The case study carbonate field has been developed by waterflooding. Presently, only the produced water is re-injected into 50 wells with voidage replacement ratio being less than unity at field level. Declining pressure trends due to suboptimal voidage replacement balancing and continuous drilling activities require rigorous monitoring to avoid dropping below a critical reservoir pressure (13000 Std. Units ) level at which the hydrostatic head could initiate fractures in the reservoir. Injection under fracturing condition is not an option for field recovery furthermore preventing artificial fractures will be safeguarding 40% of expectation reserves in three specific injection patterns.
A geo-mechanical study was conducted to understand the dependency of fracture pressure on reservoir pressure depletion, including temperature effects. A geo-mechanical model was generated using well logs such as density & sonic that was calibrated with lab tests for rock mechanical properties and leak-off tests together with pore pressure for stress model. This model was then used in the estimation of fracture pressure dependency on reservoir pressure using poro-elastic theory. A pragmatic diagnostic tool was then developed for establishing the field pressure operating envelope, where the injection pressures are plotted against reservoir pressures and compared to the field operating envelope at pattern level.. This tool helps identify which patterns are at potential fracturing risk. i.e. above fracture pressure model. Consequent to identification of risky patterns, mitigation measures are implemented by reducing water injection rates and/or reducing offtake rates to adjust sector pressure within the operating envelope. This remediation is practiced rigorously every quarter for all the patterns with the availability of updated reservoir pressure data. So far no indication of induced fractures from downhole temperature gauges or anomalous water-cut behavior is seen. With the effective injection pressure/rate management, field ultimate recovery is estimated to be 33%, exceeding the top quartile recovery factor (32%) and a sustained expectation oil production forecast for the coming 7+ years. To further enhancing the voidage replacement, two supply water wells were drilled for supplementing water injection
In summary, better management of waterflood projects dictates controlling injection strictly below fracture pressure compared to maintaining injection closely above bubble point pressure. The proposed waterflood field management methodology adopted is generic and the diagnostic tool developed can be utilized routinely for preventing induced fractures in matrix waterflood projects.