Horizontal Wells in Yemen Make a Marginal Field Economic

Abstract

Abstract This paper documents the development of a high permeability, medium-heavy oil, bottom water drive field in Yemen from its discovery through to the staged development of the field using multilateral horizontal wells. The 16.5 MM barrels of reserves in the field are only economic when developed by horizontal drilling due to severe coning from the very active underlying aquifer. Vertical wells in the field produced at initial rates of about 400 stb/d of oil. Horizontal wells have resulted in productivity index increases of over ten times the vertical well values. Initial trilateral horizontal well production rates were as high as 8,500 stb/d of oil. The key technology that makes the project successful is the multilateral open hole completions in soft, friable sand. Bilateral and trilateral horizontal wells with 3,000 feet and 4,500 feet of horizontal section were drilled using PDC bit technology in a soft sandstone formation. The horizontal laterals of each well were drilled without tripping for a new bit and hole integrity was maintained. Introduction Canadian Occidental Petroleum Ltd. signed a six year exploration and development agreement with the Yemeni government in 1987. Exploration work in the Masila Block, Yemen, covered the period from 1988 to 1991 and involved geological field studies, airborne gravity surveys and a reconnaissance seismic program. The first exploration well was spudded in September, 1990 and commerciality declared in December, 1991, following discoveries in the Sunah, Heijah and Camaal fields. This paper describes the development of the Hemiar field, which covers an area of 3,300 acres and contains estimated recoverable reserves of 16.5 MM stb of 170 API oil overlying a strong bottom water drive. An estimated 24% of the oil in-place will be recovered with full field development. Initial development consisted of two vertical wells, which rapidly developed high water cuts due to coning and pointed to the use of horizontal wells. A previous study of horizontal well analogs in Western Canada had already suggested that horizontal wells would improve performance several fold. Subsequent development has consisted entirely of multilateral horizontal wells, with two bilateral wells followed by two trilateral wells. Several more multilateral wells are in the planning stage and await drilling rig availability. Geology Canadian Occidental Petroleum Ltd. was granted a production sharing agreement for the exploration and development of 14,370 sq. mi. on the Masila plateau of south central Yemen. Figure 1 shows the developed area of the Block 14 concession. The plateau is 2,000 to 4,600 feet above sea level, and is incised with innumerable wadis, canyons and limestone fault scarps. A field study was undertaken to determine the geological evolution of the Masila Block concession and corroborate earlier findings concerning source rock potential and general stratigraphy. The possible existence of a Jurassic age graben in the north of the block was revealed and subsequent seismic surveys focused on the northwest sector of the concession. The regional depositional environment was inferred from analysis of cores from 14 wells in the Masila Block. The major producing zone is the Upper Qishn Clastics, a Cretaceous age formation, which consists of the S3 to S1 depositional sequence, which was deposited on the northeastern passive margin of Afro-Arabia from Hauterivian to Campanian time (125–75 m.y). The dominant reservoir in Hemiar is the S2 sandstone (Figure 2), consisting predominantly of estuarine channels cut into tidal flat deposits. P. 215