Hydratable shales often lead to a variety of drilling problems such as, wellbore instability, stuck pipe, and solids buildup in drilling fluids. It is important that a timely measure of hydratable clay content in shale formations be available during drilling so that appropriate action can be taken to minimize their detrimental effects. A new dielectric constant measurement (DCM) technique was developed to allow rapid, quantitative determination of shale properties at the wellsite, using a small sample of cuttings. Specific surface area is a measure of the total hydratable surface in a rock and is dominated by the hydratable smectite clay present. The dielectric constant for a shale is also strongly influenced by the presence of hydratable clays and can be correlated with specific surface area. A dielectric constant to surface area correlation was developed based on tests on hundreds of cuttings samples from many wells from around the world and thus shale surface areas can be obtained from DCM data. The wellsite DCM test is run with an inexpensive, portable test kit. Field tests on several Exxon wells showed that DCM data can be collected routinely on-site by the mud logger. This paper introduces the wellsite DCM test procedure and presents field results. procedure and presents field results Introduction Shales are fine grained sedimentary rocks that contain significant amounts of clay minerals. The types and amounts of clays in a shale plus the degree of clay hydration have important effects on the chemical and mechanical behavior of the shale. Shales make up over 75% of drilled formations, therefore, the determination of the swelling clay content of a formation is important in both the exploration for and the production of hydrocarbons. production of hydrocarbons. In exploration, the clay content is useful in determining water and hydrocarbon saturations in shaly reservoir formations and in correlating different formations. During drilling, knowledge of the swelling-clay content is useful in determining the type and amount of shale inhibitor needed in the drilling fluid to provide wellbore stability. The swelling clay content also provides information about drilling problems such as wellbore instability, stuck pipe, bottom-hole problems such as wellbore instability, stuck pipe, bottom-hole fill, bit balling, mud rings, torque, drag, and solids build-up in the drilling fluid. Completion problems such as formation damage in shaly sands, logging and coring failures, hole wash outs, and poor cement jobs are attributable to the hydratable clay content of the formation. It is desirable, therefore, to be able to obtain, at the wellsite, timely estimates of the swelling clay content of formations being drilled. A new dielectric constant measurement (DCM) was developed to quantify hydratable clays in shales. A standard test procedure was developed to minimize effects of pore fluid, interlayer cation, temperature, and frequency on the dielectric response. The technique was refined so that now shales can be characterized routinely by dielectric constant measurements using a simple wellsite test kit. Applications of the new DCM technique include determination of the hydratable clay contents, correlating offset wells, and distinguishing changes in lithology. P. 401
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