Multi-Round Development of Underground Gas Storage Converted from Depleted Oil Reservoirs: Physical Properties Changes and Capacity Expansion
Authors: Huan Qi , Yiqiang Li , Samba Mohammed A. , Yu Sun , Tong Liu 
Published: September 2023

ABSTRACT
This study analyzed the physical properties changes for natural cores under combined effects of alternating stress and multiple rounds of gas flushing. Full-diameter cores were selected to mimic the development history of the actual reservoir and the entire process of transforming the UGS development. In addition, different gas injection and production methods in UGS operation were designed to compare the difference in the effect of expansion and enhanced oil recovery rate. The results show that the multiple rounds of gas flushing resulted in fine migration and fracture formation, which improves the physical properties of the reservoir, and helps to improve the gas seepage and the capacity expansion. According to the characteristics of oil recovery and gas storage, the process of UGS operation can be divided into two stages, the cooperation stage and the gas storage stage. The change from gas injected and produced in a small volume at the beginning of UGS construction and then gradually shifting to a large volume is more helpful for reservoir expansion, and the ability to enhance the recovery rate of the reservoir during the development of UGS is also stronger.

INTRODUCTION
Underground gas storage (UGS) has attracted worldwide attention for its benefits in terms of sufficient peak shaving capacity, energy security reserve, low cost, and high safety factor through regular storage and redevelopment of energy-based gases (Guo et al., 2006; Evans and Chadwick, 2009; Eren and Polat, 2020; Mazarei et al., 2019). In recent years, a new type of gas storage construction idea has been proposed, which is to effectively combine the oil injection process, especially the Gas Assisted Gravity Drainage (GAGD) method, with the gas storage process to form an integrated oil injection and gas storage construction method, which can make full use of the completed infrastructure to smoothly transition to the production conversion stage and significantly save economic investment, both the oil development and gas storage stages will benefit from it (Jiang et al., 2021). The integrated construction method can be achieved in the following ways. Firstly, GAGD can first be applied in the early or late development stage of the reservoir, based on gravity segregation, miscibility, viscosity reduction, and other mechanisms to enhance the recovery rate further, gas continuously drives the fluid to make full use of the pore space for gas storage. After the formation of a certain amount of secondary gas cap and the production of gas-oil ratio is relatively high, part of the wells should be gradually closed in order to adjust the gas injection and development method to the operation of the gas storage reservoir for transformation. In this process, some crude oil will be developed with the operation of the UGS, which could improve the ultimate recovery rate of the reservoir until the reservoir reaches capacity and production. This type of UGS is usually called the oil reservoir-type UGS (Tang et al., 2021; Zhu et al., 2021; Liu, 2021).