Gas injection flooding for EOR and its impacts on the proposed underground gas storage capacity: A case study from the Xinggu 7 Buried Hill Oil Reservoir in the western Liaohe Depression

At present, it is unusual to find underground gas storage reconstruction based on an oil reservoir, especially, at its early development stage, and the theoretical system of storage capacity calculation has not yet been formed. From the material balance theory, we know that the cumulative underground produced oil, gas, water volume quantity is equivalent to all the elastic cumulative volume expansion quantity plus natural and manual accumulated water influx and the injected gas volume. Based on this theory, and in light of underground gas storage reconstruction based on the Xinggu 7 Buried Hill Reservoir developed by gas drive in the western Liaohe Depression, a calculation method of underground gas storage capacity was established suitable for such similar cases. According to the calculated results and quantitative analysis of influencing factors, such as the crude oil recovery percent, the elasticity of formation rocks, water invasion, etc., we obtained the following findings. First, the crude oil recovery percent is in a direct proportion to the storage capacity; under a certain formation pressure, the crude oil recovery rate is the most important factor, when it increases by 1%, the underground gas storage capacity of 105 million m3 will be added, accounting for 3.65% of the total storage capacity. Second, due to the decline of formation pressure, the elasticity of inducible water and rocks will result in the shrinking of the volume of pores and fractures, so the storage capacity decreases. Third, when water production occurs in oil reservoirs, the storage capacity will be also shrinking due to the bottom water-coning effect.