A Practical Approach to Maximise UGS Storage Efficiency in Order to Secure Energy Supply

Abstract

The Underground Gas Storage (UGS) working gas capacity is one of the important design parameters for a UGS project. The objective of this paper is to demonstrate a case study from Sichuan area, China, where a fractured carbonate gas reservoir was used for UGS and show the key factors that should be considered to optimize UGS working gas capacity and help to secure energy supply during peak season. The dynamic simulation model was history matched on both depletion and storage. This model was coupled with 4D Geomechanics effects. It was then employed to capture the key parameters that will affect the working gas capacity. Based on the 4D Geomechanical integrity and facility assessment results, the forecast model was created using the subsurface and surface facility constraints. The study workflow included setting up the forecast model, uncertainty analysis and sensitivity analysis to capture the key driving parameters for the working gas volume. This study also included sensitivities of emergency gas supply during peak winter season. This study showed that the main factors that determine the UGS working gas capacity are the initial gas in place, tubing head pressure (THP) during withdrawal phase, bottom hole pressure (BHP) during injection phase, well pattern and well count. It was observed that each injection/production cycle is affected by the previous cycles. The simulation showed that the incremental working gas volume represented as a trend can be better fitted by a power function with both THP and BHP. It also demonstrated that lowering the THP limitation is more effective than increasing the BHP limitation. However, this may require surface facility upgrading. The optimum operating condition was identified by considering all engineering and geomechanical constrains such as erosion, wellbore integrity and cap rock integrity. This is used to guide the UGS daily operations with optimum production/injection rate of each well in a safe manner. The study recommended to implement a stepwise strategy to reach the working gas capacity. This paper presents a novel practical workflow and methodology to implement increased UGS reservoir working capacity. It also provides a practical way to forecast the operational pressure range and quickly predict the deliverability of emergency gas supply under various market demands.