First Full Field Steam Injection in a Fractured Carbonate at Qarn Alam, Oman

Abstract

Abstract Gas Oil Gravity Drainage (GOGD) of the Qarn Alam fractured low permeability carbonate reservoir is being enhanced by steam injection in the world's first full field development carbonate thermal development. Unlike a normal steam flood, the steam is used as a heating agent to enhance the existing gravity drainage mechanisms, and project has proved to be viable based on encouraging pilot results. The elegance of the thermally assisted - GOGD is that the fracture network is both used for distribution of steam and recovery of the oil. The number of wells can therefore be kept to a minimum compared to conventional matrix steam floods. Whereas the primary production performance of the Qarn Alam under cold GOGD is only expected to recover 3–5 % of the oil in place, studies to date indicate that the recovery factor under steam injection at 18,000 tonne per day will be in the range 20–35 % with Oil Steam Ratio of 0.16 -0.3 m3 oil /tonne of steam. The learning from the Pilot has not only helped understand the subsurface uncertainties but also provided significant insight into the engineering design and operational issues which are being managed right upfront during design. Energy consumption in the full field development of this project is reduced by banking on the benefit from co-generation of power and steam. Utilisation of co-generation will minimise CO2 emission and reduce gas import. The project will be under construction from 2007, with full rate steam injection reached by late 2009. The paper not only discusses how the project addresses the reservoir management challenges of this complex recovery mechanism but will address some of the engineering and operational challenges that are being managed. Introduction Qarn Alam Field is located in central Oman south of the western Hajar Mountains. This large oil accumulation is trapped in shallow Cretaceouslimestone units at a depth of around 200–400m sub sea. The anti-clinal structure is a result of a deep salt diaper, with significant crestal faulting and fracturing. The field was discovered in 1972 and placed on primary production in 1975. The 16° API oil with a viscosity of 220cP has been produced from the 29% porosity, low permeability (5–14mD) limestone. . During the primary production period from 1975 to 1995, the first year showed a large peak in oil mainly from emptying of the fracture network with a minor contribution from fluid expansion due to pressure reduction. At the end of the first year, production had declined to a very low sustainable rate interpreted to be from gravity drainage, from a combination of gas-oil (GOGD) from the secondary gas cap and oil-water (OWGD) below the fracture gas-oil contact (FGOC). The reservoir then consists of a matrix with very little drainage and a fracture network with a thin oil rim below the secondary gas cap and above the fracture oil-water contact (FOWC), figure 1. Primary production performance such as that of Qarn Alam is only expected to recover some 3–5% of the oil in place over any reasonable time frame due to low matrix permeability and high oil viscosity on gravity drainage rates. Recoveries via matrix floods of water, polymer or steam were discounted as development options due to the pervasive fracturing observed in the field which would encourage the flooding agents to completely bypass the matrix.