Comparison of modified Waxman-Smith algorithms and Archie models in prospectivity analysis of saturations in shaly-sand reservoirs. A case study of Pennay field, Niger-Delta

Abstract

The presence of shales can cause severe problems for geoscientists in saturation analysis in reservoirs because of their conductivity nature. Shales are generally conductive, so as a result of this, they may consequently conceal the high resistivity distinctive of the presence of hydrocarbon in a reservoir. Shale crystals attract water that is absorbed onto the surface and some cations such as sodium Na^t which are naturally surrounded by hydration water. This will result in an unexpected excessively high conductivity compared to rocks that the shale crystals are not present, and such available vacuum might else be occupied by hydrocarbon. This study evaluates a shaly-sand reservoir’s saturation properties using the Maxwan-Smith model and compares it with Archie models for shaly-sand reservoirs in the case study field. The models were coded in a MATLAB environment with users friendly graphic interface. The four shaly-sand reservoirs delineated in the Pennay field have computed net-to-gross ranges from 0.88 to 0.94. The average hydrocarbon saturation for the reservoirs is 0.60, 0.71, 0.62, and 0.59 for Archie models, while 0.64, 0.74, 0.67, and 0.62 were derived using Waxman-Smith models for reservoirs 1 to 4, respectively. A poroperm relationship of y = −0.0036 ln (x) + 0.335 was derived for delineated reservoirs. A mathematical relationship model of Sw_archie = 0.88*Sw_maxwan-smith + 7.7786 with R = 0.72 was developed to relate Archie to Waxman-Smith derived water saturation in the field. The inferences in this study were premised around the available data for validation of the models. Reservoir characterization using Archie’s equation in shaly-sands resulted in figures of water saturation that are spuriously high. However, the shaly-sand reservoirs delineated were significant in hydrocarbon prospecting. Application of Maxman-smith yields slightly more hydrocarbon saturation by 7% -9% more than Archie models.

Appendix

CODING OF THE PROJECT AND GUI DESIGN