Summary. There are ample incentives and opportunities to improve current mud-testing equipment and to develop new instruments to measure mud properties not previously tested. This paper discusses three innovative devices for testing drilling muds: the automatic shearometer unit, the high-temperature/high-pressure (HTHP) dynamic filtration tester, and the filter-cake penetrometer. Each discussion includes a summary of previous technology, current API standards (if available), equipment description, and selected case studies.
Introduction Because of their complexity, drilling fluids require frequent performance testing. Five factors must be considered when test procedures and equipment are chosen: accuracy requirements, personnel availability, time constraints, operating environment, and investment and operating costs. The relative importance of each factor depends mostly on whether the drilling-mud tests are run at a field-support laboratory, at an R and D laboratory, or at the wellsite. Small field-support laboratories are occasionally housed in trailers at the wellsite to supplement routine field testing. This paper discusses three innovative, bench-top devices for measuring drilling muds. The automatic shearometer unit measures shear strength of a column of mud after static aging at temperature and pressure. The HTHP dynamic filtration tester measures dynamic filtration at high temperature and high differential pressure. The filter-cake pentameter determines characteristics and thickness of a filter cake obtained from a low- or high-pressure, static or dynamic filtration test. Fully functional prototypes of these units are being used effectively for research and field-support activities.
Automatic Shearometer link Prolonged exposure to elevated temperatures can affect physical and chemical mud properties-particularly the rheological, thixotropic, and shear-strength characteristics. Shear strength is developed when a mud is left static in the hole for an extended period of time. Excessive shear strengths can cause difficulties with breaking circulation, tripping, logging, cementing, fishing, and other operations. The automatic shearometer unit (ASU) was developed to provide more complete data than obtainable by current testing methods. The API standard recommendation for determining shear strength requires that the mud be statically aged at high temperature in a pressure cell and then be cooled to room temperature. The aging temperature ordinarily is selected to be near the expected bottomhole temperature of the well. The time period can vary but is usually set at 16 hours for convenience. The test involves the use of a thin-wall, stainless-steel shearometer tube. The 3.5-in. [89-mm] -long, 1.4-in. [36-mm] tube is forced into the mud column by gram weights placed on a platform on top of the tube. A reading is taken after the downward movement has stopped and the tube penetration is at least one-half the tube length. Shear strength then is calculated by
(1)
Despite the simplicity, the API test has several deficiencies. First, the single data point taken is limited to the top 2 to 3 in. [50.8 to 76.2 mm] of the mud sample. Second, a probe or spatula is needed to check weighted muds for weight material settling or sag. Comparing the densities of the upper and lower halves of the aged sample can help to monitor settling, but the results are still somewhat qualitative. Third, it can be difficult to keep the tube vertical while weights are added. Inaccurate results may be obtained if the tube leans excessively.
Equipment Description. Fig. 1 is a diagram of the ASU. The shear tube is pushed into the mud sample at a slow, constant rate by a motorized drive mechanism. The tube is attached to a shear-force sensor (load cell) that measures a resistant force proportional to the mud shear strength. A position sensor monitors vertical movement. Although signals from the two sensors (resistant force and mud penetration) can be sent directly to a strip chart recorder, the preferred method is use of a small computer capable of providing data acquisition and control. The computer automatically controls the movement of the shear tube. A rapid increase in resistance during downward movement signals the top of the mud column. Thereafter, 20 data sets are taken for every inch of mud penetration. When the tube reaches the bottom of the cell, the motor reverses and the tube to the starting position. Upon completion of the test, the computer analyzes the data and plots a shear-strength curve on an x-y plotter. A standard shear tube initially was mounted on the ASU to calibrate the results with the API test. A 7-in. [177.8-mm] tube was required, however, to use the taller, 500-mL static-aging bombs and the more common 260-mL cells. Manufacturing the longer tube proved troublesome and costly, so a readily available, 1.26-in. [32-mm] -diameter, 0.025-in. [0.635-mm] -thick tube was selected. Although the bottom edge had to be sharpened to minimize end effects, calibration was not a problem. The geometry differences were compensated by changing Eq. 1 to
(2)
Case Studies. The ASU was used to improve shear-strength analyses in many field service situations. Two case studies demonstrate this application. In Case 1, the ASU helped to solve an apparent barite sag problem on a 16.2-lbm/gal [1941-kg/m3] gypsum mud from offshore south Louisiana. The mud had a yield point (YP) of 3 lbf/100 ft2 [1.4 Pa] and 10-second and 10-minute gel strengths of 1 and 5 lbf/100 ft2 [0.48 and 2.4 Pa], respectively. The shear-strength curve for this mud is the lower curve in Fig. 2. The rapid increase in shear strength at about 3.5 in. [88.9 mm] is the apparent top of the main barite layer. The mud was treated until it had a YP of 15 lbf/100 ft2 [7.2 Pa] and gel strengths of 5 and 10 lbf/100 ft2 [2.4 mill 4.8 Pa]. The mud was hot rolled at 150F [66C] for 16 hours and then statically aged for 24 hours before it was tested on the ASU. The shear-strength curve for the treated mud (the upper curve in Fig. 2) does not show any settling. The general shapes of these curves are typical of many muds. Shear strengths near the surface are high (probably because of dehydration) and level off near the recommended API penetration for muds in good condition. Settling or sag causes the curve to slope upward near the end of the test. The API values are superimposed on Fig. 2 to show the close agreement of the ASU data and the API standard. The difference rarely exceeds 10%. In Case 2, the ASU was used to help formulate a mud for a difficult south Texas well. High temperatures and unexpected formation contamination were causing gelation problems in a 16.9-lbm/gal 2025-kg/m3] water-based mud. Fig. 3 shows the effects of temperature on shear strength of this mud after static aging for 24 hours at temperatures up to 400F [204C].
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