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

摘要

An effective caprock is crucial for safe operation of an underground gas storage(UGS).However,thecaprock's initial seal capacity can be changed due to many factors such as micro-deformation of the caprock'spore structure and fatigue damage under cyclic loading caused by UGS annual storage which may all leadto gas leakage.Consequently,the caprock's dynamic sealing capacity including capillary sealing effciencyand 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 preparedto perform laboratory tests.Nitrogen breakthrough(BT)pressure was both measured before and after50 cycles’loading-unloading on the fully kerosene-saturated plugs.Specially,the cyclic amplitude wasdesigned based on the UGS planned operational pressure bounds and local dynamic in-situ stresses.Triaxialcompression fatigue and subsequent failure tests were conducted to investigate the strain dynamic evolutionand its effect on the mechanical behaviors of clayey caprocks.Seventeen sandstone reservoir plugs werealso 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 whichis much higher than the critical value(~2MPa)for the HB trap seal.Average reduction of the BT pressureis 14.8% after 50 cycles’loading demonstrating that the alternating in-situ stresses may have a relativelyminor effect on the capillary sealing capacity of the HB UGS caprock.This finding is also supported bythe 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 disturbedin-situ stresses variation during the HB UGS operations according to the dynamic evolution of the axialand lateral strain.However,the stress-strain curves exhibit significant hysteresis especially within the firstseveral cycles and the plastic strain continuously develops.The average cumulative plastic strain is around0.14% after 50 cycles’loading which is much lower than the 1% benchmark of caprock failure suggested bySchlumberger.However,the cyclic loading has a more severe weakening effect on the caprock mechanicalstrength parameters and it cannot be neglected in geomechanical simulation compared with the sandstonereservoir plugs.This study gives an in-depth understanding of the dynamic capillary sealing capacity and mechanicalproperties 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.