Sealing evaluation of Liuzhuang UGS in Subei Basin

The evaluation of underground gas storage（UGS） tightness is a critical aspect of the construction and operation phases. The Liuzhuang UGS in the Subei Basin, characterized by a fault-controlled fault nose structure and edge water UGS, underwent a comprehensive evaluation of its sealing performance. This assessment focused on both cap rocks and faults, incorporating rock mechanics experiments and theoretical analysis. The evaluation analyzed the sealing strength of the cap rock from macroscopic, microscopic, and mechanical perspectives. It was determined that the thick cap rocks are primarily mudstone from the upper part of the second member and the lower part of the third member of the Funing Formation. These cap rocks are widespread throughout the area with stable regional distribution. Their lithology is favorable, containing over 30% clay minerals. The cap rock's mechanical properties, including a Young's modulus generally below 20 GPa and a brittleness index typically under 45%, contribute to its effective sealing capabilities. The predominant clay minerals in the cap rock are illite-montmorillonite mixed layers, which easily expand in water, enhancing the oil and gas seal effectiveness. Furthermore, the fault analysis focused on both lateral and vertical sealing capabilities. The Liu① fault, characterized by significant fault throw and steep dip angles, demonstrated robust vertical sealing obtained through the combination of the positive pressure method on the fault plane and regional experiences with overlying strata depths. The lateral seal of the fault was substantiated using both qualitative methods, like sand-mud docking, and quantitative assessments, including the shale smear factor. These methodologies confirmed that the Liuzhuang reservoir-controlling fault possesses strong lateral sealing properties.