Drilling Mud Tests
The field tests for rheology, mud density, and gel strength are accomplished in the same manner as outlined for water-based drilling mud. The main difference is that rheology is tested at a specific temperature, usually 120◦F or
150◦F. Because oils tend to thin with temperature, heating fluid is required and should be reported on the API Mud Report.
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Sand content measurement is the same as for water-base drilling mud except that the mud’s base oil instead of water should be used for dilution. The sand content of oil-base mud is not generally tested.
HPHT Filtration The API filtration test result for oil-base drilling mud is usually zero. In relaxed filtrate oil-based muds, the API filtrate should be all oil. The API test does not indicate downhole filtration rates. The alternative high-temperature–high pressure (HTHP) filtration test will generally give a better indication of the fluid loss characteristics of a fluid under downhole temperatures The instruments for the HTHP filtration test consists essentially of a controlled pressure source, a cell designed towithstand a working pressure of at least 1,000 psi, a system for heating the cell, and a suitable frame to hold the cell and the heating system. For filtration tests at temperatures above 200◦F, a pressurized collection cell is attached to the delivery tube.
The filter cell is equipped with a thermometer well, oil-resistant gaskets, and a support for the filter paper (Whatman no. 50 or the equivalent). A valve on the filtrate delivery tube controls flow from the cell. A nonhazardous
gas such as nitrogen or carbon dioxide should be used as the pressure source. The test is usually performed at a temperature of 220 – 350◦F and a pressure of 500 psi (differential) over a 30-minute period. When other temperatures, pressures, or times are used, their values should be reported together with test results. If the cake compressibility is desired, the test should be repeated with pressures of 200 psi on the filter cell and
100 psi back pressure on the collection cell. The volume of oil collected at the end of the test should be doubled to correct to a surface area of 7.1 inches.
read also Testing of Drilling Systems
The electrical stability test indicates the stability of emulsions of water inoilmixtures. The emulsion tester consists of a reliable circuit using a source of variable AC current (or DC current in portable units) connected to strip electrodes . The voltage imposed across the electrodes can be increased until a predetermined amount of current flows through the drilling mud emulsion-breakdown point. Relative stability is indicated as the voltage at the breakdown point and is reported as the electric stability of the fluid on the daily API test report.
Liquids and Solids Content
Oil, water, and solids volume percent is determined by retort analysis as in a water-base drilling mud. More time is required to get a complete distillation of an oil mud than for a water mud. The corrected water phase volume, the volume percent of low-gravity solids, and the oil-to-water ratio can then be calculated.
The volume oil-to-water ratio can be found from the procedure below:
Oil fraction 100 × % by volume oil or synthetic oil / (% by volume oil or synthetic oil−% by volume water)
Chemical analysis procedures for nonaqueous fluids can be found in the API 13B bulletin available from the American Petroleum Institute.
Alkalinity and Lime Content (NAF)
The whole mud alkalinity test procedure is a titration method that measures the volume of standard acid required to react with the alkaline (basic) materials in an oil mud sample.
The alkalinity value is used to calculate the pounds per barrel of unreacted, “excess” lime in an oil mud. Excess alkaline materials, such as lime, help to stabilize the emulsion and neutralize carbon dioxide or hydrogen sulfide
Total Salinity (Water-Phase Salinity [WAF] for NAF)
The salinity control ofNAFfluids is very important for stabilizing water-sensitive shales and clays. Depending on the ionic concentration of the shale waters and of the drilling mud water phase, an osmotic flow of pure water from the weaker
salt concentration (in shale) to the stronger salt concentration (in mud) will occur. This may cause dehydration of the shale and, consequently, affect its stabilization
Other, more advanced laboratory-based testing is commonly carried out on drilling fluids to determine treatments or to define contaminants. Some of the more advanced analytical tests routinely conducted on drilling fluids include:
Advanced Rheology and Suspension Analysis
FANN 50 — A laboratory test for rheology under temperature and moderate pressure (up to 1,000 psi and 500◦F).
FANN 70 — Laboratory test for rheology under high temperature and high pressure (up to 20,000 psi and 500◦F).
FANN 75 — Amore advanced computer-controlled version of the FANN 70 (up to 20,000 psi and 500◦F).
High-Angle Sag Test (HAST)
A laboratory test device to determine the suspension properties of a fluid in high-angle wellbores. This test is designed to evaluate particle setting characteristics of a fluid in deviated wells.
Laboratory test device to determine the suspension properties of a drilling fluid under high angle and dynamic conditions.
Specialized Filtration Testing
FANN 90 Dynamic filtration testing of a drilling fluid under pressure and temperature. This test determines if the fluid is properly conditioned to drill through highly permeable formations. The test results include two numbers: the dynamic filtration rate and the cake deposition index (CDI).
The dynamic filtration rate is calculated from the slope of the curve of volume versus time. The CDI, which reflects the erodability of the wall cake, is calculated from the slope of the curve of volume/time versus time. CDI and dynamic filtration rates are calculated using data collected after twenty minutes. The filtration media for the FAN 90 is a synthetic core. The core size can be sized for each application to optimize the filtration rate.
The PPT test is accomplishedwith a modified HPHT cell to examine sealing characteristics of a drilling fluid. The
PPT, sometimes known as the PPA (particle-plugging apparatus), is key when drilling in high-differential-pressure environments.
Aniline Point Test
Determine the aniline point of an oil-based fluid base oil. This test is critical to ensure elastomer compatibility when using nonaqueous fluids.
Particle-Size Distribution (PSD) Test
The PSD examines the volume and particle sizedistribution of solidsinafluid.This test is valuable indetermining
the type and size of solids control equipment that will be needed to properly clean a fluid of undesirable solids.
This test is used to determine if contamination of a synthetic-based mud has occurredwith crude oil during drilling
Various lubricity meters and devices are available to the industry to determine how lubricous a fluid is when exposed to steel or shale. In high-angle drilling applications, a highly lubricious fluid is desirable to allow proper transmission of weight to the bit and reduce side wall sticking tendencies.