Shale In CO2 Seepage-stress Coupling Experimental Study Under Action

Seepage-stress coupling effect is a very important natural phenomenon during the shale gas development and design, but we haven't known the rule for gas production and how to produce shale more effectively till now. At present, using CO2 fracturing technology to develop shale gas has been accepted worldwidely, but the key problem is that seepage-stress coupling rule and multi-scale seepage law is still unclear, when the shale is under the effect of CO2. In this paper, based on the systematically review of shale seepage-stress coupling effect, we study the Southwest region Longmaxi Silurian and Ordovician Wufeng formation shale. We carry out the test of shale basic physical parameters to understand the basic physical properties of reservoir conditions. Shale stress sensitivity was obtained by permeability stress sensitivity experiments. The seepage-stress coupling rule during fracturing was obtained by CO2 complete stress-strain-seepage coupling test, the main works and findings are as following:(1)Five blocks outcrop shale sampling of Longmaxi Silurian and Ordovician Wufeng were obtained, and 84 specimens were prepared for the test;(2)The comprehensive evaluation of this region was medium, with SEM test, X diffraction whole rock analysis, organic analysis, porosity and permeability test.(3)The experimental program of effective pressure for permeability is designed. The stress sensitivity test and the effective stress for permeability test of 2 specimens were finished, where the permeability is measured by transient pulse method.(4)Shale permeability and effective stress meet the power relationship. Nonlinear effective stress factor was obtained by response surface method, and the accuracy was examined.(5)Based on sensitivity evaluation method, two matrix specimen were evaluated as moderate stress sensitivity;(6)The experimental program of complete stress-strain-seepage coupling test was designed and 4 specimens were tested;(7)Shale complete stress-strain-seepage coupling test results show that shales'seepage peak lags the stress peak. The specimen exists partial stress adjustment and release before peak stress, besides some specimen hasn't the compaction stage or the stage is not obvious;(8)The test, shale triaxial stress-strain and after peak test under the CO2 seepage, shows that CO2 can significantly reduce shale's peak strength and elastic modulus. CO2 contributes to form joint fracture network and the permeability after rupture can increase 2-6 orders of magnitude. Besides,the retention amount of liquid CO2 is 2.57 times of N2.Through this test, we deeply know the permeability stress sensitivity, CO2 can reduce reservoir mechanical properties, and CO2 can get complex fracture network. So, this can provides a theoretical basis for shale development.