Evaluation of Infill Drilling in the Third Sand Upper Reservoir, Greater Burgan Field, Kuwait

Abstract

The Third Sand Upper (3SU) is one of the three sub-reservoirs in the Third Sand of the Greater Burgan field, the world's largest sandstone oil field. Initial oil production begun in 1948 and 3SU field development has not been aggressive due to its poor reservoir quality and productivity. After 60 years of primary production, only 7.5% recovery has been achieved. Infill drilling was identified as a key development strategy in 3SU. In 2008, a simulation study was initiated to investigate infill drilling potential and its impact on production and recovery. We opted for a sector model mainly due to practicality and time constraint. The 780,800 cells sector geological model was scaled-up to 421,632 cells for flow simulation. Due to the sand-to-sand contact with the lower Burgan sands, it is imperative to include these reservoirs in the model to achieve proper energy balance. Accordingly, four pseudo layers were added to the simulation model to allow fluid migration from the lower reservoirs. The 3SU sector simulation model has 100m × 100m areal cells and individual layers with 4–6 feet thickness. Overall, the sector model has 30 times refinement compared to previous 3SU models (Ambastha et al, 2006). The history match has been carried out for 37 3SU historical wells with 60 years of production history. Detailed study of interactions among field permeability distribution, aquifer strength, fluid migration and fault transmissibility specifications on simulation results was key in developing meaningful history match. Water cut match was less than satisfactory for wells located in the dome area due to modeling deficiency introduced by the pseudo layers. Three infill drilling spacing scenarios were set up to evaluate prediction performance of 800-meter, 400-meter and 200-meter well spacing. Results of the 50-year prediction runs indicated that an incremental recovery of 11% can be achieved by reducing the current well spacing of 800-meter to 400-meter. Introduction Greater Burgan field, which is located in southeastern Kuwait, covers a surface area of about 320 square miles and has been ranked as the largest clastic oil field in the world. The four main reservoir units comprising the Greater Burgan Field complex are Wara, Mauddud, Burgan Third Sand (3S) and Burgan Fourth Sand (4S). The massive 3S is further subdivided into Third Sand Upper (3SU), Third Sand Middle (3SM) and Third Sand Lower (3SL). The 3SU reservoir is sandwiched by a tight Mauddud formation above and a permeable 3SM sand below. Figure 1 shows the corss-section of the major reservoir-horizons in the Greater Burgan field. 3SU reservoir communication occurs mainly through sand-to-sand contact with 3SM but extensive faulting also allows communication between Wara, Mauddud, 3S and 4S reservoirs. The Greater Burgan Field is separated into three producing areas, Burgan, Magwa and Ahmadi. No structural, geologic or reservoir features distinguish these areas, although PVT differences are assigned for areas north and south of the Graben fault. Figure 2 shows the areal view delineating these 3 areas. Initial 3SU production begun in early 1948. Despite of its significant STOOIP, 3SU has not been a dominant producer due to its inferior productivity. Overshadowed by the prolific 3SM reservoir, 3SU development has not been the priority and its potential was not fully assessed. In 2007, Kuwait Oil Company (KOC) has started revitalization of several low priority reservoirs in order to achieve the corporate production growth by 2020. In 3SU reservoir, two new wells were drilled in 2008 and 2009 to evaluate the performance of infill drilling. At the same time, a 3SU sector model was built to investigate the incremental recovery of infill drilling. This simulation effort was carried out by the KOC Greater Burgan Studies team with consulting assistance from Schlumberger.