Comprehensive Evaluation of the Dynamic Sealing Capacity of Clayey Caprocks in a Large Underground Gas Storage

Abstract An effective caprock is crucial for safe operation of an underground gas storage (UGS). However, the caprock's initial seal capacity can be changed due to many factors such as micro-deformation of the caprock's pore structure and fatigue damage under cyclic loading caused by UGS annual storage which may all lead to gas leakage. Consequently, the caprock's dynamic sealing capacity including capillary sealing effciency and mechanical integrity under alternating stress must be comprehensively evaluated. A total of 21 clayey caprock plugs drilled from the HB large UGS in western China were prepared to perform laboratory tests. Nitrogen breakthrough (BT) pressure was both measured before and after 50 cycles’ loading-unloading on the fully kerosene-saturated plugs. Specially, the cyclic amplitude was designed based on the UGS planned operational pressure bounds and local dynamic in-situ stresses. Triaxial compression fatigue and subsequent failure tests were conducted to investigate the strain dynamic evolution and its effect on the mechanical behaviors of clayey caprocks. Seventeen sandstone reservoir plugs were also selected to carry out mechanical tests as a comparison. Experimental results indicate that the BT pressure of the HB caprock ranges from 3.88 to 8.79MPa which is much higher than the critical value (~2MPa) for the HB trap seal. Average reduction of the BT pressure is 14.8% after 50 cycles’ loading demonstrating that the alternating in-situ stresses may have a relatively minor effect on the capillary sealing capacity of the HB UGS caprock. This finding is also supported by the mechanical tests that the maximum cyclic loading is always below the yield point of the HB caprock. Compaction is the main deformation behavior and shear expansion has not occurred within the disturbed in-situ stresses variation during the HB UGS operations according to the dynamic evolution of the axial and lateral strain. However, the stress-strain curves exhibit significant hysteresis especially within the first several cycles and the plastic strain continuously develops. The average cumulative plastic strain is around 0.14% after 50 cycles’ loading which is much lower than the 1% benchmark of caprock failure suggested by Schlumberger. However, the cyclic loading has a more severe weakening effect on the caprock mechanical strength parameters and it cannot be neglected in geomechanical simulation compared with the sandstone reservoir plugs. This study gives an in-depth understanding of the dynamic capillary sealing capacity and mechanical properties of the clayer caprocks under cyclic loading-unloading. Based on the above experimental finds, the more accurate evaluation will be obtained through 3D geomechanical model simulation.