Comparative Evaluation of Single vs. Multi-Mandrel Gas Lift System for an India Offshore Field

摘要

Mumbai High field is located in Western Offshore ofIndia, 160 km South West off Mumbai City. The field wasdiscovered in the year 1974 and put on production inMay 1976. Massive development plan was undertakenin phases and a peak production rate of about 400,000bopd was achieved during year 1984. To maintainreservoir pressure, suitable water injection scheme wasput in place in 1984-85. The conventional multi-mandrelgas lift system was applied as artificial lift mode andoperating in the field since 1987.In multi-mandrel conventional gas lift system, 4-6mandrels are used in the well. The GLVs in uppermandrels work as unloading valves and orifice valve inlast mandrel act as operating valve. As mentionedearlier, the gas lift started in the year 1987 in this fieldand in 1994-95, a significant number of wells (more than100 wells) were put on gas lift. The system utilizescompressed gas at 1000 to 1350 psi (at well head) indifferent complexes. The existing compression system isan integrated system, which supplies gas to gas-liftnetwork and balance gas to sales line leading to onshoreplant. Presently, 381 wells are operating on gas lift (outof 454 wells completed with gas lift) and contributingabout 80% of total oil production from Mumbai High.During the incipient stages of gas lift, CFPrecommended single mandrel single point gas lift systemwith a gas lift pressure of 1800 psi for unloading and1400 psi for normal gas lift operation. Considering theavailability of present gas compression system at about1400 psig, as per worldwide practice, conventional gaslift system with multi-mandrels in place was accepted byONGC. By doing so, major compression system revampand consequent delays were avoided.In the recent past, some of the major operators,especially in North Sea, have applied the Single MandrelGas Lift System (referred here as Single Point Gas Lift)to overcome the inherent disadvantages of conventionalgas lift system. Presently, Mumbai High is undergoingmajor additional re-development plan to augment oilproduction, the present conventional gas lift systemneeded re-look to draw a learning curve. It is in thiscontext, a detailed exercise has been carried out toevaluate the present conventional gas lift systeminvolving its efficacy and well servicing costs.The present paper analyzes advantages of single pointgas lift over conventional gas lift, the requirements ofsingle point gas lift for Mumbai High, efficacy ofconventional gas lift system, present major and minorwell servicing costs and other related issues forimplementation in Mumbai High. A detailed technoeconomicanalysis is presented. It is observed that themulti-mandrel system of gas lift is more effective for thisfield and should be pursued for future developmentschemes of this field.The field is currently undergoing a major redevelopmentexercise. Hence, a cost-economicevaluation of present system is essential for futuredecision process.