients already there.

Viscometer 19 generates a signal sent through line 20 which is used to control the speed or energy input of flow-controlled cavitation mixer 17 as a function of viscosity. Viscometer 19 also generates a signal sent through line 21 which is used to control the introduction of viscosity-modifying agent from source 22. A process controller, not shown, can manage the viscosity inputs and regulate the mixer and the viscosity-modifying agents according to programmed instructions.

It is thus not necessary to rely on the drill bit to perform the highly desirable function of shear mixing. And, the drilling mud is at all times at the desired viscosity. The shear mixing action of the cavitation mixer 17 will be further explained with respect to FIG. 3.

FIG. 3 is a partly sectional view of a flow-controlled cavitation mixer, or FCCM. The FCCM comprises a substantially cylindrical rotor 31 within a housing having an inlet end 41, an outlet end 39, and encasement 33 defining a cylindrical internal surface substantially concentric with that of rotor 31. Rotor 31 is mounted on shaft 32 which is turned by a motor not shown. Shaft 32 is set on bearings 45 and 46 in extension 38, and its position may be adjusted horizontally (as depicted) to vary the spaces between rotor 31 and housing ends 41 and 39 as indicated by arrow 47. Rotor 31 has cavities around its cylindrical surface; the cavities are illustrated as sections 34 aand as openings 34 b.Rotor 31 also has a flow director 37 on its inlet side. While rotor 31 rotates, fluid from a source not shown enters through inlet 35 and encounters flow director 37 which spreads it to the periphery of rotor 31 as indicated by the arrows. The fluid then passes through cavitation zone 40, a restricted space where cavitation is induced if the rotor is rotating fast enough, as explained elsewhere herein. Cavitation can be controlled to increase the temperature of the fluid to a desired value by controlling the speed of rotation of the rotor. Conversely, energy input to the FCCM can be controlled by direct measurement of rotation speed, a very useful datum to have for fluids of varying viscosity and rheology such as drilling mud.

to obtain the viscosity μ. However, some reports on the spindle viscosity formula are concerned with the effects of the space at the end of the spindle, and various workers have calculated additional formulas for them. In the present invention, not only are relatively large surfaces present on both “ends” of the rotor 31, but also, the fluid continually flows through the cavitation mixer while the calculations are made. Although the non-cylindrical faces of rotor 31 (the “ends” of the “spindle”) are relatively large compared to the width of the rotor, their effects on the calculation of viscosity are reduced by two features of the FCCM construction: first, flow director 37 spreads the incoming mud evenly over its surface so that when the mud enters cavitation zone 40 it will follow a helical path in substantially laminar flow over the cylindrical surface of rotor 31. In the non-cavitation mode—that is, when the rotor 31 is not rotating fast enough to cause cavitation, the cavities 34 aand 34 bare nevertheless filled with fluid which tends to remain in the cavities, providing surfaces over which the fluid passes. As indicated in FIG. 3, the profile of flow director 37 is a smooth curve tending to reduce turbulence and encourage laminar flow. The smooth curve profile of flow director 37 may be parabolic, elliptical, hyperbolic or a more complex smooth curve, generally campanulate and asymptotic toward the neck of rotor 31. Second, helical flow through cavitation zone 40, even in the absence of cavitation, is somewhat assisted by the position of outlet 36 near the periphery of rotor 31, as the mud passes quickly to outlet 36 from cavitation zone 40 without establishing a significant flow pattern on the outlet side of rotor 31.

Viscosity of slurries has been successfully measured in a helical flow instrument. See, for example, T. J. Akroyd and Q. D. Nguyen, Continuous Rheometry for Industrial Slurries,14thAustralasian Fluid Mechanics Conference, 10-14 Dec. 2001. The authors recognized a tangential component to the shear stress as well as an axial component, incorporated into their calculations. See also Shackelford U.S. Pat. No. 5,209,108. Because laminar flow is encouraged across the cavitation zone when measuring viscosity, pressure drop across the cavitation mixer may be used, according to the classical Poiseuille formula explained below, to modify the calculation of viscosity.

In FIG. 4, a flow diagram is presented for a loop of the invention. In this configuration, the cavitation mixer 53 performs two separate functions. In one function, it is operated with power input sufficient to cause cavitation in the fluid until a desired temperature is attained in the fluid. In the cavitation mixer"s second function, the power input is reduced so that no cavitation takes place and the cavitation mixer acts as a viscometer.

In the optional “straight-through” mode, which does not employ the recycle loop, the drilling mud ingredients pass through valve 51 on conduit 50 to pump 52, through valve 62, and then into cavitation mixer 53, where they are heated and mixed, then through conduit 54 to Coriolis meter 55 and viscosity meter 61 before passing through valve 56 to a well, or to storage or other use not shown. Coriolis meter 55 (which may be an E+H Coriolis meter) may measure density in conduit 54. Viscosity meter 61, which may be a Brookfield TT-100 viscometer, may be programmed to continually read viscosity at all Fann 35 speeds.

But an important feature of the invention is that an aliquot of fluid (drilling mud) can be isolated in the loop defined by closing valves 51 and 56 and opening valves 58 and 59, thus flowing an isolated, known quantity of fluid continuously in the loop through cavitation mixer 53, conduit 54, conduit 57 and again through conduit 54 to cavitation mixer 53. This may be referred to as the “loop mode.” In accordance with the invention, the cavitation mixer is operated in the cavitation mode to quickly heat the mud aliquot to a desired temperature (measured by a transducer or other device not shown), and then it is operated in the non-cavitation, or shear, mode so it can shear the aliquot and be utilized as a viscometer. Acting on the same aliquot of drilling mud as it circulates in the loop, the cavitation mixer 53 may be programmed to heat the mud, by cavitation, to a second temperature and then, without cavitation, to shear it. While shearing the mud, the cavitation mixer may be utilized as a viscometer employing Couette principles. The isolated aliquot may be further heated to a third temperature and viscosity measurements obtained as described elsewhere herein, as a function of torque on the mixer"s shaft and angular velocity of the rotor.

When viscosity-modifying agents or other chemicals are to be added to the mud, valve 62 may be closed and valves 64 and 70 opened, causing mud to flow through additive conduit 65. Additive conduit 65 passes through an eductor 67 which assists the feeding of dry chemical (such as dry polymer) from hopper 66 if such a feed is required by the controller. Conduit 65 also is associated with liquid feeder 68, which can, on command, deliver doses of liquid chemical (such as dissolved polymer) into additive conduit 65 through inlet 69. Additives introduced to the mud in additive conduit 65 will be thoroughly mixed into the mud when it passes into cavitation mixer 53.

A dashed-line rectangle bearing the reference number 63 on conduit 57 in FIG. 4 is labeled “Mud Check Instruments.” This represents any or all of meters, probes, instruments and transducers for detecting or measuring density, flow, viscosity, pH, percent solids, water cut or oil/water ratio, electrical stability, particle size, temperature and other properties of the mud. Such devices are not limited to positioning in or on conduit 57. They may be anywhere in the system; for example, temperature probe 71 and pressure probe 72 are illustrated in conduit 54. Included in Mud Check Instruments 63 are (one or more) computers, processors or controllers necessary or useful to monitor and modify the properties of the mud in the loop. For example, computers, processors, or controllers may be programmed to vary the power input and/or angular velocity of the shaft of cavitation mixer 53, or to open and close valves so that hopper 66 or liquid feeder 68 can deliver prescribed amounts of additives. Data about the mud and the well"s operation may be accumulated to provide increasingly accurate refinements to be used possibly in the “straight-through” mode. Additives are proportioned to the aliquot in the loop and circulated to confirm the modifications made to its properties. The “straight-through” mode may be modified to take the illustrated detour through additive conduit 65 for continuous proportionate injections of additive(s).

Viscosity may be measured by a viscometer, not shown, in conduit 54 or conduit 57. Optionally, viscosity may be read by pressure difference as is known in the art. The reduction in pressure between points Pr1 and Pr2 may be ascertained by any acceptable pressure reading devices and the difference used to reinforce the calculations according to the spindle viscosity formula described above and/or viscometer 61. Poiseuille"s pressure drop equation for viscosity μ for a fluid flowing in a tube is:

where R is the radius of the tube, gc is the gravitational constant, P1is the measured upstream pressure in the tube, P2is the measured downstream pressure in the tube, C is a constant conversion factor for expressing viscosity in poises, L is the distance on the tube between P1and P2, and Q is the flow rate of the fluid in the tube. So, where the radius of the tube is fixed and the flow is steady, and because everything else is a constant except the measured pressures, the viscosity μ is directly proportional to the pressure difference.

One of the advantages of my process is that data may quickly be accumulated for more than one temperature for one or more aliquots of the mud. The aliquot isolated in the loop is easily ramped up from, for example, 100° F. to 150° F. to 175° F. In this example, the aliquot is first heated by the cavitation mixer in the cavitation mode to 100° F., the viscosity is measured either by Couette principles applied to the cavitation mixer or by a separate viscometer, or both, then the mud is heated to 150° F. and the viscosity is again measured by one or more devices, and the mud is further heated by the cavitation mixer to, say, 175° F., after which the viscosity is again measured by at least one device, which may be the cavitation mixer itself. Additional temperature levels may be included, or not. As Couette principles require inputs of torque and angular velocity of the rotor 53, these are monitored and sent to the process controller along with the temperature and other properties.

Thus, whether viscosity is measured in the loop at one temperature or at more than one temperature, the viscosity measurements can be stored (along with any other properties found by other instruments) and then used in the straight-through mode to heat the fluid and adjust the viscosity to the desired value until it is determined that additional data are needed. Converting from the loop mode to the straight through mode may be accomplished either by the programmed controller or by a human operator.

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