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Rubber Lining

Cement

Cement r Sv p'essure ? Fracture

Figure 3-22. Simulated test fracture. (From Howard and Scott.*1 Copyright 1954 by SPE-AIME.)

minerals present; adsorption isotherms: dispersion of shale chips in candidate muds; and performance of candidate muds under simulated downhole conditions. These tests are described in Chapter 8.

Lubricity

Extreme pressure lubricants were originally added to drilling muds as a means of increasing the life of bit bearings.44 A Timken Lubricant Tesier was modified to permit the mud under test to circulate between the rotating ring and the block upon which the ring bears, as shown in the diagram in Figure 3 23. The load-carrying capacity is indicated by the maximum weight that can he used without seizure. Film strength is calculated from the areu of the scar and from the load which is used on the block which passes the highest load test.

With the advent of sealed bit bearings, extreme pressure lubricants arc no longer added to drilling muds to reduce bearing wear, but they, and other surfactants, are added to reduce drill pipe torque, as discussed in Chapter 9. For this purpose the Timken tester has been further modified45 as shown in Figure 3- 24. The recommended procedure is to apply a 150-lb load with the torque arm, adjust the shaft speed to 60 rpm. and read the amperes on the meter. Amperes are converted to the lubricity coefficient by means of a calibration chart.

Figure 3 25 shows a more versatile lubricity tester:10 Mud is continuous!) circulated through the hole in the sandstone core; the stainless steel shaft rotates under load against the side of the hole; and torque in the shaft is monitored by a torque transducer, and automatically plotted against lime

Figure 3-23. Timken lubrication tester. Diagram of loading lever system showing test block and cup in place.
Figure 3-24a. Lubricity tester for drilling muds.

Figure 3-24b, Ring and block for lubricity tester. (Courtesy of NL Baroid.)

Figure 3-26. Schematic diagram of low differential-pressure test apparatus. (From Haden and Welch.™ Copyright 1961 by API.)

Corrosion Tests

Tests for corrosivity may be made in the laboratory by putting steel coupons and the mud to be tested in a container, tumbling the container end over end or rotating it on a wheel for a prolonged period, and then determining the weight lost by the coupon. If the test is made at temperatures or pressures that require the use of a steel cell, the coupon must not be in electrical contact with the cell (see Figure 3 28). Results are reported as loss

of weight per unit area per year, or as mils per year (rapy). With steel coupons of specific gravity 7.86, the formula is:

weight loss, mg x 68.33

area, in x hours exposed

Steel rings machined to fit into a tool joint box recess are commonly used to measure the corrosion that occurs in a drilling well.541 Recommended exposure times in the well vary from 40 hours to 7 days. The rings are then removed, cleaned, examined for type of corrosion, and the loss of weight determined,

A sensitive test for hydrogen embrittlement is obtained by using solar steel roller bearings that have been permanently stressed by a 40,000 psi (3,000 kg crrr ) load, instead of the steel coupons.55 Rubber or Teflon O rings may be placed around the bearings to simulate the corrosive conditions thai prevail under scale or mud cake on the drill pipe.

Floeculants

The standard method of determining flocculation value is described m Chapter 4. A variation suggested by Lumraus56 for evaluating polymer flocculants is to add 0.01 lb/bbl of the flocculant to a 4% clay suspension, allow to stand quiescent, and note the level of clear supernatant liquid alter various intervals of time.

Empirical tests for determining the degree to which treating agents will disperse clay mineral aggregates are described in Chapter 4.

Foams and Foaming Agents Rheological Properties of Foams

The rheological behaviour of foams depends on foam quality (ratio of gas volume to total volume). In the quality range of interest in oilfield operations the flow model of foams is that of Bingham plastic (see Chapter 7), and the parameters yield stress, plastic viscosity, and effective viscosity are used to describe foam consistency curves. The Fann V-G meter, the Bendix Ultra-viscoson, and capillary viscometers have been used to determine these parameters.57,58 Mitchell59 used a capillary viscometer in which the flow velocity was measured by the transit time of a dye between two photoelectric cells, and the pressure drop across the capillary was measured by means of a differential transducer or a high-pressure manometer.

Evaluation of Eoaming Agents

The apparatus recommended in API publication RP4600 for evaluating foaming agents is shown in Figure 3 29. The foams are tested in four standard solutions whose composition is shown in Table 3-2. To lest the effect of solids on the stability of the foam, 10 grams of silica flour are placed in the bottom of the tube. One liter of test solution is poured into the tube, and the remainder put in the reservoir. Air and solution are then flowed down the tube at the rates shown in Figure 3 29. The volume of liquid carried out of the top of the tube in 10 minutes is taken as a measure of the effectiveness of the foam.

For specific field applications, samples of the liquids and solids from the well of interest should be used instead of the solutions and solids recommended above. Since the results of the tests are comparative, any apparatus embodying the essential features shown in Figure 3 29 will give satisfactory results

Figure 3-29. Laboratory apparatus for testing foam agents. (From API RP 46 60 Copyright 1966 by API.)

Table 3-2* Evaluation of Foaming Agents

Component Fresh Water Fresh Water plus 15% 10",, Brine Kerosene

10% Brine plus 15%

Kerosene

Distilled water icm') Kerosene (cm1» Sodium chloride (g) h earner (".. by volume)

0 0

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