4D Seismic Feasibility Study for Well and Reservoir Monitoring in a Deepwater Nigeria Field

Abstract

Abstract A 4D feasibility study was carried out to evaluate seismic detectability and the application of 4D technology in a green field in deepwater Nigeria. A 3D seismic survey, acquired prior to production, will serve as a baseline for future follow-up 4D survey if the production induced changes are sufficiently detectable. The objective of this study was to assess if production-induced changes and fluid movement, using time-lapse seismic technology, are detectable. The seismic differences between monitor seismic and modeled time stamps are analyzed for 4D responses in the presence of realistic noise levels. This 4D feasibility study will provide technical basis for incorporating 4D seismic monitoring into the reservoir surveillance plan. The 4D seismic will assist in placement of new wells, to optimize fluid flow between producer/injector pairs, and to locate by-pass oil areas. This will further support the current effort to improve the ultimate recovery by optimising phased development activities and in-field drilling using time-lapse monitor survey. A dynamic reservoir simulation model was used to generate several time-stamps, the first being base and later time-stamps acts as monitor models. These models were converted to acoustic properties through a suitable rock and fluid model, and acoustic impedances were calculated. Synthetic seismograms were subsequently generated for the various base and monitor combinations, and the difference between the base and monitor synthetic seismic was analyzed for production-induced 4D signals (after the baseline synthetic was calibrated to the existing baseline seismic). The synthetics were corrupted with realistic noise levels, derived from the existing baseline seismic and existing 4D difference seismic volume from a nearby deepwater field. The results showed that production-induced 4D signals due to changes in fluid saturation are clearly mapable in the field after one or two years of production in the presence of expected noise (NRMS of 15%) levels. However, the pressure drawdown is of relative small magnitude and has no significant impact on the 4D response. This information serves as a good guide for planning 4D acquisitions and reducing costs of unnecessary intervention in the field. The results of the study will be applied in updating Wells/Reservoir surveillance plan and optimising field development. Based on the study, the plan is to acquire a minimum of three (3) seismic monitor surveys in the life cycle of the field. The first monitor will take place after two years from start of production and should show already significant 4D (time-lapse) signals.