Uncertainty failure risk quantitative assessments for underground gas storage near-wellbore area

Abstract

The injection and withdrawal wells of underground gas storage (UGS) are prone to weakening the sealing performance of faults and cementation planes, under multiple rounds of injection-withdrawal alternations. To comprehensively quantify the stability of these weak planes under injection-withdrawal, the uncertainty quantification method is used to analyse the risk sensitivity of failure in the near-wellbore area (NWA). Based on the inversion technique of uncertain crustal stress intervals, three-dimensional Mohr-Coulomb and polar projection charts, high-precision geological modelling fault dip sampling point data and multidimensional technical evaluation index models are established. An uncertainty quantitative index of the shear slip failure risk of a borehole section under a coupled dynamic fluid pressure alternating with in-situ stress is constructed. A mechanical integrity analysis of the NWA weak planes, formation wellbore, and cement sheath is conducted. The stress change interval in NWA under alternate injection-withdrawal changes is then determined. Based on the quantitative analysis of the deterministic parameters and uncertainties, approximate results are obtained. The interval range of the wellbore stress change in the NWA provides a reference and optimization for the design of injection-withdrawal wells. The results show that a comprehensive quantitative analysis could be used to more comprehensively evaluate the sealing performance of NWA. The critical activation stress ratio and the opening angle have a high specificity for the opening and closing characteristics of the section. It is thus concluded that the proposed uncertain risk analysis method provides a comprehensive and quantitative tool for assessing the risk of slip at the fault level in the injection and withdrawal wells in UGS.