Summary Shales make up more than 75% of drilled formations and cause at least 90% of wellbore-stability problems. Physical measurements of shale properties are required to develop realistic constitutive relationships and to understand and define shale strength and behavior under downhole conditions. Quantitative shale-strength data are needed to improve wellbore-stability model predictions. This paper describes a research test program to predictions. This paper describes a research test program to determine the mechanical properties of different classes of shales and to build a database on their characteristics and mechanical behavior. Shale samples from undisturbed block samples were prepared under controlled conditions. Mechanical test techniques prepared under controlled conditions. Mechanical test techniques were developed to measure effective-stress/strain properties accurately for shales. The tests require a heavy-duty, triaxial test load frame and other specially designed equipment to obtain precise pore pressure measurements. During the tests, pore precise pore pressure measurements. During the tests, pore pressure, stresses, and strains were monitored accurately by an pressure, stresses, and strains were monitored accurately by an automatic data-acquisition system. The test techniques described can be used to test shales with different characteristics: soft, hard, brittle, plastic, etc. On the basis of results of several shale tests, high-quality, effective-stress/strain data and failure criteria (shale strength relationships) can be obtained routinely. Introduction Compressive wellbore failure is the major cause of stuck pipe, hole enlargement, poor log quality, poor primary cement jobs, and excessive drilling costs. Most wellbore-stability problems occur in shales. In the Gulf of Mexico and many other areas, some drilled openhole intervals are predominantly shale formations. Advances in wellbore-stability and shale technology have long been sought because shale instability consistently has produced one of the highest drilling trouble costs. In the current economic climate, however, major improvements in wellbore-stability and shale technology are required to affect drilling risks and costs significantly, particularly in expensive wells under high-angle, extended-reach, and severe tectonic conditions. In addition, incremental cost benefits from timely response to shale-related wellbore-stability problems in many of the more routine wells can add up to large savings in total drilling costs. Compressive wellbore failure occurs when the wellbore stresses exceed the strength of the rock. This usually happens in weaker rocks like shales. Much past work has been directed toward the chemical and mechanical aspects of wellbore stability. Nevertheless, most of the benefits have been realized by chemical means through application of inhibitive drilling fluids. Still, even with the most inhibitive drilling fluids available, compressive wellbore failure occurs when the wellbore stress exceeds the shale strength. Under these conditions, rock-mechanical approaches are required to keep the wellbore stable - i.e., to increase the wellbore pressure by increasing the mud weight so that the wellbore stress is less than the shale strength. Keep in mind, however, that excessive mud weights can reduce drilling rates and increase the risks of differential-pressure sticking and lost circulation. Thus, a method to predict quickly the minimum mud weight required to stabilize the wellbore is quite important. An understanding of shale behavior (constitutive relationships) and quantitative shale strengths under downhole conditions is needed to develop a predictive, rock-mechanical wellbore-stability model. In 1979, Bradley proposed a theoretical rock-mechanical model approach based on work by Fairhurst to predict the proper mud weight to stabilize the wellbore.
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
A series of centrifuge model tests was performed to study the behavior of shallow foundations on soft normally consulidated clay. The model tests included testing of foundation models with one-dimensional, plane strain, and axially symmetric geometries. The nonlinear consolidation properties of the soil were determined using specially developed laboratory testing techniques. The centrifuge test data were then compared with conventional and fir1jte strain theories for consol idation assessment, as well as a 1imit analysis solution for foundation stability considerations..Ĩ t is found that centrifuge testing coupled with careful laboratory testing providesca useful tool to validate analytical procedures. It has demonstrated that the finite strain theory and the limit analysis solution are valid procedures for the determination of consolidation settlement and foundation penetration of·shallow foundations on soft soil, respectively.
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