Multilateral Horizontal Well Improves Oil Recovery in a Mature Field in Libya
Authors: Bybee Karen 
Published: February 2015

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 112125, "Multi Lateral Horizontal Well Application for Improving the Oil Recovery of a Mature Field, Intisar N. Field, Concession 103 - Libya," by M.M. Gharsalla and M.B. Elghmari, Zueitina Oil Company, originally prepared for the 2008 SPE North Africa Technical Conference and Ex hib ition, Marrakech, Morocco, 12-14 March. The paper has not been peer reviewed.
The full-length paper examines the performance of a multilateral completion in a mature thin carbonate reservoir. The initial field development with two vertical wells resulted in a poor recovery of only 9% of the original oil in place. 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 horizontal wells. The stabilized oil-production rate of the multilateral horizontal well was approximately three times higher than that of the vertical well.

Introduction
Intisar N field is in Concession 103 in Libya approximately 360 km south of Benghazi in the prolific Sirte basin. The field was discovered in 1968 by drilling Well N1 to a total depth of 10,318 ft in the Lower Sabil. The Intisar N field comprises two separate carbonate reservoirs, the Upper Sabil (Zelten) at a depth of 9,230 ft and Lower Sabil (Heira) at 9,820 ft, approximately 600 ft apart. Fig. 1 shows a map of formation tops of the Upper Sabil and Lower Sabil formations. Both reservoir structures are composed of two anticlinal saddles believed to be separated by a fault. The field is relatively small in size, with initial oil in place of 19.0 million STB of which 14.4 million STB is contained in the Zelten (Upper Sabil) reservoir and 4.6 million STB in the Heira (Lower Sabil) reservoir.

Stratigraphy and Trapping Mechanism
The general stratigraphy and structure in the Concession 103 area has been controlled by the shelf edge in Upper Paleocene times between the Marsa Brega Trough and the Gialo/Amal high. Reefal bioherms developed on this shelf edge through the Upper Paleocene, after which there was a transgression or subsidence in the very late Paleocene resulting in the deposition of the Kheir marls and shale. During the Eocene, there was predominantly shelf carbonate deposition, with the grain size determined by provenance and water depth. Locally, the sedimentation has been influenced by drape over the reef, resulting in locally shallow water depths in the immediate area of the reef, resulting in rather gentle homoclines providing up to 50 to 70 ft of closures. The oil accumulations at Concession 103N in the Zelten and Heira reservoirs are trapped by the homoclines.

Petrophysical Properties
The reservoir quality varies laterally significantly in both the Upper and Lower Sabil; the quality improves toward the crest and deteriorates toward the flanks. Most parts of the Upper Sabil formation have fair porosity, and good solution porosity is locally present. The oil systems in the Upper and Lower Sabil reservoirs are light and highly undersaturated.