Investigation of H2S Migration In Marjan Complex

Abstract Marjan complex is a large offshore oil field located in the Arabian Gulf and is composed of four fields: Marjan 1, Marjan 2, Marjan 3 and Marjan 4. Currently production in the complex is limited to the Khafji reservoir. For several years, it was known that the H2S concentration in the Khafji reservoir varied across the complex. A comprehensive study was initiated to map the concentration profile of H2S across the complex and to address the migration of H2S within it. This included area-wide well-stream H2S measurement and geochemical fingerprinting of Marjan oils. For the first time, the concentrations of H2S were mapped across the complex. The southern and southwestern wells show relatively high concentrations of H2S, and the northern wells show no, or negligible amounts of H2S (less than 10 ppm). The migration of H2S into the northern part of the field has serious implications because the crude handling facilities, or the gas-oil separation plant (GOSP), were designed for sweet crude processing. If migration is proven, the facilities in the northern part of the field need to be upgraded to handle sour crude. The results of this study indicate that there is a ‘hot’ spot of high H2S located in the southwestern part of the complex. There is a significant H2S gradient across the Marjan complex with H2S decreasing from the southwest to the northeast. H2S concentration profiles with time also indicate that there is an increase in H2S concentration with time in the hot spot. The data negate the possibility of H2S generation in the Khafji reservoir from either sulfate reducing bacteria (SRB) or thermochemical sulfate reduction (TSR). Therefore, it is suggested that the H2S is migrating into the Khafji reservoir from somewhere else, probably from the Ratawi or other, deeper reservoirs. The geochemical analyses show that the hydrocarbon composition is very uniform across the complex, and there is no evidence for barriers to fluid flow within the Khafji reservoir. It is proposed that the lateral migration of H2S within the reservoir is arrested due to the presence of H2S-scavenging iron minerals. Two hypotheses are proposed for the migration of H2S into the Khafji reservoir from the Ratawi reservoir: (a) through inter-reservoir faults, or (b) through channeling leaks behind well casings. These two hypotheses are discussed in the paper.