Steam Injection in Fractured Carbonate Reservoirs

Abstract

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 102186, "The Physics of Steam Injection in Fractured Carbonate Reservoirs: Engineering Development Options That Minimize Risk," by G.T. Shahin Jr., SPE, Shell E&P Technology; R. Moosa, SPE, PDO; and B. Kharusi and G. Chilek, Shell E&P Technology, prepared for the 2006 SPE Annual Technical Conference and Exhibition, San Antonio, Texas, 24–27 September. The economic success of the Qarn Alam project depends on the ability to predict steam requirements and oil production. Two key oil-production mechanisms are heat transport through the fractures and into the matrix and gas-cap generation resulting from thermal volatilization of the oil. The process mechanisms involved in thermally assisted gas/oil gravity drainage (TAGOGD) were validated through laboratory experiments, while the field-forecast model results were validated by history matching pilot-performance data. Introduction The Qarn Alam field was first brought on production in 1975. When the field came on stream, the net oil production peaked at 6000 m3/d, despite the high oil viscosity. Water breakthrough followed rapidly, and by 1978, net oil production declined and remained low thereafter. During the initial peak in oil production, the oil contained in the highly permeable and pervasive network of fractures was produced and replaced mostly by formation water. After this period, a rim of oil remained, separating the newly formed fracture water/oil contact (WOC) and the newly formed fracture gas/oil contact (GOC). Under primary production, the reservoir produces on average approximately 100 m3/d of 16°API oil, at a 10 m3/m3 gas/oil ratio (GOR). The purpose of TAGOGD in Qarn Alam is to develop additional oil by increasing the field recovery from 4% under isothermal gas/oil gravity drainage (GOGD) to 27% of original oil in place. TAGOGD is a novel enhanced-oil-recovery development concept that has as yet no full-field analog. Despite the lack of commercial analogs, several field trials have occurred on a pilot scale. Isothermal GOGD in this viscous (220 cp)-oil reservoir is economically unattractive because of the low oil mobility. Reservoir heating by steam injection significantly lowers the oil viscosity to approximately 2 cp, increases the gas cap as a result of thermal vaporization of the oil, and increases the GOGD rate by a factor of approximately 60. This development concept has been piloted in the Qarn Alam field and was found to be economically attractive. The attractiveness of the concept stems from the fact that the dense fracture spacing in the reservoir effectively distributes the steam and is a conduit to collect the oil in the oil rim. This type of development requires significantly fewer wells than an equivalent conventional steamflood.