Overcoming Challenges in the Development of Underground Gas Storage

Subsurface gas storage is a strategic tool used to balance seasonal sales gas supply and demand fluctuations. Developing and managing gas storage reservoirs requires the application of standard reservoir engineering tools and practices; however, a number of additional challenges are specific to Underground Gas Storage (UGS). This paper addresses these challenges and present both modelling, design, and operational solutions. There are important considerations and challenges that can be associated with areas such as reservoir selection, surface-subsurface modelling, and optimum number of wells with the best design in gas storage reservoirs. Nevertheless, operational challenges can also be very critical and lead to jeopardizing the success of the project, if not mitigated properly. Due to the cyclic nature of gas storage during injection and re-production, cyclic stress effects can be a concern and should be studied via appropriate geomechanical models and laboratory tests (Thick-Walled-Cylinder) to address any imapct on wellbore stability and potential sand/solid production. Although mature gas reservoirs are good candidates for underground gas storage, drilling any new wells can be challenging and has to be addressed using state of the art technologies such as managed-pressure-drilling (MPD) or under-balance-coiled-tubing drilling (UBCTD). The well completion in terms of material selection and design will also impact the workover frequency and productivity/injectivity of the gas storage wells. As such, accurate evaluation of flow assurance and completion accessories are essential to ensure long term suitability of the gas storage wells. Last but not least, due to the lean nature of the gas in these developments, the risk of hydrates formation is very likely and should be analyzed and mitigated with the right engineering tools. This work presents the basic theory and applications of the above-mentioned methods and evaluations with the ultimate goal of proposing general guidelines for the development of UGS. The results and interpretations of geomechanical modelling and laboratory testing are presented as well as the drilling design and its challenges. Well integrity and erosional velocity assessment are discussed as part of the flow assurance as well as hydrates formation envelopes and its prediction methods. Gas storage projects are strategic for gas operating companies and require careful planning and economic feasibility evaluation. The challenges and lessons learnt, discussed in this paper, are required to guarantee the success of such initiative.