Implementation of Waterflood in a Mature Oil Field – How Surveillance Supports Success

Field A is one of the largest oil fields in the South of the Sultanate of Oman. The field was discovered in 1975 and has been in production since 1989 under depletion drive utilizing the aquifer support. The main producing reservoirs are part of the Haima Group, which are characterized by good quality homogeneous sandstone, but the oil is viscous up to 1200 cP. After over 20 years of production, depletion levels in the field were getting high and a new development strategy was initiated by introducing water injection in this mature oil field. The waterflood development concept chosen for this mature field was injection deep into the aquifer at about 100 meters below the oil-water contact. 38 vertical water injectors were drilled at regular spacing across the field area. A key risk identified during the field development plan was vertical fracture growth from the injection depth to the oil column, hence short- circuiting injection water to the producers. Geomechanical modelling confirmed this risk, but also showed a significant range in the predicted fracture growth driven by uncertainty in the fracture pressure in depleted sandstone and shale baffles. The water injection started in 2014 and ramped up over the following years to the target level. The pressure response from the injection scheme was initially much slower than expected especially in the crestal area of the field. A comprehensive water injection performance analysis was conducted, and several remedial actions were initiated by the team to improve waterflood performance. As part of the remediation, 8 injector wells were recompleted just below the oil-water-contact as part of a shallow injection trial. These wells are fitted with fiber optic cables. A surveillance strategy was developed to address the main risks around short circuiting in both the deep and shallow injectors. Acquisition of cased hole logs and fiber optic surveys has played a key role in the success of the waterflood implementation. Production logging tools, temperature logs and Distributed Temperature Sensing helped assess fracture growth and injection conformance. Combined with pressure data acquisition in existing oil producer wells and new infill wells, the surveillance data show a clear response to the waterflood. Reservoir pressure is increasing in all areas of the field, and water injection is on target due to limited signs of upwards fracture growth. The acquired surveillance data have been instrumental to this success. The estimated recoverable volumes increased by about 10%, a very significant addition. The increase in reservoir pressure has also helped to conduct many production optimizations like well bean up, artificial lift optimizations and well stimulations. This allowed increased gross offtake and higher net oil production. As a result, the field achieved record production over 2023, the highest level in 6 years. FIELD