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This paper describes the successful implementation of a lean gas injection trial in a long horizontal well and the subsequent expansion of hydrocarbon gas injection in the low-permeability Kharaib B carbonate reservoir of the Al Shaheen field, offshore Qatar. The main objectives of the injection trial were to determine whether premature gas breakthrough would occur in the neighbouring production wells and whether water injectivity after the gas flood would be reduced as a result of the presence of high gas saturation around the injection well. During a 6-month trial period, some 1.7 Bscf of produced gas was reinjected into a trial well. The neighbouring production wells, located some 1,000 ft away, have seen increased production due to pressure support, without suffering from premature gas breakthrough. Data suggest that the reinjected gas may have redissolved into the reservoir oil and that the pressure response is linked entirely to water injection. Furthermore, water injectivity was comparable to the nearby water injection wells; hence, gas injection did not seem to have a negative impact on the subsequent water flooding efficiency. Encouraged by the positive results of the trial, gas injection is being expanded to a larger number of wells belonging to five different platform locations and encompassing three carbonate reservoirs of the Al Shaheen field. Prediction of gas injection performance has been carried out with compositional reservoir simulation models, which required non-equilibrium initialisation to account for lateral reservoir fluid property variation, a tilted free water level, and a large transition zone. The project started early 2010 using the currently available lift gas pressure. High-pressure gas injection, implemented as a water-alternating-gas (WAG) scheme, will take over once the required surface modifications have been made. |
