Structural and petrophysical controls on the remaining fluid distribution in the reservoirs of Gharif Formation before abandonment

A thorough study of reservoirs is required for the planning of effective production strategy with careful analyses of the areas, in which the oil can be potentially unswept before any field abandonment. This study presents a modelling workflow to evaluate changes in water saturation due to a long oil production period to localize the remaining oil. The static model of water saturation at the beginning of oil production is compared with the water saturation model before the abandonment obtained using dynamic simulation. Structural, lithological and petrophysical factors controlling the fluid distribution are studied for two reservoirs: the Middle Gharif consisting of Upper Haushi Sand (UHS), Dolomite Drain (DD), and Sandstone Drain (SD) and the Lower Gharif consisting of Lower Haushi Sand (LHS). Following the transition from proximal to basinal settings across the marginal slope, the fractions of sand and shale in all reservoir units decrease while calcite and dolomite increase from the eastern and southeastern parts toward the western and northwestern parts of the field. Controlled by the geologic structure, petrophysical properties and OWCs, water saturation before abandonment varied significantly in various directions. The oil is fully extracted in UHS from the areas with the best porosity and permeability, while overall oil recovery from the reservoir is very low because of the poor rock properties. Oil recovery was high from the DD, but the oil remains in two domes due to the absence of producing wells. Oil is concentrated in the elevated part of the ridge in SD while water saturation reached more than 90% in other parts. Although the LHS with the best petrophysical properties was the most prolific, it experienced early water breakthrough with oil remaining in the ridge and two domes.