Maximising offset well information in unravelling onshore geohazards indicators: Case study of the Gale field

Abstract

An attempt is made in this study towards maximising offset wells information in unravelling onshore geohazards indicators in the Gale field. The Gale field is located about 100km north-west of Port Harcourt, Nigeria. The field consists of a highly faulted and elongated rollover anticline, bounded to the north by a regional growth fault. The data used for this study integrates the quadrature and reflectivity amplitude attributes from seismic data, with offset well data. The conventional reflectivity seismic data was 90° phase rotated to derive the quadrature volume. The quadrature seismic was considered a more appropriate reflectivity seismic attribute for use in shallow geohazard analysis as it is known for its characteristic preservation of high frequency spectrum inherent in the data. Offset wells (GALE-01, GALE-03, GALE-04, GALE-05, GALE-06, and GALE-08) analysis revealed mud losses, stuck pipe, overpull and gas cut as gathered from the daily drilling reports. These could translate to potential triggers of some geohazards where poorly managed. Review of field geotechnical report did not reveal any geohazards issues. Based on the geohazards assessment carried out for these wells; chances of encountering shallow gas for all the units as shown in the well summary is rated low. Results from a geohazards analysis indicate the presence of possible shallow gas within the area of interest and particularly along the shallow section of planned well trajectory. This is further supported by the presence of faults within the vicinity of gas bearing reservoirs at deeper level and a potential for these faults extending to the shallower interval. These faults are likely to serve as migration pathways for gas to seep to the shallower section, hence forming a potential geohazard. In addition, some of the offset wells targeting deeper gas reservoirs penetrated pockets of gas at the shallower interval that stratigraphically correlates with the shallow section that would be penetrated by the planned wells. The results of this work were used to move the proposed drilling location of the Gale planned wells to a nearby area free of shallow gas signatures.