The objective is to present an evaluation of waterflood management in a mature multi-layered carbonate reservoir using classical reservoir engineering techniques. The contrast in effectiveness of the waterflood performance is illustrated by incorporating case studies of two blocks in the field, characterized by high degree of heterogeneity with large scale vertical permeability variance. Simulation studies along with classical analysis to envisage the best way forward to improve the performance of waterflood are presented.
Successful implementation of waterflood necessitated enhancement of areal sweep and distribution of injection pattern using horizontal injectors. Studies indicate that an effective waterflood leads to additional oil recovery and improved reservoir pressure. Oil recovery envisaged before and after initiation of waterflood was established for Block-I& II. Pressure ratio (P currentto Pinitial) was plotted against recovery efficiency and compared with the standard drive mechanisms plot. Additionally, various diagnostic plots were incorporated to establish the effectiveness of waterflood. New opportunities were identified in pressure sink areas and their impact on performance of waterflood has been demonstrated using simulation.
The response to waterflood is favorable in Block-I with oil recovery more than 35%. Continuous Injection at controlled rates was possible and the created voidage was compensated appropriately. Enhancement of areal sweep employing horizontal water injectors resulted in better displacement which could be observed from sustained liquid production rates. The reservoir pressure in the area has noticeably improved resulting in reduction in Gas-Oil ratio. In addition, steep decline in oil production per string could be arrested with waterflood. Persistence of a pressure sink area in Block-I indicates the presence of a structural barrier. Block-II presents a contrasting scenario as early water breakthrough in producers resulted in poor sweep thereby adversely affecting the performance of waterflood. An analysis of Halls plot indicated that water injection at higher rates caused formation parting. Shutting of injection from highly permeable layers and controlling injection rate below threshold limit to avoid formation parting had a positive impact on producers in the area; hence profile modification of injectors could be a viable option. Moreover, for addressing the issue with isolated pressure sinks the effect of conversion of few wells into injectors has been studied. Commercially available software package has been used to simulate the field behavior after proposed injector conversion.