Benefits of Early Life Water Flooding in Oil Reservoirs Near to Saturation Pressure – A Case Study

Abstract

This paper explains the importance for implementation of early water flood in near saturation pressure oil reservoirs particularly for the case having solution gas as dominant drive mechanism. The depletion in case of solution-gas drive (having no or minor support) with low to moderate in-place volumes is relatively fast. It is commonly observed that no pressure maintenance program is implemented till the reservoir pressure has been severely exhausted. This delay is generally caused by time consumed during understanding of fluid and reservoir behavior, and ultimately symbolizes the phrase ‘missing the train’. The objective of this study is to present the importance of early water flood and its impact on oil recoveries. A field was discovered in South Indus Basin which has half graben and four-way fault bounded structure with numerous splay faults. A well was drilled which encountered Sands ‘A’ and initially produced ~1360 bopd having ~4500 psia initial reservoir pressure. A detailed study was carried out when the reservoir pressure had depleted from 4500 to 1200 psia after draining ~400 MBO with ~1 Bscf associated gas. Based on the outcomes of the study, water flood was implemented by drilling an injector well ‘Inje-1’ which increased the pressure from 1200 psi to 4500 psi in the later life of field. Despite the pressure had rose to initial reservoir pressure, the recovery from the reservoir remained sub optimal. To understand the importance of implementing early water flood at higher pressures, a numerical simulation model was developed, history matched, and various sensitivities were run to see the impact of water flooding at various reservoir pressures during the life span of the field. It was observed that the recovery would have been more if the water flooding was implemented when the reservoir pressure was above bubble point. The reason being liberation of gas and shrinkage of oil resulting in high viscosity and low mobility oil remaining behind. If this liberation of gas is prevented by injecting water and conserving reservoir energy, both viscosity and mobility of oil would remain favorable due to delay in arriving at saturation conditions. Hence the recovery of these types of reservoirs can be enhanced by taking advantage of low viscosity and higher mobility of oil during early life. If the waterflood is implemented after exhausting the reservoir pressure, then the increased viscosity restricts oil flow and causes water channeling due to higher mobility contrast. As a result, leaving behind bypassed oil zones and very high residual oil saturation. In the present case study, it was observed that if the water flooding was implemented prior to reaching bubble point, recoveries would be 7-15% higher as compared to previous recovery. The early implementation would have added value to the overall project. Implementing the lesson learned, recent new discoveries are being evaluated to initiate water flood in early life. Early implementation of water flood in the oil reservoirs closed to saturation pressures will always be beneficial. Appropriate field development plan of the field and right decisions at right time will aid to enhance oil recovery. Once the energy in the oil reservoir is drained after producing gas, it is very difficult to regain the same energy.