Discussion and Comparison of Performance of Horizontal Wells in Bouri Field
Authors: Jafar A.S. , Al-Attar H.H. , El-Ageli I.S. 
Published: December 2007

Seven horizontal wells were drilled in Bouri field, offshore Libya, and put on production between 1989 and 1992. This paper presents the experience gained in producing and monitoring these wells over the past few years. Comparison of wells basic characteristics and performances with offset conventional wells indicates their higher productivity and general superior performance. Different monitoring activities conducted on these wells are discussed including transient test analyses and production logs. Free gas production was found to be related to the phenomena of coning and fracture flow mechanism. These discussions show that, in most of the cases, production of water and gas is inevitable, as long as economical oil rates are to be maintained, and that the targets put for these wells, as recovery increase and production troubles reduction, were at least partially fulfilled.

Introduction

Bouri field is an elongated E-W orientated anticline located in the Libyan offshore about 130 km. NW of Tripoli. The field was discovered in the seventies, and started on production in 1988, from the eastern two sectors 3 and 4. The producing pay interval is the uppermost member of the Metlaoui formation from Early Eocene age, at an average depth of 8000 ft. subsea level, with a lithology dominated by Nummulitic Limestones. The reservoir comprises a thick oil ring of around 300 ft. overlaid by an extended primary gas cap and underlain by water, with the resultant drive mechanism of a combination of aquifer activity and gas cap expansion. Different fault sets cut the structure with varied local intensity, affecting more dominantly the southern flank, where water advance is mostly appreciated. These faults and their associated network of fractures and microfractures affect the petrophysical characteristics of the rocks and influence, to one degree or another, the productivity and the flow dynamics in the reservoir.

Horizontal Wells

Out of a total of 55 wells drilled for field development, seven were drilled as horizontals (HWS), while the others, to be called here as conventionals (CWS), comprise slanted wells with up to 65 degrees, and two verticals. The original objective was to efficiently drain the reservoir oil below the gas cap (Fig.1), by optimizing production rates while keeping low drawdowns, thus minimizing the gas coning tendency.

Geological Brief. The particular target of all horizontal drilling in the field was the stratigraphic layer of Metlaoui Formation designated as U2, which forms the lowest interval of the pay zone. It is further divided into three subunits, from bottom to top, U2a, U2b, and U2c. The first subunit represents a zone of good petrophysical properties, with an average thickness that exceeds 80 ft. in the interested area and excellent lateral homogeneity, containing some 20% of the OOIP of the reservoir in the developed area, hence it was the target of most of HWS in Sector-4. The unit gradually diminishes towards the West, and disappears completely in Sector-3 area. As the intermediate subunit U2b is composed mainly of cemented Nummulitic limestone that forms a compact 20–30 ft. well traceable layer extending all the way above U2a; it represents a presumed natural barrier to gas expansion, further promoting U2a as a favourable position for HWS. The upper subunit U2c has a much wider areal extension than the lower two, covering the total developed area, with an average thickness of 80 ft., and with fairly good properties. It was the target of the two horizontal wells in Sector-3 and one in Sector-4. Relevant wells' information is found in table-1.

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