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| Experimental study on fault activation of different infiltration interface types in gas energy storage process of abandoned oil and gas reservoirs |
| HU Minghui，LIU Richeng，LI Shuchen，ZHU Xinjie，QIU Kai |
| (1. State Key Laboratory of Intelligent Construction and Healthy Operation and Maintenance of Deep Underground Engineering， China University of Mining and Technology，Xuzhou，Jiangsu 221116) |
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Abstract Through conducting the triaxial unloading-induced slip tests of different types of rock structural planes with dry，wet and crude oil interfaces，the effect of interface types on the activation process of rock structural planes was explored，and the correlations between the microscopic characteristics and macroscopic performance of structural planes during the activation process were revealed. The results show that the activation of rock structural plane is divided into three stages，i.e.，stable stage，activation stage and dynamic slip instability stage. During shear sliding of rock structural plane，rock fragments resulting from the shearing and exfoliation of rock structural plane accumulate in the form of bedding，with greater damage observed at the edges of the structural plane compared to the interior. The JRC degradation rate of rock structural plane with dry interface is 62 % after the triaxial tests，which is greater than that of dry interface(33.6 %) and crude oil interface(30.5 %). For the structural plane with dry interface，the asperities on the structural plane are strongly self-locked，and the average slip rate is low(0.13 μm/s) in the stable stage. While in the stick-slip stage，the damage of the asperities on the structural plane is primarily brittle failure with suddenness，which leads to the average slip rate of stick slip reaching 9.7 μm/s，74 times larger than that in the stable stage. For the structural plan with wet interface，the presence of water promotes the occurrence of stick-slip events. During stick-slip，the damage of the asperities is mainly ductile failure，and the slip rate transition exhibits a sharp increase followed by a falling process. The mixing of rock fragment and water during shear slip increases the contact area between the adjacent structural plane，improves the intermolecular adsorption force，and strengthens the friction strength of the rock structural plane. For the structural plan with crude oil interface，a layer of colloidal crude oil is attached to the rock structural plane，which fills the void space on the rock structure surface as ductile gouge layer，weakens the hardness of the surface asperity and reduces the friction strength of the rock structure structural plane. The existence of crude oil also makes the friction strengthening effect of the structural plane and the self-locking effect of asperities weak，and the structural plane is more prone to undergo dynamic slip compared to structural plane with dry or wet interfaces. The interface type controls the increase of friction coefficient and the slip rate at the start of dynamic slip. The friction coefficient of dry，wet and crude oil rock structural planes increased by 5%，11.2% and 0.7% respectively during activation process，and the slip rate at the start of dynamic slip are 1.1 mm/s，0.27 mm/s and 0.023 mm/s respectively. Our research may provide a theoretical basis for evaluating the fault instability of structural plane with different interface types in the oil and gas reservoirs，and have important implications for better understanding the occurrence of extraction induced fault activation.
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