p3 may then be calculated from either of the first two equations.

Oil and Solids Content

The volume fractions of oil, water, and solids in a mud are determined in a retort such as that shown in Figure 3-20. It is important that any air or gas entrained on the mud be removed before retorting; otherwise, the solids content will be in considerable error. Removal of gas by substantial dilution is undesirable because of the loss of accuracy involved, especially with low-solids muds. Gas may often be removed by adding a deloamer such as aluminum stearate, or a high molecular weight alcohol, plus a thinner if necessary to break the gel. If, as indicated by the dilution test described above, this procedure fails to remove all the gas, a vacuum should be applied. Portable hand vacuum pumps are available for use in the field.

Retorting involves placement of a precise volume of mud in a steel container, and heating it in the retort until no more distillate collects in the graduated cylinder. The volume of oil and water are read in the graduated cylinder, and their sum subtracted from the volume of the mud sample to obtain the volume of solids. The method is rather inaccurate with low solids muds because the result depends on the difference between two largo numbers.

If the mud contains substantial amounts of salt, the volume occupied by the salt must be subtracted from the volume of solids. The volume of salt (as NaCl) in the mud may be estimated with sufficient accuracy by multiplying the grams of chloride per 100 cm1 of filtrate (determined by titration) by 0.6 and by the volume fraction of water in the mud

Electrical Properties Stability of Water-in-Oil Emulsions

The stability of water-in-oil emulsions is tested in an emulsion tester (see Figure 3-20a), which permits a variable voltage to be applied across two electrodes immersed in the emulsion.41 The voltage is increased until the emulsion breaks and a surge of current flows between the electrodes. The voltage required for breakdown is regarded as measure of the stability of the emulsion.

Research by Ali et al.41a has shown that results depend on a number of variables: emulsifier concentration, hot roll, oil/water ratio, mud density, and composition variables. They recommend that only trends in electrical stability be used for making treatment decisions.

Accurate measurement of small amounts of oil in offshore discharges is highly desirable. According to Jones and Bagnall,4115 the smallest amount of oil that can be measured accurately is 1%, and lesser amounts can be detected. But to achieve this capability requires modifications of the API instructions, addition of an oil-soluble dye, and centrifugation of the distillate.

Figure 3-20A. Fann Model 23 C, emulsion stability tester. {Courtesy Fann Dresser.)

Resistivity of Water Muds

Measurements of the resistivity of water muds, filtrates and filter cakes are routinely applied in electrical logging. Under some conditions, better evaluation of formation characteristics may be had by controlling resistivity while drilling is in progress. Resistivity measurement provides a rapid means of detecting soluble salts in barite and in waters, such as makeup or produced waters.

Resistivity is measured by placing the sample in a resistive container having two electrodes spaced so that electrical current can flow through the sample. The resistance is measured by a suitable meter (see Figure 3-20b). [f the instrument indicates the sample resistance in ohms, the cell constant must be de

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