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 mm in diameter and 100 mm 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.

非线性应力应变行为和破坏包络是评估和开发气藏的关键岩石力学数据，尤其是在包括孔隙压力在内的实际储层条件下测量基本岩石特性时。对于煤层气（CBM）储层，煤的力学性能在实际储层条件下可能会有所不同，因为原位煤是流体饱和的。煤的多级三轴测试可以使用单个岩心而不是一组破坏性岩心测试来生成完整的破坏包络-单级测试就是这种情况。由于实际油藏标本稀缺，因此需要这种多阶段技术。因此，在这项研究中，对流体饱和的煤样品进行了一系列的多级三轴压缩试验，以确定储层煤层的力学性能与应力，流体组成和压力的关系。制备直径为50mm，长度为100mm的干燥煤芯。测试中使用了不同的流体，包括非吸附气体He和吸附气体，例如N ₂，CH ₄和CO _2。，和也可为h ₂ O.煤吸附行为的三轴压缩试验过程中研究，测定在各种孔隙压力和围压条件下的峰值应力。此外，还讨论了流体密度引起的煤芯硬化与吸附引起的软化之间的竞争。气体饱和条件下的煤强度对围压很敏感。在较高围压下的煤表现出较硬的响应，并且峰值载荷随着围压的增加而增加。对于N ₂，CH ₄，CO ₂和注水，观察到由于气体吸附而导致的强度减弱。CH ₄和CO _2均煤的刚度变化有很大的影响，主要是因为煤对煤的吸附能力更高。当涉及吸附气体时，应力-应变关系呈非线性。注入CO ₂的软化作用由于在较高压力条件下的CO ₂相变而变得复杂。这项研究适用于广泛的工程应用，包括油藏应力响应，渗透率演化，水力压裂，煤矿开采，井眼稳定性和断层滑动。