Investigation of Fluid-Injection-Induced Coal Stiffness Alteration using a Single-Core Multistage Triaxial Test

Abstract

Abstract Nonlinear stress-strain behavior and failure envelopes are critical rock mechanical data for gas reservoir evaluation and development, especially when the basic rock properties are measured under actual reservoir conditions that include pore pressure. For coalbed methane (CBM) reservoirs, the mechanical properties of coal may be different under actual reservoir conditions because the in situ coal is fluid saturated. Multistage triaxial testing of coal can generate a full failure envelope using a single core instead of a set of destructive core tests—as is the case with single stage testing. This multistage technique is necessary because of the scarcity of actual reservoir specimens. Consequently, in this study, a series of multistage triaxial compression tests on a fluid-saturated coal specimen was conducted to determine the mechanical properties of reservoir coal formations as a function of confining stress and fluid compositions and pressures. An oven-dried coal core 50\u202fmm in diameter and 100\u202fmm in length was prepared. Different fluids were employed in the test, including the non-sorbing gas He and sorbing gases, such as N2, CH4 and CO2, and also H2O. Coal sorption behavior was investigated during the triaxial compression test, and the peak stresses under various pore pressures and confining pressure conditions were measured. In addition, the competition between the fluid-density-induced hardening and adsorption-induced softening of the coal core is discussed. The coal strength under gas-saturated conditions was sensitive to the confining pressure. The coal under a higher confining pressure exhibited a stiffer response and the peak load increased with higher confining pressure. Strength weakening due to gas adsorption was observed for N2, CH4, CO2, and water injection. Both CH4 and CO2 had a strong influence on coal stiffness alteration mainly because the higher adsorption capacity of coal for them. Stress-strain relationships behaved in a non-linear when a sorbing-gas was involved. The softening effect of CO2 injection was complicated by a CO2 phase change at higher pressure conditions. This study is applicable to a wide range of engineering applications, including reservoir stress response, permeability evolution, hydraulic fractures, coal mining, and borehole stability, and fault slipping.