In order to design and maintain a mud to perform a given function, it is necessary to measure the mud properties which control its ability to perform that function. For this reason, measurements are normally made on a variety of mud properties. Considerable research has been done to design tests and to correlate these tests with the functions of a mud.
Normally, the functions of a mud are carried out downhole under conditions not easily duplicated in a mud test. Over the years, attempts have been made to either simulate more closely the downhole conditions or to predict the downhole mud properties from surface condition measurements.
Measurements of both physical and compositional properties are made in a complete mud check. Some functions are controlled directly by the mud composition, but even those which we correlate with physical properties are controlled by adjusting the mud composition. Since we normally have a multifunctional requirement for a drilling fluid, we need an extensive series of both physical and
Relationship of compositional tests in order to properly monitor a mud system. Mud Properties to Properties should be measured on the mud going in at the suction Functions, and on the same mud coming out at the flow line.
Density The starting point of pressure control is the control of mud density.
The weight of a column of mud in the hole necessary to balance formation pressure is the reference point from which all pressure control calculations are based. The required weight of the mud column establishes the density of the mud for any specific case.
Fortunately, density is one of our most accurate measurements. With a simple mud balance we are able to weigh a mud to the nearest 0.1 lb./ gal, which is equivalent to 5.2 psi per 1000 ft. of mud column. Mistakes in measuring density account for most of the inaccuracies. The most frequent mistakes are:
Density The calibration of a mud balance should be checked at two densities
Continued that span the range of densities to be measured. One convenient calibration check point is to measure the density of fresh water. At temperatures between 75° and 95°F, the density of water should range between 8.32 and 8.30 lb./gal. A completely suitable means of checking the calibration at the high end of the scale has not been devised. High weight muds can be used if their density is accurately known and if a representative sample can be transferred to the mud balance. Settling of solids in the sample makes it questionable whether a representative sample is used in the calibration check. Some high-density solutions have been used for this purpose, but they are highly corrosive.
If a mud balance is found to be in error by more than 0.1 lb./gal at either end of the scale, it would be advisable to obtain a new, correctly calibrated balance rather than re-calibrate or correct the readings of the old balance. The caps on mud balances should never be interchanged. A cap may weigh differently from the original and cause an error in density measurements.
When air or gas is trapped in a mud sample, it will decrease the measured density. This is especially troublesome in muds that have high yield points or gel strengths, but it can occur in any mud. There are devices which can be used to degas a mud sample. In less
Continued severe cases the sample can be stirred on a Fann viscometer at medium speed to help "break out" the entrained gas. A pressurized mud balance can also be used to obtain correct mud densities.
It is important to fill the balance 100 percent with mud in order to obtain an accurate density. An air bubble can be trapped under the lid and cause low density measurements or granular material may hold the lid up, resulting in overfilling and a high density measurement.
The mud balance should be level and out of the wind. Air currents are often sufficient to tip the balance and affect the reading.
Density is not greatly affected by downhole conditions. Increased temperature causes the density to decrease, but increased pressure causes an increase in density. Downhole, these effects oppose one another and tend to equalize. Problems can result from large changes in measurement temperatures. If a new mud is carried to location to displace a circulating system, there may be a large difference in temperatures at which the densities of the old and new muds were measured. The cooler mud may decrease in density from 0.1 to 0.3 lb./gal when it returns to the flow line after the first circulation. To avoid this problem, the densities of both muds should be checked at the same temperature.
Density The density of a mud is increased by adding barite, a commercial
Continued grade of barium sulfate. Barite is used as a standard weighting agent because of its low cost, high specific gravity, inertness, and low abrasiveness. Commercial barite is a mined product that undergoes very little processing other than grinding. Its specific gravity averages about 4.25. Pure barium sulfate has a specific gravity of 4.5, indicating that some impurities are present in the commercial grade. The impurities vary depending upon the source of the barite. The color is not indicative of the purity or quality of the barite. Soluble alkaline earth metals such as calcium and magnesium are among the more detrimental types of impurities and have been limited by API specification to a maximum of 250mg/l expressed as calcium. The amount of calcium introduced into the mud system from addition of API grade barite should not be detrimental, but a calcium check on the barite should be run to assure that an abnormal amount of calcium is not present. This can occur through poor quality control at the grinding plant or from cement contamination during transporting. Equations, charts, and tables necessary to calculate required material for weighting a mud are included in Appendix C.
Muds maintained at densities higher than required to control formation pressures cause a variety of problems. They decrease penetration rates, increase differential pressure sticking, increase
Density possibility of lost circulation, increase mud costs, and thus increase
C°ntinued overall well cost. Consequently, density should be controlled even in unweighted muds. This amounts to solids control, which is discussed in another section.
Flow Properties The flow (or rheological) properties of a mud are those properties which describe the flow characteristics of a mud under various flow conditions. In a mud circulating system, flow occurs at a variety of rates in conduits of different sizes and shapes. In order to know or predict the effects of this flow, we need to know the flow behavior of the mud at the various points of interest in the circulating system. To simplify the measurement procedure, we make only a limited number of measurements.
When a fluid flows, it exerts a frictional drag - called the shear stress - on the surface of the conduit. The magnitude of the shear stress depends on the frictional drag between adjacent "layers" of fluid traveling at different velocities, and the difference in velocities of adjacent layers next to the wall of the conduit. The difference in velocities between adjacent layers is called the shear rate. We are interested in the effect of the flow at the wall where both shear rate and shear stress are a maximum.
F|ow Properties If the shear stress is known at all shear rates, the complete flow Continued behavior of the fluid has in effect been described. For very simple fluids such as water or oil, the ratio of shear stress to shear rate is a constant. In these fluids, called Newtonian fluids, measurement of shear stress at one shear rate is sufficient to predict flow behavior at all shear rates. The ratio of shear stress to shear rate is the viscosity.
The viscosity is a single number that characterizes the flow behavior of a Newtonian fluid. When solids are added to a Newtonian fluid, the irregular shapes of these particles resists flow in a manner not directly proportional to rate of shear. The interparticle interference to flow is more pronounced at low shear rates than at higher shear rates. As a consequence, the ratio of shear stress to shear rate, or the effective viscosity, is high at low shear rates and decreases with increasing shear rate. This is known as "shear thinning." In other words, the increase in effective viscosity over that of water decreases with increasing shear rate.
The shear stress is directly proportional to the pressure required to produce the flow. In a Newtonian fluid, the smallest possible amount of pressure will cause the fluid to flow. In fluids that contain solids which link together to form a structure, flow will stop when the pressure or shear stress is reduced to a point which is less than the
Flow Properties shear strength of the structure. This point is called the yield stress of Continued the fluid. These non-Newtonian fluids, when allowed to remain still for a period of time, continue to develop this semi-rigid structure and the shear stress required to initiate flow increases. This shear stress is called gel strength. The structure becomes more rigid with time, causing the gel strength to increase with time.
Basically, in the mud circulating system, we are interested in the flow behavior of the mud at the bit where shear rates are extremely high, in the annulus where shear rates are relatively low, and in the pits where shear rates are almost zero. We are also interested in the flow behavior inside the drill pipe and drill collars, since this controls the amount of hydraulic horsepower that a pump can deliver to the bit. Figure l shows the approximate shear rate ranges for the various parts of the circulating system and the effective viscosity of a bentonite suspension at these various shear rates.
Continued on next page
Flow Properties Our basic design criteria for flow property control are to: Continued 1. Minimize viscosity at high shear rates in order to maximize penetration rates.
Obviously, this list of requirements has points of conflict. A degree of compromise will always be necessary.
Several instruments are used to secure information of the flow characteristics of mud. A brief description of the various tests and interpretation of the test data follows.
Marsh Funnei The Marsh funnel is a crude method for measuring the consistency of a fluid. Although listed on the mud check sheet as viscosity, the Funnel viscosity is not in the true sense a viscosity at all.
The test consists of filling the funnel to the bottom of the screen with mud (1500 ml) and timing how long it takes for one quart to flow out of the funnel. The time in seconds is reported as the funnel viscosity. Fresh water at 70°F will have a funnel viscosity of 26 seconds.
This test has the advantage of being quick, simple, and requiring very little equipment. It is useful in showing gross changes in the overall "viscosity" of a fluid, but it does not measure specific flow parameters. It can be changed by changes in plastic viscosity, yield point, gel strength, or density. For this reason it should be used only to monitor a mud and not to diagnose problems or to prescribe treatment.
Fann V-G Meter The V-G meter is a rotational type viscometer in which the fluid is contained between coaxial cylinders. The outer cylinder rotates at a constant speed and the viscous drag of the fluid on the inner cylinder or bob exerts a torque that is indicated on a calibrated dial. The torque is proportional to shear stress and the rotational speed is proportional to shear rate. The indicated dial reading times 1.067 is equivalent to shear stress in lb./100 sq. ft. And the rotational speed
Fann V-G Meter in rpm times 1.703 is equivalent to shear rate in recipical seconds.
Continued Two models of the V-G meter in common use are the Fann 35 and
34. The Model 35 is a six-speed model (600, 300, 200, 100, 6, and 3 rpm) and the Model 34 is a two-speed model (600 and 300 rpm). These instruments provide measurements of the actual flow parameters of shear rate and shear stress and also provide a means of making gel strength measurements. With this information we are better equipped to diagnose flow behavior and prescribe mud treatment than with the funnel viscosity.
After the shear stress/shear rate data are collected, they can be handled and reported in a number of ways. Traditionally, these data have been used to calculate plastic viscosity and yield point in the Bingham plastic rheological model, and these parameters have been reported on the mud check sheet. The difference in the V-G Meter dial readings at 600 and 300 rpm is the plastic viscosity, and the plastic viscosity subtracted from the 300 rpm reading is the yield point (see Figure 2).
Since the Bingham plastic model does not truly represent the shear rate/shear stress behavior of most muds, the calculated yield point is not equivalent to the true yield stress and the plastic viscosity is not a true viscosity. However, the wealth of experience we have acquired in the use of these parameters make them quite useful in predicting mud performance and diagnosing mud problems.
Figure 2 Bingham Plastic Mode/
Figure 2 Bingham Plastic Mode/
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