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Abstract
BP has developed a range of innovative techniques to maximize economic oil recovery from its global miscible gas floods and the results have been reported in a series of publications over the past three decades. The purpose of this paper is to provide an overview of BP’s experience of establishing, managing and optimizing a miscible gas flood.

Prudhoe Bay (Alaska) is the world’s largest miscible gas project. Conventional and unconventional methods have been applied in a variety of different settings. An extensive surveillance program has facilitated a good understanding of the processes operating at field scale and surveillance data are used to optimize the flood. In 2000, a large-scale gas cap water injection project was implemented to slow the decline in field pressure. This project has made the vaporization process more efficient at higher pressure, resulting in additional recovery. Miscible gas injection has been extended to numerous other fields on the North Slope of Alaska.

BP has two active miscible gas projects in the North Sea: Magnus and Ula. Tertiary miscible water-alternating-gas (WAG) flooding in Magnus field started in 2002 and its impact on reservoir performance is significant and well understood. Tertiary miscible WAG injection in Ula field started in 1998 and has played a key role in arresting production decline. The WAG scheme in Ula is currently being expanded. In addition to these projects, BP operates a CO2 injection and storage project at In Salah, Algeria, where more than 3.2 million tonnes of CO2 have been stored since 2004.

Miscible gas injection has generated considerable benefits for BP over the past three decades and will continue to do so. The potential availability of large sources of CO2 in the future, supplied by carbon capture, could help maintain a leading role for miscible gas injection for years to come.

Introduction
Miscible flooding is a proven method for enhancing oil recovery. The principle is to reduce the interfacial tension between the displacing solvent and displaced oil and thereby achieve a significant reduction in the residual oil saturation compared to immiscible water flooding and primary depletion. Under ideal conditions, miscible flooding can recover almost 100% of the oil originally in place. Under field conditions, this limit is seldom achieved owing to imperfect volumetric sweep, incomplete displacement of oil in rock that is swept and inadequate capture of displaced oil (Stalkup, 1983). In addition, commercial factors may limit the amount of miscible gas that is available. Despite these limitations, there are many successful miscible gas projects around the world and the prospects for miscible flooding in the future look bright if new sources of CO2 become available for enhanced oil recovery.

BP operates the world’s largest miscible gas project (Prudhoe Bay, Alaska) and has been actively involved in planning and operating many other miscible and immiscible gas floods. Miscible gas EOR forms part of BP’s suite of Designer Gas™ technologies, which is being developed as part of the Pushing Reservoir Limits program. Designer Gas EOR is strategically focused on bringing about a radical increase in economic recovery by improving pore scale displacement and sweep using existing well stock. The purpose of this paper is to review BP’s experience in this important area of enhanced oil recovery. Several key factors are addressed using case examples, including: (1) identification of a suitable source of injection gas; (2) reservoir and fluid characteristics required for a successful gas flood; (3) planning, surveillance and reservoir management requirements for field trials of new technologies and deployment of proven technologies.