Heat-Front Monitoring in the Qarn Alam Thermal GOGD Pilot

Abstract

Abstract Steam is currently being injected into the naturally-fractured Shuaiba carbonate in the Qarn Alam field, Oman in a five-year pilot project operated by Petroleum Development Oman (PDO). The objective of the project is to test the ability of steam to significantly lower the viscosity of the heavy and viscous (16 API, 220 cP) crude and by so doing accelerate the Gas-Oil Gravity Drainage Process which is currently taking place under cold conditions. One of the key monitoring objectives of the thermal pilot is the shape and rate of progression of the heat front within the fracture system. Gravity, fracture anisotropy and fracture oilrim thickening will all influence the steam zone geometry, which will in turn influence decisions regarding the expansion of the pilot project for full-field exploitation. The monitoring of steam zone growth will be accomplished by a unique combination of different technologies. The objective of this paper is to describe the various monitoring methods and techniques and include some basic estimation of results and examples of surveys completed to date. The techniques are surface deformation monitoring, time-lapse VSP, 2D and 3D seismic, and temperature and saturation surveys in offset observation wells. Introduction The Qarn Alam Shuaiba/Kharaib/Lekhwair reservoir is a salt-induced dome structure (Fig. 1) with densely-fractured chalky limestone containing 213 million m3 of heavy viscous (16 API, 220 cP) crude. The field has produced since 1975 with initial production benefiting from depletion of the fracture network at oil rates up to 6,000 m3/d. Within two years of production, the watercut increased sharply to 95 % and has remained virtually constant. The water level within the fracture system rose quickly to a level controlled by the producing intervals, while at the same time a secondary gas cap of 30 m formed in the fracture system leaving a fracture oil rim of 40 m. These factors have resulted in a predicted Ultimate Recovery of 4 million m3, or 2 % of STOIIP. To date 3.5 million m3 oil have been produced, with at least 75 % of this amount estimated to have come from the fracture system. The current oil production rate of 200 m3/d is assumed to be dominated by water-oil gravity drainage and gas-oil gravity drainage. The crest of the field is heavily faulted and fractured at the Shuaiba/Kharaib levels. Core and FMS studies showed that the open fractures appear to be mostly sub-vertical, extensional and NE-SW trending. At the crestal part of the structure, the dominant fault orientation is NW-SE. The structure is also intersected by a more subdued N-S trending fault pattern. The Shuaiba/Kharaib/Lekhwair formations were deposited during the lower Cretaceous. They consist mostly of porous, chalky limestones deposited in a shallow sea. The Lekhwair and Upper Kharaib formations are argillaceous and exhibit poorer reservoir quality than the overlying Shuaiba formation which consists predominantly of wackestones with a variable content of algae, forams and rudists. This heavy oil reservoir has for a long time been identified as a candidate for thermally assisted recovery. Similar thermal recovery schemes in carbonate reservoirs have already been successfully implemented elsewhere, e.g. in the Lacq Superieur reservoir. Qarn Alam Thermal GOGD Pilot Due to the high oil viscosity, the gravity drainage rates for the field are low. A thermal pilot has now been implemented with the objective of testing the viability of reducing the oil viscosity by means of injecting steam into the crest of the field and thereby accelerating the GOGD process. Extensive reservoir modeling has been completed in preparation for the Qarn Alam steam injection pilot. The radial extent of the heat-front in the Qarn Alam field can be approximated by the following expression, assuming radial movement and injection at 1,000 T/d (1) where t is time in days which indicates that after 5 years of injection, the heat front will have reached a distance of 250 m. Steam injection commenced in July 1996 from two injection wells located 25 m apart at the crest of the field. The locations of injection and observation wells are shown in Fig. 2. Two observation wells, QA-16 and 17, are cased and cemented and are used for temperature and saturation logging. P. 223^