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CO_2–水两相条件下的岩石力学行为是CO_2地质封存中的关键科学问题之一。利用自主研发的厚壁圆筒式两相致裂仪进行CO_2–水两相条件下砂岩压裂试验,并研究CO_2–水两相有效应力模型。通过向试样加载内压、围压和轴压形成与井壁围岩类似的最小主应力为负的真三轴应力状态,采用半透隔板法进行CO_2驱水试验以精确控制试样孔隙中CO_2–水两相流体的组分压力和饱和度,最后通过内压致裂试样获得致裂压力。还进行了不同有效围压下含单相水孔流试样的致裂试验,得到其致裂压与有效围压的拟合函数。为研究CO_2–水两相有效应力模型,选用4种可应用于岩石的非饱和介质有效应力公式与拟合函数联立推导得到破裂压力预测模型,与试验结果对比以间接验证这4种有效应力公式对于表征CO_2–水两相流体条件下有效应力的适用性。结果表明,考虑Biot系数的Bishop非饱和有效应力模型能够较好地描述CO_2–水两相流体下所测试砂岩的张性开裂行为,可直接作为CO_2–水两相有效应力模型使用。
Rock mechanics behavior under CO_2-water two-phase conditions is one of the key scientific problems in CO_2 geological storage. The self-developed thick wall cylindrical two-phase fracturing instrument was used to test the sandstone fracturing under CO_2-water two-phase conditions, and the effective stress model of CO_2-water two-phase was studied. By applying internal pressure, confining pressure and axial pressure to the specimen, a true triaxial stress state with a minimum principal stress similar to that of the surrounding wall of the borehole is assumed to be negative. The semi-permeable diaphragm method was used to carry out CO 2 flood test to precisely control the pore size In the CO_2-water two-phase fluid component pressure and saturation, and finally obtained by internal pressure fracture test specimen fracturing pressure. The fracture test of single-phase water-hole flow specimen with different effective confining pressure was also carried out, and the fitting function of the fracture pressure and the effective confining pressure was obtained. In order to study the CO_2-water two-phase effective stress model, four kinds of effective stress formulas of unsaturated media which can be applied to rock are selected to derive the prediction model of fracture pressure with the fitting function. Compared with the test results, the four effective stress Applicability of the formula to characterize the effective stress under CO_2-water two-phase fluid conditions. The results show that the Bishop unsaturated effective stress model considering the Biot coefficient can well describe the tensile cracking behavior of the sandstone under the CO_2-water two-phase flow and can be directly used as the CO_2-water two-phase effective stress model.