Thermal Effect on the Compaction Behavior of Tight Shale by Uniaxial Strain Test

Abstract

In order to investigate the compaction behavior of tight shale and obtain the apparent preconsolidation stress and critical stress of brittle-ductile transition, uniaxial strain tests were conducted at various temperatures. This method utilizes a cold-stretched 45# carbon steel sleeve to restrain the lateral deformation of tight shale, which successfully simulates the rigid constraints of rock stratum and overcomes the challenging issues of measuring the apparent preconsolidation stress and brittle-ductile transition based on existing tests. The results show that (1) the test curve can be divided into three stages: the pore compaction stage, the elastic compaction stage, and the cataclastic flow stage; (2) there is no obvious relationship between the front in situ stress coefficient and temperature, whereas the middle and later in situ stress coefficients increase at elevated temperatures; and (3) the apparent preconsolidation stress and critical stress of brittle-ductile transition decrease as the temperature rises, as does the elliptical yield surface of tight shale. In addition, the thermal stress distribution law of rock units was thermodynamically analyzed. The average increase in axial thermal stress is nearly equivalent to the decrease in apparent preconsolidation stress at various temperatures, and thermal stress has a compensating effect on the axial load. A new method is provided for studying the compaction behavior of tight rock under various coupling conditions.