Novel poroperm characteristics design method for cement sheath to improve the long-term sealing integrity in gas storage well

Abstract

Poroperm characteristics of the cement sheath are the crucial factors affecting the long-term safe and stable storage of underground gas in gas storage wells. In this paper, the permeability, connected porosity, and average connected pore radius of the cement sheath were characterized using a permeability test system, mercury intrusion, and computed tomography (CT). A dynamic permeability calculation model, which considered the pore compaction effect, was established and validated by test results. On this basis, a gas seepage length analysis model was further derived. The risk and influencing factors of gas seepage were explored and the critical poroperm characteristics indexes of the cement sheath were proposed. The results found that the connected pores inside the cement sheath will cause the risk of gas seepage. Decreasing the gas storage pressure, average connected pore radius, connected porosity, and permeability of the cement sheath can shorten the gas seepage length. The poroperm characteristics requirements charts were designed, and the average connected pore radius (r) and permeability (K_a) of the cement sheath under various well depths (H) and gas storage pressures (P_e) were quantified, that when the r and K_a meet K_a (10³ P_e – 9.8 H–121034 r⁻¹)/(13400 + 1.52 H) < 0, the gas sealing of the gas storage wells' cement sheath can be ensured under different H and P_e. The research presents a novel approach for predicting the cement sheath sealing and proposes a set of specific performance indicators to effectively guide the design of the cement sheath's poroperm characteristics in gas storage wells, which is significant for guaranteeing the long-term safe operation of gas storage wells.