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IMPACT OF INJECTING INERT CUSHION GAS INTO A GAS STORAGE RESERVOIR (Sudheer Reddy Lekkala) Underground natural gas storage is a process which ensures constant supply of natural gas by storing the excess gas produced and quickly supply when required. The underground storage makes use of depleted reservoirs to store the natural gas. Cushion gas assists in delivering the natural gas (working gas) and maintains the adequate pressure in the reservoir. The key issue is that cushion gas cannot be produced and remains as a permanent inventory which accounts to about 30% of development cost in a typical storage reservoir. A part of cushion gas may be replaced with an inert gas such as nitrogen or carbon dioxide to reduce the investment cost. The impact of replacing the cushion gas by nitrogen was studied by simulating nitrogen injection into a storage reservoir. The technical difficulty in replacing the cushion by nitrogen is the mixing which would corrupt the quality of natural gas. Some mixing is unavoidable when two dissimilar gases come in contact, but could be controlled with close monitoring and strategic planning. The reservoir parameters such as porosity, permeability and pressure were varied to study the mixing trends. Storage reservoirs are usually considered to serve for long time periods and therefore multiple withdrawal and injection cycles were simulated to study the mixing trend in detail. Different scenarios were considered in simulation by altering well placement, reservoir shape, injection of nitrogen in stages, and distance between the injector and producer wells. It has been found that the optimum percentage of cushion that can be replaced by nitrogen gas is 20. The degree of mixing is function of withdrawal rate and the fraction of cushion gas replaced in the storage reservoir. Reservoir pressure and permeability may affect the degree of mixing in early cycles but do not have significant impact on mixing. Reservoir porosity appears to have minor impact on the mixing. The mixing effect decreases to a great extent if distance between the producer well and the nitrogen injector is increased. iii ACKNOWLEDGEMENTS It is a pleasure to express my gratitude to my research advisor Dr.Kashy Aminian for providing me an opportunity to work on this topic. I would like to thank him for his valuable guidance, encouragement and support throughout this work. I would like to extend my thanks to my other two committee members Dr.IIkin Bilgesu and Dr.Samuel Ameri, who also is the chair of Petroleum & Natural Gas Engineering department for being a part of my research. I would also like to thank Dr.Shahab Mohaghegh and other faculty for their help during the course of my study. There needs to be a special mention for Ms.Beverly Matheny who has helped with the administrative matters. Thanks to my friends for helping me during the course of study.

Impact of injecting inert cushion gas into a gas storage reservoir