Baroid Sand Content

600 rpm reading

Tl022)'r

The theory underlying the above calculations is discussed in Chapter 5.

The shear rates prevailing in the two-speed direct-indicating viscometer are somewhat higher than those usually prevailing in the annulus. The plastic viscosity and K value of many muds increase at low rates of shear; the yield point is less than predicted from the 600 and 300 rpm readings; and n decreases. Therefore, when flow parameters are being determined for the purpose of calculating flow rates or pressures in the annulus, it is advisable to use a multispeed viscometer, such as the Fann Model 35A. This model is similar to the Fann 34 except that it has six speeds: 600, 300, 200, 100, 6, and 3 rpm. A modification by Walker and Korry5 covers shear rates in the annulus more evenly. The Fann Model 50 enables the rheological parameters to be measured at temperatures up to 500 F (260 C) and pressures up to 1000 psi (70 kg cm-). A recorder, which plots shear stress versus time on a strip chart, is available for use with the direct-indicating viscometers. It is useful for observing hysterisis loops (see Chapter 5) and for observing changes in shear stress with time at constant speed and temperature.

Another type of concentric cylinder viscometer, known as the BHC viscometer6 (see Figure 3 -8), operates inside a high pressure cell which permits the viscous parameters to be determined at temperatures up to 650 F (343 C) and pressures up to 20,000 psi (1400 kg/cm2). Drive through a magnetic couple provides shear rates of 11, 21, 32, 64, 96, 191, 286, 573, and 860 reciprocal seconds. Torque is measured by an electrical dynamometer containing the spring on which the bob is suspended. Changes in resistance are transmitted to an external meter. Shear stresses from 7 to 840 dynes/cm2 can be detected.

A knowledge of effective viscosity at the very high rates of shear prevailing in the bit nozzles is necessary when maximizing rates of penetration. Concentric cylinder viscometers are not suitable for this purpose because the high rotational velocities required cause centrifugal disturbances. High shear rates arc best obtained in a capillary viscometer, which, in its simplest form, consists of a pressure vessel from which the mud is discharged through at least 12 inches (30 cm) of capillary tubing with an internal diameter in the

Shearometer Diagram
Figure 3-8. BHC viscometer. (Courtesy of Oil and Gas J.)

magnetically controlled bob (see Figure 3-9). Maximum pressure is 20,000 psi (1400 kg/cm2) and maximum temperature is 500 F (260 C). The disadvantage of the consistometer is that the rate of shear cannot be determined, so the data obtained are empirical. Correlations with the Fann Model 50 have been used to report the data as "equivalent viscosity".'1

Figure 3-9. Simple section diagram of modified Fann consistometer. (From SinhaCopyright 1970 by SPE-AIME.)

Cíe! Strength

Gel strengths are determined in the two-speed direct-indicating viscometer by slowly turning the driving wheel on top of the instrument by hand and observing the maximum deflection before the gel breaks. The same procedure is followed in the multispeed viscometer, except that the cylinder is rotated at 3 rpm by the motor. Gel strengths may be measured after allowing the mud to stand quiescent for any time interval of interest, but they are routinely measured after 10 seconds (initial gel strength) and 10 minutes. The dial reading gives the gel strength directly in pounds per hundred square feel

Formerly, gel strengths were sometimes measured in an instrument known as the shearometer. A duraluminum cylinder guided by a graduated scale v>as allowed to sink into the mud, and the gel strength in lb/100 ft-' read from the scale. Nowadays, the instrument is seldom used, but the cylinders are useful for determining gel strength in situ after the mud has been aged statically. as described later in this section.

Filtration

Static Filtration. The low-pressure static filtration press in use today is based on an original design by P.H. Jones.12 The essential components are shown in Figure 3-10. Several modifications of this cell are commercially available. The standard dimensions are: filtration area, 7.1 in2 (45.8 cm2); minimum height, 2.5 in (6.4 cm); and standard filter paper, Whatman 50, S & S No. 576, or equivalent. Pressure of 100 psi (7.0 kg/cm2), from either a nitrogen cylinder or a carbon dioxide cartridge, is applied at the top of the cell. The amount of filtrate discharged in 30 minutes is measured, as is the thickness of the filter cake to the nearest 1/32 in. (1 mm) after washing off the excess mud with a gentle stream of water.

Filtration properties at high temperatures and pressures are usually measured in a cell similar to that shown in Figures 3-11 and 3 12. The working pressure is 1000 psi (70 kg/cm2), and the maximum temperature is 450 F (232 C). In order to avoid flashing or evaporation of the filtrate at high temperatures, a back pressure of 100 psi (7.0 kg/cm2) is held on the filtrate discharge when the test temperature is less than 300°F (149 C), and 450 psi (31.6 kg/cm2) when the temperature is between 300 and 450 F (149 and 232 C). For temperatures up to 400 F (204 C), a Whatman 50 filter paper is used, and for temperatures above 400 . a new stainless steel disc (Dynalloy X-5 or equivalent) is used.

f iltration time is 30 minutes at the temperature of interest, but the volume of filtrate collected is doubled to allow for the difference in filtration area between the high and low pressure filtration cells. 22.9 cm2 versus 45.X .cm-.

Figure 3-10 Fanrs Model 12 B, low temperature filter press. (Courtesy Fann Dresser.)

Strict safety precautions must be followed in making filtration tests at high temperatures and pressures. The procedure recommended in API RP 13B should be closely followed. In particular, the cell must not be filled above the manufacturer's recommendions and, at the conclusion of the test, the cell must be allowed to cool to room temperature before disassembly.

A filtration cell with a maximum pressure of 2,425 psi (170 kg/cm') at temperatures up to 300 "F (140 C) was designed by Shremp and Johnson."

Dynamic Filtration. In order to simulate filtration in the drilling well more closely, it is necessary to limit the growth of the filter cake by either liquid or mechanical erosion. Over the years, a number of investigators have studied dynamic filtration in specially designed apparatus.i4- 15' l" The most meaningful results were obtained in systems that either closely simulated conditions in a drilling well, or which permitted the rate of shear at the

Figure 3-11. High pressure high temperature filter tester with pressure receiver. (Courtesy of NL Baroid.)

surface of the cake—which is the critical factor limiting growth—to be calculated. Ferguson and KIotzls came close to simulating well conditions by measuring filtration rates through permeable lumnite cement and sand cylinders in a model well using full-size drilling tools. Horner, el al,l(' used a microbit drilling machine and rock cores. Novak and Krueger1' observed filtration rates through cores exposed on the side of an annulus through which mud was being circulated. A mechanical scraper enabled filtration conditions under the bit to be simulated when desired.

Figure 3-12. Assembled high pressure high temperature filter tester. (Courtesy of NL Baroid.)

Figure 3-12. Assembled high pressure high temperature filter tester. (Courtesy of NL Baroid.)

The rate of shear at the cake surface can be calculated in systems, such as that of Prokop,18 in which mud is circulated under pressure through a permeable cylinder. The internal diameter of the cylinder should be large relative to the thickness of the filter cake so that the growth of the cake does not change the internal diameter significantly, and thereby change the rate of shear. Bezemer and Havenaar19 developed a compact and very convenient dynamic filtration apparatus, in which mud was filtered into a central core or sleeve of filter paper, while being sheared by an outer concentric cylinder

permeable cylinders. WyantV" circuit (Figure 3 14) includes high-shear valves, which rapidly break down clay aggregates and rigid gel structures, and a filtration cell and a corrosion unit. A system designed by Kelly includes a pipe viscometer, and means of adding treating agents while circulating. Maximum temperature is 350 F (177 C) (Figure 3-15). A more elaborate system by de Lautrec— enables rheological properties, and static and dynamic filtration, to be measured at flow rates up to 12 ft/sec (4 m/sec), temperatures up to 480°F (250°C), and pressures up to 7250 psi (510 kg cm ).

Aging at High Temperature

Many mud constituents degrade slowly at high temperature. Such degradation occurs while circulating, but is more severe with the mud left in the lower part of the hole when making a trip, because of the higher temperature involved. Consequently, the effect of aging at elevated temperatures should be observed on all mud compositions and additives.

Such tests are usually made in stainless steel or aluminum bronze pressure cells, 1 which are commercially available in 260 or 500 cm1 sizes (Fig ? I 6).

For Liquid-Chemicals

To Stum Supply and Control«

I To Condensate Line

To Presaure-Sensitive Alarm

For Liquid-Chemicals

To Stum Supply and Control«

To Presaure-Sensitive Alarm

I To Condensate Line

Figure 3-15. Multifunctional circulating system. {From Kelly and HawkP Courtesy of Oil and Gas J.)

Figure 3-15. Multifunctional circulating system. {From Kelly and HawkP Courtesy of Oil and Gas J.)

B. Low pressure cell.

A. High pressure cell.

Figure 3-16. Celts tor aging muds at elevated temperatures in roller oven.

To prevent boiling of the liquid phase, the cells are pressurized with nitrogen or carbon dioxide through connections provided for the purpose.JJ The applied pressure must be at least equal to the vapor pressure of the liquid at the test temperature.

To simulate aging of the mud while it is circulating in the well, the cells are rolled in an oven, such as that shown in Figure 3 17, for at least 16 hours at the average well circulating temperature. The ceils are then cooled to room temperature, and the rheological and filtration properties are measured and compared to the same properties before aging.

When a mud is left in a high temperature hole during a round trip, the crucial factor is the undisturbed gel strength, which determines the pressure required to break circulation. When testing, therefore, the cells are aged statically in an oven heated to the temperature of interest for the required length of time; cooled to room temperature; and the undisturbed gel strength

B. Low pressure cell.

A. High pressure cell.

Figure 3-16. Celts tor aging muds at elevated temperatures in roller oven.

(Courtesy ot NL Baroid measured in the ceil with a shearometer tube.2S In the case oflime muds that may set to semi-solid, it may be necessary to use a modified cement penetrometer.20

Particic Size Determination

Fhe API Sand Test

The sand content is a measure of the amount of particles larger than 200 mesh present in a mud. Even though it is called a sand test, the test defines the size, not the composition, of the particles. The test is conveniently made in the apparatus shown in Figure 3-18- The mud is first diluted by adding mud and water to the respective marks inscribed on the glass tube The mixture is

Figure 3-17. Roller oven. (Courtesy of NL Baroid.)

Table 3-1* Definition of Particle Sizes

Particle Size

Particle Classification

Greater than 2000

2000 - 250 microns

- 74 microns

74 - 44 microns

44 - 2 microns

2 - 0 microns microns

*} rom API Bui. 13C (June, 1974). American Petroleum Institute, Dallas.

Sieve sizes to suit the particular material being tested may be chosen, bul the mesh sizes should correspond to the American Society for Testing Materials specification El 1-26. The ASTM has recommended procedures for sieve tests.1" Table 3 1 shows the sieve sizes suggested for use in parliclc size classification of solids removed by drilling fluid processing equipment The API committee on standardization of Drilling Fluid Materials has recommended a procedure to determine the particle size distribution of drilling fluid solids.28

Sedimentation Methods

The importance of the sub-sieve particles in the performance of drilling muds has long been recognized, but actual measurements were rarely made.2" Methods for measuring the particle size distribution of mud solids have been taken mainly from studies on ceramic clays and soils.'" No attempt will be made here to list the numerous publications dealing with such methods: Recent books on the subject can be consulted for details.31 32

Particle sizes in the range of 44 microns to 0.5 micron are generally determined by observing sedimentation velocities.33 Particle radius is related to sedimentation velocity by Stokes' law, which states:

9d/i

0 -1

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