This paper examines the performance of a horizontal well with multi-lateral completion in a mature thin carbonate reservoir. The Upper Sabil reservoir in Zelten formation of Paleocene age in Intisar 103N Field has a structure of anticline that housed 14.4 MMSTB of OOIP. The reservoir was highly under-saturated with oil of 42° API.

The initial field development with two vertical wells resulted in a poor recovery of only 9 % of OOIP. The recovery mechanism of Upper Sabil reservoir is mainly rock/fluid expansion with marginal support from flank aquifers. The poor recovery can be attributed to thin pay thickness, poor reservoir quality, large well spacing, and lack of pressure support from aquifers.

The existing two vertical wells revealed a near constant production rate for the last 17 years indicating the drainage area is much larger than the wells can deplete in a reasonable time frame. The incentive of reducing well spacing was there but the question was how to reduce the well spacing: by drilling many vertical wells or less number of horizontal wells.

A reservoir simulation study in 2003 indicated that drilling vertical wells in a very thin reservoir of poor quality was not cost effective and recommended drilling of horizontal well. Hence, a horizontal well was drilled and completed at the top of the porosity with two lateral legs.

The stabilized oil production rate of the horizontal well was approximately three times higher than that of the vertical well, whereas the drilling cost of the horizontal well was about 1.5 times higher. The production from the horizontal well showed no negative impact on the production performance of the existing vertical well and the reserve was doubled.

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