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| Experimental study of fracturing characteristics of sandstone under CO2-water two-phase condition and effective stress model |
| LIU Mingze，BAI Bing，LI Xiaochun，GAO Shuai，WANG Lei，WU Haiqing |
| (State Key Laboratory of Geomechanics and Geotechnical Engineering，Institute of Rock and Soil Mechanics，Chinese Academy of Sciences，Wuhan，Hubei 430071，China) |
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Abstract The rock mechanical behavior under the two-phase pore fluid condition of CO2-water is one of the key scientific issues in geological storage of CO2. The fracturing experiments of sandstone in two-phase fluid of CO2-water were conducted with the test device named hollow-cylinder tensile tester in two-phase fluid developed in-house. An effective stress model in two-phase fluid of CO2-water was studied based on the experimental results. The internal，confining and axial pressures were imposed on the hollow cylinder sample to build a true triaxial stress state so that the minimum principal stress was negative，which was similar to the stress state in surrounding rock of borehole. A semi-permeable disk method was used to flood water to precisely control the partial pressure and saturation of two-phase fluid of CO2-water in the sample. The fracturing pressure was measured by fracturing the sample with internal pressure. The function of fracture pressure and effective confining pressure was fit with the results of the fracturing tests of samples in water under different effective confining pressures. Finally，the prediction models were deduced based on the fitting function and four types of unsaturated effective stress formula were proved to apply to rock. The prediction models were compared with the experimental result to indirectly verify the applicability of these formulas for describing the effective stress under the two-phase fluids of CO2-water condition. The results indicated that Bishop?s unsaturated effective stress formula was the most appropriate to describe the issue for the tested sandstone，which can be directly used as the effective stress model in two-phase fluids of CO2-water.
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2016, Vol. 35 
