Quantitative Determination of the Mechanical Properties of Shales

Abstract

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.