ABSTRACT:

The changes in reservoir pore pressure due to hydrocarbon production induces stress changes within the reservoir and its surrounding rocks. These stress changes lead to reservoir compaction, which in turn can trigger potential geomechanical problems such as surface subsidence, fault reactivation, wellbore instability, pore collapse, casing deformation and seismicity. In this study, we present a 4D seismic, ant-attribute assisted, interpretation of induced-stress change in hydrocarbon producing reservoir in Kolo-Creek Field, Coastal Swamp Niger Delta, Nigeria. The analysis integrates 3D seismic interpretation, antattribute evaluation, well-log analysis, pressure and production data. Time-lapse seismic surveys acquired in 1997 (base) and 2009 (monitor) show clear 4D responses with a root-mean-square repeatability ratio (RRR) of 0.38, indicating excellent survey repeatability. The seismic interpretation reveals fault reactivation and fracturing associated with production-induced stress changes. Geophysical well logs from seven wells were used to delineate and correlate three reservoir zones (Sand A, Sand B, and Sand C). Petrophysical analysis indicates low shale content ranging from 7.74–37.44%, high porosity values between 0.19 and 0.36), and excellent permeability varying from 375–3327 mD, which is consistent with high-quality, coarse-grained sandstones. Production and pressure data provided by SPDC show a decline from 1592.55 to 400.34 bbl/day and from 4766 to 3103 psi over 12 years, respectively, corroborating with the ant-tracker attribute interpretation. Variance attribute was used to delineate faults and the result obtained from variance attribute were used as an input in running the Ant tracker attributes to delineate subtle faults in both base and monitor that are difficult to see easily. The results obtained from the ant attribute monitor volume indicate that the subtle faults seen in the base volume are fractured, reactivated in the monitor causing reduction in the porosity of the reservoir and in turn resulting to reservoir compaction and subsidence. The integration of ant tracker attribute with structural analysis reveals reservoir stress changes on fault behavior and reservoir performance. This study could lead to Wellbore Stability Management; Using stress change predictions to guide well placement and drilling orientation, minimizing risks of shear failure, casing deformation, and production losses.

Keywords: Seismic attributes, induced-stress, Ant trackers attributes, 3D seismic cubes, structural mapping, faults and fractures Nigeria.