Planning Optimal Horizontal Wells in Clastic Reservoirs: Case Histories from the South Oman Nimr Field

Abstract

Abstract Case histories from the Nimr Field are cited to demonstrate how enhanced 3D reservoir characterization and multi-disciplinary approaches in well planning, targeting and monitoring have improved productivity and oil recoveries and reduced development costs. Over 140 horizontal wells have been drilled targeting the complex Gharif, Al Khlata and Haima clastic reservoirs. In the early horizontal drilling campaign, vertical and deviated pilot holes were required prior to horizontal drilling in the unpredictable Al Khlata glacial reservoirs. The success ratio was initially low but has greatly improved with the application of high resolution 3D seismic and reservoir modelling techniques which have improved understanding of the 3D geological model. These tools have permitted the prediction of reservoir geometry, sand development and hydrocarbon distribution and reduced structural uncertainties. Initial productivity and ultimate recoveries of successful horizontal wells have been improved by a factor of 3 and 1.5 respectively over that of vertical wells. Well costs have been reduced by improving the completion design and applying recently developed drilling technologies and practices. The subsurface geology model is constantly updated with new well data thus continuing the trend of better prediction and planning of horizontal wells. Introduction The Nimr Field, one of the largest oil field in South Oman, was discovered in 1980 and brought onto production in 1985 (Fig. 1). The field is a northeast-southwest trending, elongated anticline bound by major faults to the southwest and northwest. It extends over an area of approximately 200 km2 with a relatively thin oil column (20-85 m). To date 340 wells have been drilled, of which 140 are horizontal. The field is currently producing at an average daily rate of 15,600 m3/day oil with 80% water cut. The main reservoirs are Haima, Al Khlata and Gharif sandstones containing some 430 million m3 of STOIIP with an Ultimate Recovery of 46 million m3. The oil is heavy (density 927 kg/m3, 210 API) and viscous (400 mPa.s). More than 80% of the STOIIP is in the relatively thin oil columns of the Haima and Al Khlata reservoirs which directly overlie a massive Haima aquifer while the Gharif produces under moderate edge water drive. Initially the field was developed with a production rate of 5,000 m3/d and an Ultimate Recovery of 7% by vertical wells on 600 m spacing. A comparison of actual performance of vertical and horizontal wells was undertaken after the drilling of 22 horizontal wells during 1990–92. These wells were drilled in areas of different geology and column length. The results were encouraging: the initial oil production potential of the horizontal wells was on average a factor of 4.3 higher than for the nearby vertical producers. The cumulative oil produced at water breakthrough (20% water cut) is on average a factor of 2.1 more than for vertical wells. At present, the field is developed with horizontal wells on 172 m spacing with an average recovery factor of 18%. Over 140 horizontal wells have been drilled so far with an initial productivity estimated to be 3 times that of vertical wells and the Ultimate Recovery 1.5 times greater. This paper focuses on how the enhanced 3D reservoir characterization and multi-disciplinary approaches in well planning, targeting and monitoring have improved productivity and oil recoveries. It will also demonstrate how the implementation of recently developed technologies together with improved drilling practices have had a direct impact on cost reduction. Three case histories of wells drilled in different geological settings will be demonstrated, which highlight the impact of these technologies and techniques on the planning of optimal horizontal wells. Reservoir Characterisation: Hi-Resolution 3D Seismic The first 3D seismic survey in the Nimr Field was acquired in 1989 and revealed a complex structural framework of the dissected field culminations separated by Al Khlata glacial valleys with heterogeneous fill. P. 403^