Experimental Study On The Failure Of Capping Mudstone Of Western Hubei-eastern Chongqing Suffered Uplift And Erosion

Western Hubei-Eastern Chongqing region is a major exploration block of shale gas. but the region had been suffered long term uplift and erosion after hydrocarbon generation,which may result in cap rock generating fractures,and then gas escape. Therefore,it is necessary to fully understand the mechanism of tectonic deformation due to uplift and erosion、the failure law of capping mudstone and reveal the law of crack initiation and crack propagation in the cap rock. To investigate the mechanical properties of mudstones during geological evolution, field survey、theoretical analysis、experiment and numerical simulation are adopted and applied to this study.Based on analysis of regional structural section, the non-coordinated deformation between different stratum is produced after the extrusion movement,which leads to the differential uplifts and denudation. It is important to note those formations deposited in different geologic ages are the material foundation of deformation. Basic mechanical properties of rock are obtained by uniaxial compression, direct shear test and Brazil splitting test.The differences of rock mechanical properties in different groups come from mineral composition and structure. Mineral composition, micro fracture development and pore structure characteristics of different mudstones was analyzed by X- ray diffraction and SEM technique.The mechanical properties of rock samples also reflect the mechanical properties of rock strata. So, difference of rock’s rheological character can be characterized by the uniaxial compressive strengths and shear strengths of rock samples deposited in different geological era. It could be helpful to find the critical detachment layer during extrusion tectonism, and then put forward a new interpretation of deformation difference of Western Hubei-Eastern Chongqing region.After a long period of deposition and burial, sedimentary mudstone turns into capping mudstone, but uplift and erosion may result in failure of capping. The deformation, fracture and strength characteristics of mudstone in the process of unloading were observed by unloading test to simulate the uplift and erosion process of cap rock. Also,Simulation of the deposition and burial process of cap rock by uniaxial strain test, nominal pre consolidation pressures of different mudstones are obtained. Simulation of overlying mudstone in different depth by triaxial compression test with different temperature, mechanical parameters of mudstones are obtained. Integrated test parameters, the relationship between brittleness of mudstone and rock uplift amplitude is established.Sealing effectiveness of capping rock is the key of preservation of shale gas. At present, the process of crack initiation and crack propagation in the mudstone under compression conditions could be monitored by Acoustic emission and CT technology. RFPA3 D software can simulate the real process of crack initiation of capping rock under compression load. The law of crack initiation and propagation of mudstone under different temperature and pressure was obtained. And, this software was used to simulate the process of crack propagation of region under compression with regional geological features.