In the absence of a gas market, the management of the gas associated with oil production often poses serious challenges to the development of oil fields, resulting in potentially stranded gas resources. While gas flaring was the default solution, more stringent carbon constraints have rendered this solution unattractive, owing to its negative health, safety, environmental and socio-economic impacts. One potential solution entails injecting the otherwise stranded associated gas (AG) into underground geologic systems for temporary storage, but re-producing same when a viable gas market becomes available.

Faced with the challenge of providing temporary AG solution for a new oil development in the shallow offshore area of the Niger Delta, this paper presents an overview of candidate solutions, which include (i) venting, (ii) flaring, (iii) injection for improved oil recovery, (iv) underground sequestration (AG injected but not re-produced), and (v) on-site conversion to natural-gas hydrates to facilitate evacuation to other outlets. Their relative strengths and weaknesses are examined relative to this case study.

The paper details the key aspects of a reservoir-engineering assessment of temporary AG storage in a non-associated gas (NAG) reservoir for later monetisation of both the native NAG and alien (injected) AG resources. Rigorous numerical-simulation studies highlight the key factors that control both the injection and production phases of the subject reservoir. For a given set of abandonment conditions, effects of the gas volume injected prior to the production phase on the ultimate recovery factors of the native and alien gas resources are presented. Additionally, the relative advantages of matrix, pseudo-fractured, and fractured injection modes are explored.

Compared against the above-mentioned alternative solutions in terms of (i) effective sales volume of hydrocarbon, (ii) infrastructural requirements, and (iii) net carbon savings, underground storage is the most attractive solution to address stranded AG in this example field, and possibly others.

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