Underground Gas Storage Thermal Effects on Wellbore Integrity and Operation Safety – a Case Study from a Carbonate Reservoir in High Tectonic Stress Region

Abstract

Abstract High injection and production rate is a typical feature of underground gas storage (UGS) wells at each seasonal circle, which cause temperature change due to Joule-Thomson effect. Through an UGS case study of Southwest China, this paper demonstrates that cooling effect augments pressure related in-situ stress changes, which has impact on wellbore and cap rock integrity and UGS operation safety. Cooling effect was clearly observed in high-rate wells at the crest of the reservoir during injection and production cycles. Thermal analysis was carried out to investigate the risks associated to the UGS operation. The results of analytical Energy Balance Equation (EBE) indicates that the 20°C-temperature drop can happen at the near wellbore 15-80m region, and the 10°C-temperature drop can happen at the near wellbore 30-150m region. Rreservoir thermal modelling which accounts for heat transfer between the reservoir and its overlying and underlying formations show a temperature reduction of 4 ~ 18 °C in vicinity of the well, with an impact radius of 15~150 m from the injection boreholes. This was more pronounced around the higher injection rate wells. From thermoelastic equations, these results are used to evaluate the range of stress reduction induced by thermal contraction of overlaying and underlaying rocks. Subsequently, the upper pressure limit for caprock integrity was modified. It is not paid much attention to thermal/cooling effect in the many of UGS operations, and its impact on rock stress change by high-rate injection and production is ignored. This paper showed that the cooling effect shouldn't be ignored during UGS safety operation to ensure the wellbore integrity and formation sealing integrity. The methodology established in this case study can be referred by other UGS and gas storages projects.