Reservoir Management for World's First Thermal Gas/Oil Gravity-Drainage Project

Abstract

This article, written by Editorial Manager Adam Wilson, contains highlights of paper SPE 153967, ’A Reservoir Management System for the World's First Thermal Gas/ Oil Gravity-Drainage Project,’ by Keith Rawnsley, SPE, Augustine Ikwumonu, SPE, Solenn Bettembourg, Pristwanto Putra, SPE, and Ibrahim Al-Ismaili, SPE, Petroleum Development Oman, prepared for the 2012 SPE EOR Conference at Oil and Gas West Asia, Muscat, Oman, 16-18 April. The paper has not been peer reviewed. The world’s first full-field thermally assisted gas/oil gravity-drainage (TAGOGD) project has begun in Oman (Fig. 1). Historically, the field was operated at very high water cut because of aquifer influx through the fractures to the producers. The recovery process was a mixture of gas/oil gravity drainage and water/oil gravity drainage, but the high oil viscosity and low matrix permeability prevented recoveries from reaching more than a few percent. To attempt to unlock the oil in place, a TAGOGD development concept has been piloted in the field over approximately 10 years. This led the way to the full-field project that has now begun. Introduction Steam, mainly generated by heat-recovery steam generators, is injected into the natural fractures at the crest of the reservoir, and the fractures distribute the steam in the reservoir. As the oil in the matrix is heated, its viscosity decreases by a factor of 100 and it drains down by gravity through the matrix into the fracture oil rim. From there, it is sent to the production facility (Fig. 2). The field’s life can be divided into two phases. The first phase, now under way, is the transition period during which the oil rim in the fractures is lowered from its position near the top of the reservoir to a position close to the matrix oil/water contact. This increases the oil column height exposed to TAGOGD to more than 100 m. This period is expected to last approximately 2 years, and production is through gas lifted producers, which are wells inclined 45° to intersect fractures. The second phase is the plateau/tail period, when the oil rim will be maintained at its deep position. Production in the second phase will be through horizontal producers drilled close to the oil/water contact. Modeling The thermal gas/oil gravity process will be the first application of the technique at field scale in the world. Modeling, therefore, has taken and will continue to take a more substantial role than it has in more conventional fields, both to build confidence and to act as an interface between the reservoir surveillance data and opportunity generation. The modeling strategy is designed to ensure that predictions are accurate, to capture ongoing uncertainties, and to improve with experience. As more data become available, the understanding will improve, which will be reflected in changes in the modeling strategy.