Calculation methods on formation pressure of underground gas storage rebuilt from depleted oil and gas reservoir

The dynamic change of reservoir pressure is the key factor that influences the injection and production capacity of underground gas storage (UGS). In order to study the dynamic change relationship between formation pressure and storage capacity in the process of UGS injection and production, three fast calculation methods (i.e., production data fitting method, constant volume method and dynamic calculation method) were established according to production data and material balance principle to evaluate the dynamic change of UGS formation pressure with its storage capacity. Then, three methods were used to calculate the change of UGS formation pressure with the storage capacity, as well as the variation of formation pressure during one dynamic injection-production cycle. Besides, the reasons for the difference between constant volume method and dynamic calculation method in terms of predicted UGS formation pressure were analyzed. Finally, sensitivity analysis was performed on the effect of injection rate fluctuation, production rate fluctuation and production-injection cycle on the UGS formation pressure. It is shown that all the three methods can be used for the dynamic prediction of UGS formation pressure change. The accuracy of the production data fitting method depends on the production data acquisition and change trend analysis. The constant volume method is applicable to predict the formation pressure of UGSs whose edge water and oil rim change are negligible. The dynamic calculation method is applicable to various UGSs for formation pressure prediction, but more formation parameters are needed. It is indicated that the formation pressure is influenced less by the fluctuation of daily gas injection and production rate, but more by the injection-production cycle.