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

Abstract

Abstract Mumbai High field is located in Western Offshore of India, 160 km South West off Mumbai City. The field was discovered in the year 1974 and put on production in May 1976. Massive development plan was undertaken in phases and a peak production rate of about 400,000 bopd was achieved during year 1984. To maintain reservoir pressure, suitable water injection scheme was put in place in 1984–85. The conventional multi-mandrel gas lift system was applied as artificial lift mode and operating in the field since 1987. In multi-mandrel conventional gas lift system, 4–6 mandrels are used in the well. The GLVs in upper mandrels work as unloading valves and orifice valve in last mandrel act as operating valve. As mentioned earlier, the gas lift started in the year 1987 in this field and in 1994–95, a significant number of wells (more than 100 wells) were put on gas lift. The system utilizes compressed gas at 1000 to 1350 psi (at well head) in different complexes. The existing compression system is an integrated system, which supplies gas to gas-lift network and balance gas to sales line leading to onshore plant. Presently, 381 wells are operating on gas lift (out of 454 wells completed with gas lift) and contributing about 80% of total oil production from Mumbai High. During the incipient stages of gas lift, CFP recommended single mandrel single point gas lift system with a gas lift pressure of 1800 psi for unloading and 1400 psi for normal gas lift operation. Considering the availability of present gas compression system at about 1400 psig, as per worldwide practice, conventional gas lift system with multi-mandrels in place was accepted by ONGC. By doing so, major compression system revamp and consequent delays were avoided. In the recent past, some of the major operators, especially in North Sea, have applied the Single Mandrel Gas Lift System (referred here as Single Point Gas Lift) to overcome the inherent disadvantages of conventional gas lift system. Presently, Mumbai High is undergoing major additional re-development plan to augment oil production, the present conventional gas lift system needed re-look to draw a learning curve. It is in this context, a detailed exercise has been carried out to evaluate the present conventional gas lift system involving its efficacy and well servicing costs. The present paper analyzes advantages of single point gas lift over conventional gas lift, the requirements of single point gas lift for Mumbai High, efficacy of conventional gas lift system, present major and minor well servicing costs and other related issues for implementation in Mumbai High. A detailed techno-economic analysis is presented. It is observed that the multi-mandrel system of gas lift is more effective for this field and should be pursued for future development schemes of this field. The field is currently undergoing a major re-development exercise. Hence, a cost-economic evaluation of present system is essential for future decision process. Introduction Mumbai High, an offshore oil field was discovered in the year 1974 and was put on production in May 1976. The main pay zone L-III is a multi-layered, anisotropic limestone reservoir with a gas cap and partial water drive. This L-III reservoir has 13 different sub-layers, which are separated by shale streaks. Location map of Mumbai High field is shown in Fig. 1.