Study on gas leakage along cement sheath interfaces in salt cavern gas storage well

Abstract

Cement sheath interfaces, which are potential gas leakage channels, play an important role in the safe operation of salt cavern underground gas storage (UGS) because the wellbore passes through different lithologies. Cement-salt rock specimens were experimentally prepared to measure the permeability changes under cyclic unloading–loading confining pressure. The results showed that after 10 cycles, the permeability decreased by 64.83% and presented a negative exponential correlation with the number of cycles, indicating that an increase in the gas injection–production cycles was beneficial to the zonal isolation of the cement sheath in the salt rock formation. Based on the permeability evolution law, a 3D geological model of the KING-1 cavern of Jintan UGS, was built considering the cavern and wellbore, to investigate the effects of internal gas pressure, operating time, and lithology on gas migration along the cement sheath interfaces. The gas leakage channel during the operation of salt cavern UGS was determined based on numerical simulation results. The results indicated that after the cavern was formed, the horizontal stress above the casing shoe became larger than the vertical stress, and the difference between the horizontal and vertical stresses increased with the number of cycles. After 20 cycles, the maximum micro–annulus widths at the cement–formation interfaces (CFI) in the mudstone–salt rock interface and mudstone layer at 3 MPa gas pressure were 3.50 and 5.34 times, respectively, of those at 9 MPa. The CFI at the mudstone–salt rock interface was a potential gas leakage channel at the early operational stage of gas storage, whereas at the later operational stage, gas leakage occurred along the CFI in the mudstone layer 20 m above the salt rock layer. The results are expected to provide a reference for gas leakage analysis, risk judgment, and long-term safe operation of salt cavern UGS.