Application of Velocity Trend and Effective Stress in Prediction of Sediment Com-paction and Overpressure for Drilling Operation in Gulf of Guinea

Abstract

This study demonstrates the effectiveness of integrating both velocity trend and effective stress in overpressure and sediment compaction prediction. Incorporating both strategies into overpressure forecasting is shown to be helpful in this study. The gamma ray, density, neutron, acoustic, and deep resistivity records from the X-Field deep offshore Niger Delta were used. These wells were X-001, 002, 003, and 004. The sediment compaction mechanism, attributed to differential sedimentation rates and grain size component, was predicted using a cross plot of compressional sonic velocity versus depth color coded with gamma ray log. Compaction disequilibrium is the dominant overpressure mechanism, as evidenced by the cross plot of sonic velocity against density, color coded with depth for wells X-001 and X-004. The wells' effective stress was plotted using density logs to create an overburden trend, and gamma ray logs were utilized to create shale volume logs. Overpressure prediction focused on the deformation behavior of shale formations since they are more sensitive to overpressure phenomena than sands, being denser and characterized by less resistant minerals. Therefore, shale bodies within the same depth range are more likely to experience loss of porosity and rise in density with depth, as demonstrated in the wells. Well X-001's computed normal compaction trend (NCT) indicates normal compaction from the surface to a depth of 2500 m; however, the NCT line deflects to the left at this depth, signaling the onset of overpressure; at this point, the well pressure is expected to exceed hydrostatic pressure. Beyond this depth, an increase in porosity in the overpressured shale was detected, leading to a decrease in sonic velocity below the usual compaction trend line. In well X-001, the TOV is located at this depth. Overpressure zones were also first detected in wells X-002 and X-004 at depths of 2400 and 2500 meters, respectively. Based on these results, it is likely that sediment compaction trends normalize at depths below 2500 m in the research area. The innovation in predicting sediment compaction and overpressure for deep-water operations in the Gulf of Guinea involves advanced geophysical and geological modeling techniques.