Increasing the working gas capacity is a goal that Underground Gas Storage (UGS) companies have been pursuing. The objective of this paper is to demonstrate a strategy and practice to broaden the operational pressure range, in order to enlarge the working gas capacity of UGS reservoirs, based on a 4D coupling simulation and integrity study. This is a real case in southwest China which has been put into effect and shown good results so far.
The overall approach is to carry out integrated subsurface and well engineering studies, numerical simulation, evaluation, step-wised implementation, and monitoring processes. This is a thoroughly integrated UGS study including: 1) Dynamic reservoir simulation which is coupled with a 4D geomechanical study of integrity evaluation of caprocks, faults and underpinning layers; 2) Research on increasing upper limit pressure and expanding storage capacity; 3) Well engineering design evaluation and integrity research. Five steps are recommended to increase working gas volume (WGV), where the most crucial step is to ensure that injection pressure exceeds original reservoir pressure in a safe manner. Thus, a rigorous monitoring, measuring and verification (MMV) plan must be proposed and carefully put into place.
This UGS project has been online since 2013 and has operated safely for 11 years, with three phases construction. The Operation Pressure Expansion and Capacity Increments (OPECI) Project was launched in Phase 3. The main purpose of OPECI is to increase WGV and storage capacity (SC) through expanded operational pressure range. The study suggests step-wise operational changes to achieve working gas volume target increase of about 10-15% through enlarging operation pressure range. This "pressure expansion" is to lower the minimum tubing head pressure (THP) and increase maximum injection bottom hole pressure (BHP). This process involves facility upgrade, optimum rate and pressure from each injector and producer, emergency supply & demand, and safety operation. From the perspective of production performance, the practice showed that the simulation forecast accuracy is above 95% both for the single well and the entire gas reservoir. The actual implementation has increased BHP to 32 MPa for most wells at the instantaneous stage, overall exceeding the original reservoir pressure (28.7 MPa) to 30 MPa at the balance stage. After raising injection BHP, the gas storage capacity increased by 2.4 × 108 m3, WGV increased by 3.0 × 108 m3, with emergency capacity increasing by 9.45 × 106 m3. The effect of the pressure expansion is very significant, and the economic benefit is obvious. The MMV implementation is on schedule and partly in place, and the forward plan is to improve the microseismic early warning functionality, strengthen dynamic monitoring and integrity management, and ensure the dynamic sealing of the UGS.
The implementation of "operation pressure expansion and capacity increase (OPECI)" - expanding range of operation pressure beyond the original reservoir pressure to increase WGV and SC, is an innovative attempt, the first in China, in the practice of construction of gas storages utility.
