Protect and Then inject: Optimized Well Fluids Successfully Drill Depleted Reservoirs To Store Gas

Abstract

A major operator has initiated the data-acquisition campaign in the southern North Sea for a future storage facility capable of holding 5 billion m 3 of gas. It is estimated this venture will double the existing gas supplies stored in the UK and represent more than 5% of its annual gas demand. As North Sea gas production decreases and the UK becomes more dependent on imports, the ability to store gas has become an important part of the UK energy policy.Drilling into depleted reservoirs for gas storage produces several major technical problems and issues that must be addressed. This field is a pressure-depleted reservoir with a differential pressure equivalent to 7.3 lbm/gal between the drilling fluid's hydrostatic pressure and the reservoir pressure. This differential must be controlled to eliminate the risk of differential sticking, downhole losses, and hole collapse.Because of the reservoir depletion, it would be impossible to backflow and clean up the near-wellbore region without a postdrill-in treatment fluid to remove the fluid filter cake and waterwet all the surfaces for gas injection. To ensure project success and usable fluid designs, reservoir conditions were simulated in the laboratory and fluid parameters were altered to provide the optimum properties to minimize the future risks.The paper discusses in full the laboratory design process, the verification of the drill-in and treatment fluids as being fit-forpurpose, and their successful application in the field. Initial well testing suggested that the expected injection rates of 500 scf/min at 300 psi were exceeded, with rates of 750 scf/min at 280 psi reported.Stephen Vickers is the Eastern Hemisphere Applications Engineering Manager at Baker Hughes, where he has worked since 2001. His main area of interest is drill-in fluid design with emphasis on minimizing fluid induced formation damage. He studied quarry and mining engineering at the Doncaster School of Mining and Mineral Resources.Stephen Bruce is a fluids service representative for Norway Operations for Baker Hughes. He studied chemistry at State University of New York at Binghamton and oilfield chemistry at RGU.Alistair Hutton is a technical sales representative for UK Operations for Baker Hughes. He studied chemistry at Aberdeen University and software technology at RGU.Paolo Nunzi is a well operations manager for Eni UK. He has been with Eni E&P for more than 25 years. He served as an engineer for 8 years in several different countries, primarily in northern Africa, northern Europe, and CIS. He holds an MS degree in mining engineering.

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