Optimizing Drilling Mud Shearing

Abstract

Viscosity and other properties are determined at desired temperatures in drilling mud and other fluids by using a versatile cavitation device which, operating in the cavitation mode, mixes and heats the fluid to a specified temperature, and, operating in the shear mode, acts as a spindle for application of Couette principles to determine viscosity as a function of shear stress and shear rate. The invention obviates the practice of adjusting rheology of a drilling fluid by passing it through the drill bit. Drilling fluid may be managed by a “straight-through” method to the well, or by placing the cavitation device in a loop which isolates an aliquot of known volume and circulating the fluid through the loop including the cavitation device. A controller may be programmed to manage the viscosity and other properties at various temperatures by controlling the power input and angular rotation of the “spindle” (which has cavities on its cylindrical surface), and feeding viscosity-adjusting agents and other additives to the fluid. Data may be collected from the loop and used in the “straight-through” mode until it is determined that conditions require a new set of data, or the loop may be used continuously. The system may be used with a supplemental viscometer, density meter, and other instruments.

Claims

1. Method of preparing drilling mud ingredients for use in a well comprising (a) pumping said drilling mud ingredients from at least one source through a conduit, said conduit including a recycle loop with valves for isolating an aliquot of said drilling mud ingredients in said recycle loop, (b) passing an aliquot of drilling mud ingredients so isolated through a cavitation mixer located on said recycle loop to heat, by cavitation, said drilling mud ingredients to a desired temperature, (c) passing said aliquot of drilling mud ingredients through said cavitation mixer, without cavitation, to shear said drilling mud ingredients at said desired temperature to obtain a desired viscosity, thus making a prepared drilling mud for a well, and (d) passing said prepared drilling mud outside said loop for storage or use in a well.

2. Method of claim 1 wherein said cavitation mixer is a flow-controlled cavitation mixer.

3. Method of claim 1 including, in step (c), determining said desired viscosity by Couette principles, optionally modified by a function of a difference in pressure across said cavitation mixer, applied to said cavitation mixer.

4. Method of claim 1 including, in step (c), measuring viscosity by an in-line viscometer within said loop and regulating, as a function of said viscosity, at least one of (i) power input to said cavitation mixer, and (ii) the addition of a viscosity modifier to said drilling mud ingredients.

5. Method of claim 1 including intermittently or continuously monitoring said drilling mud ingredients in said loop for at least one of density, flow, viscosity, pH, percent solids, water cut or oil/water ratio, electrical stability, particle size, and temperature.

6. Method of claim 1 including accumulating viscosity data for said aliquot of drilling mud ingredients in said loop at more than one temperature, adjusting said valves to convert said loop to the straight through mode, and continuously or intermittently controlling viscosity of said drilling mud ingredients during drilling of said well.

7. Method of claim 1 including adjusting said valves to isolate at least one additional aliquot of said drilling mud ingredients in said loop and repeating steps (b) and (c) on said at least one additional aliquot of said drilling mud ingredients.

8. Method of claim 7 including accumulating a data base of properties of said drilling mud from a plurality of aliquots so isolated.

9. Method of claim 1 wherein steps (a), (b), and (c) are performed while drilling of said well is stopped to add pipe to the drill string.

10. Method of preparing drilling mud ingredients for drilling a well comprising (a) pumping said ingredients into an isolation loop, (b) heating said ingredients in said loop to a desired temperature, (c) measuring the viscosity of said ingredients at said desired temperature in said isolation loop, (d) adding at least one viscosity-adjusting agent to said ingredients to obtain a desired viscosity at said desired temperature, and (e) removing the drilling mud so prepared from said isolation loop.

11. Method of claim 10 including, between steps (c) and (d), repeating steps (b) and (c) at at least one additional desired temperature.

12. Method of claim 10 including controlling valves on said isolation loop to admit said ingredients into said loop in step (a) and remove said ingredients in step (e).

13. Method of claim 10 including pumping said ingredients to circulate said ingredients though said loop during steps (a), (b), and (c).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] FIG. 1 is a diagram of the prior art method using the drill bit to shear the fluid.

[0031] FIG. 2 is a diagram of the invention method.

[0032] FIG. 3 is a partly sectional view of the flow controlled cavitation mixer.

[0033] FIG. 4 shows a basic process loop including a cavitation mixer.

DETAILED DESCRIPTION OF THE INVENTION

[0034] 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 3, 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.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] 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 34a and as openings 34b. 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.

[0039] The versatile FCCM is also able to act as a viscometer because, when it is not causing cavitation, it acts as a cylindrical spindle, a known form of viscometer employing Couette principles. For the fluid materials relevant to the invention, viscosity may be expressed as the ratio of shear stress to shear rate, or μ.=I where the shear stress T is .sub.T=T/2πRs2L and shear .sub.2wRc2Rs2 that is 2wRc.sup.2Rs.sup.2/x.sup.2(Rc.sup.2-Rs.sup.2), where Rc is the radius of the cylinder, in this case the internal width of inlet and outlet ends 41 and 39, R.sub.s is the radius of the spindle, in this case the radius of rotor 31, T is the torque of the rotor acting on the fluid, co is the angular velocity of the rotor, and x is the radial location at which shear is being calculated. As indicated above, this formula assumes there is no cavitation taking place around the rotor—that is, that the action of the cavitation mixer is limited to generating the shearing action that enables reading shear stress and shear rate without the disruption that would be caused by cavitation. I call this the “shear mode,” and when the cavitation mixer is causing cavitation, I call it the “cavitation mode.” The above described method of calculating viscosity, and similar formulas in the literature using a spindle and cylinder, I call the “spindle viscosity formula” or, sometimes, “Couette principles.”

[0040] Persons skilled in the art may observe that most presentations of Couette principles or the cylindrical spindle measurement of viscosity illustrate a spindle that is longer than the diameter of the cylinder in which it resides, and that the cavitation mixer of the present invention is illustrated as the opposite—that is, the length of the “spindle” is the width of rotor 31, which is depicted as shorter than its diameter, or even its radius. This relationship of the cylinder and the housing within which it resides does not fundamentally change the calculation of I to obtain the viscosity 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 34a and 34b are 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.

[0041] 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, 14th Australasian 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.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] Persons skilled in the art may recognize that additive conduit 65 is not essential for liquid feeder 68, which could be placed on conduit 50 anywhere upstream of cavitation mixer 53. Eductor 67 for solid additives, however, is an in-line device and accordingly is best used in a separate conduit such as additive conduit 65.

[0047] 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).

[0048] 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 II. for a fluid flowing in a tube is:

[00001]$\mu =\frac{\pi R^4\mathrm{gc}\left(P_1-P_2\right)C}{8\mathrm{LQ}}$

[0049] where R is the radius of the tube, gc is the gravitational constant, Pi is the measured upstream pressure in the tube, P2 is 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 Pi and 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 II. is directly proportional to the pressure difference.

[0050] 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.

[0051] 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.