Effects of Strain Rate and Confining Pressure on the Deformation and Failure of Shale

Abstract

Summary Previous work on shale mechanical properties has focused on the slow deformation rates appropriate to wellbore deformation. Deformation of shale under a drill bit occurs at a very high rate, and the failure properties of the rock under these conditions are crucial in determining properties of the rock under these conditions are crucial in determining bit performance and in extracting lithology and pore-pressure information from drilling parameters. Triaxial tests were performed on two nonswelling shales under a wide range of strain rates and confining and pore pressures. At low strain rates, when fluid is relatively free to move within the shale, shale deformation and failure are governed by effective stress or pressure (i.e., total confining pressure minus pore pressure), as is the pressure (i.e., total confining pressure minus pore pressure), as is the case for ordinary rock. If the pore pressure in the shale is high, increasing the strain rate beyond about 0. 1 %/sec causes large increases in the strength and ductility of the shale. Total pressure begins to influence the strength. At high strain rates, the influence of effective pressure decreases, except when it is very low (i.e., when pore pressure pressure decreases, except when it is very low (i.e., when pore pressure is very high); ductility then rises rapidly. This behavior is opposite that expected in ordinary rocks. This paper briefly discusses the reasons for these phenomena and their impact on wellbore and drilling problems. Introduction Although most of the rock encountered during drilling is shale and most hole problems are encountered in this type of rock, relatively little is known about the mechanical behavior of shale. Previous rock mechanics studies have focused on stronger crystalline and reservoir rocks because formidable technical problems arise in shale testing mainly because of the rock's extremely low permeability. Furthermore, sufficient, well-preserved materials are difficult to obtain, and adequate models to analyze the data are scarce. In re-cent years, however, more work 1–4 on shale properties has been published. In the petroleum industry, the main motivation for this work has been wellbore failure prediction, especially in highly deviated holes, where sophisticated wellbore-failure models are being developed that require comprehensive shale constitutive laws (i.e., stress/strain/time relations). Shale mechanics is also i inportant in drilling because it influences bit performance and the determination of lithology and pore pressure from drilling parameters. 8–10 For bit-performance models and drilling-response interpretation to be soundly based, a good understanding of shale behavior under the relevant conditions is needed. The major difference between wellbore deformation and deformation during drilling is the much higher deformation rate during drilling. Our investigation concentrates on behavior at high deformation rates. The strain rates pertinent to drilling can be estimated only approximately. The rock under a roller-cone-bit tooth is taken from an intact state to well beyond failure (e.g., to a strain of 10%) in the time of tooth contact. If the bit is rotating at 60 rev/min, a tooth impact might last between 0. I and 0. 0 1 second, depending on tooth position and trajectory, so the strain rates are from 100 to 1,000%/sec. With a drag bit, the tooth velocity is on the order of 1 m/s, and the length characterizing the deformation produced is, at the most, comparable with the depth of cut (e. g., 1 mm). Therefore, the strain rate is on the order of 100,000%/sec. If this strain rate is to be imposed on typical laboratory cores (roughly 50 mm long) in a controlled way, a testing machine must exert considerable load at a displacement rate of about 50 m/s. This is not feasible with present technology, but testing over a range of lower strain rates reveals trends in behavior that can be extrapolated with some confidence to roller-cone-bit drilling rates and, with more caution, to drag-bit rates.