Energy Geoscience (Oct 2022)
Physical properties of sandstone reservoirs: Implication for fluid mobility
Abstract
Core samples representing depths of hydrocarbon-bearing zones are not readily accessible for reservoir evaluations. On the other hand, wireline logs with incorporated seismic data, which can be archived over a more extended period while retaining their original forms, are typically more available for research purposes. Therefore, the study relies on wireline logs with seismic data to predict the reservoirs' fluid mobility by evaluating the hydraulic (flow) units, reservoir depths, fluid saturations, and geothermal gradients. It also indicates the associated water cut (Cw) within Ritchie oil and gas field, Niger Delta considering a three-phase (oil-gas-water-bearing) reservoir (RA) and an oil-saturated reservoir (RB) delineated across three wells (RW1, RW2 and RW3). Research activities combining the presented factors to achieve the stated objectives are not quite common within the study location. It shows lower, average and upper limits of the flow unit factors and irreducible water saturation (Swirr) within the reservoirs. The study shows the relationship between hydraulic units/fluid saturations and fluid mobility/associated Cw within the sandstone reservoirs. It maximises porosity (Ф) for the theoretical flow units' prediction during qualitative and quantitative estimation based on the adopted expressions. Therefore, the study reveals that water saturation (Sw) and hydrocarbon/water ratios substantially control Cw, and other contributing factors include thermal gradients and Swirr. The flow unit factors are also significant and will encourage fluid mobility. The evaluated reservoirs (RA and RB) are below 10 400 ft (3 170 m) across wells RW1, RW2 and RW3 within the Agbada Formation of a geothermal gradient up to 2.7 °C/100 m; therefore, they have good thermal conditions to enhance hydrocarbon mobility and increase Swirr. Hence, the reservoir should feature significant hydrocarbon extraction via primary recovery. The average water cut (Cw-avg.) (12.3%) estimated for reservoir RA is within the acceptable range; therefore, the associated water production from the three-phase reservoir will not be much of a concern. In addition, simple models are presented to aid an alternative approach for predicting reservoir quality and Cw within sandstone reservoirs, especially in the absence of core samples.
