Recently, underbalanced drilling (UBD) has been used with increasing frequency to minimize problems associated with invasive formation damage which often greatly reduces the productivity of oil and gas reservoirs. When properly designed and executed, the benefits of underbalanced drilling (UBD) are well recognized by the industry and there is increasing raise in interest in this technology. The benefits from UBD generally fall into two categories:

Cost reduction- including mitigation of conventional drilling problems such as lost circulation, differential sticking, reduction in stimulation and cleanup costs, etc.

Value adding- including productivity improvement, increase in ultimate recovery, and real-time reservoir characterization while drilling.

Arabian Gulf oil Company (AGOCO), a major energy player in the Libyan oil industry, decided to pioneer the introduction of the new technology by embarking on a pilot project to drill 6 underbalanced horizontal wells in the Sarir C-Main field. Sarir C-Main Field is located in the Libyan Desert on the south east of Sirte basin. The reservoir consists of sandstone from the Early Cretaceous age. The field was first put on production in 1967. Since 1991 ESP pumps have been used for the production. The target reservoir, Member 5AC, is partly depleted oil bearing reservoir from the initial reservoir pressure of 3995 psi to about 2400 psi (5.3 ppg EMW). 3 out of the 6 well UBD campaign in the Sarir C have been drilled so far. The first two wells identified the technology as an effective method to improve penetration and production rates. The third well built on the experience of the previous wells by emphasizing on minimizing formation damage and maximizing the production of the well. Another objective was the evaluation of production while the well is being drilled, eliminating the need for expensive post drilling production testing.

This paper discusses the objectives, planning, problems encountered and lessons learned during the first application of UBD in the Sarir C Main Field for AGOCO Libya.

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