Effects of Pore and Confining Pressures on Failure Characteristics of Sedimentary Rocks

Abstract

Published in Petroleum Transactions, AIME, Volume 216, 1959, pages 26–32. Abstract Triaxial compression tests have been performed to determine the strength characteristics of limestone, sandstone and shale rocks subjected to controlled stress conditions. This control was exercised by varying the liquid pressures within and around a plastic-encased rock specimen. The pressure in the pores of the rock was varied throughout the range from atmospheric to 15,000 psig; the external pressure was changed over the same range with various positive pressure differences between it and the internal pressure. The data show that the rock strength increased and the mode of failure changed as the pressure surrounding the rock became greater than the pressure in the pores of the rock. These observations and the results of microbit drilling experiments indicate that the increased rock strength under pressure may be an important effect in reducing drilling rate, but that other factors are probably of even greater importance. Introduction A large part of the research conducted to reduce the cost of drilling is directed toward improving present drilling technology and developing new drilling methods. In spite of the fact that drilling is essentially the act of making rocks fail, many details of the mechanism of failure are not known. It seems probable, therefore, that continuing studies of the fundamentals of rock failure can lay a foundation for future improvements in drilling technology. One of the factors which affects rock failure is the stress applied to the rock. This effect has been known for many years. As an example, stress studies have been published by engineers concerned with the strength characteristics of concrete in large dams. More recently, a number of investigators have explored the effects of stress on the strength of geologic formations and the geologic implications of their findings. During the last few years some research on rock strengths has also been conducted in an attempt to gain additional insight into the factors which affect rock drillability. In determining the strength characteristics of rocks under different stress conditions, investigators have conventionally used triaxial compression equipment, wherein a jacketed rock cylinder is uniformly loaded from all directions and then compressed longitudinally.