Reducing Uncertainties in Hydrocarbon Volumetric Estimation: A Case Study of Fuba Field, Onshore Niger Delta

Reducing uncertainties to the barest minimum before reserve estimation aids in making a better decision regarding field development. This study analyses uncertainty in hydrocarbon reserve estimation in Fuba Field using both scenario-based deterministic and stochastic methods. Two hydrocarbon reservoirs (A and I) were selected and mapped. Depth structure maps revealed fault supported collapsed crestal closures for both reservoirs. Uncertainty analysis was conducted using low case (P90), base case (P50), and the high case (P50) reservoir properties. On average, porosity, NTG and Sw are 31%, 89%, 10%, and 24%, 84%, 23% for A and I reservoirs. Hydrocarbon volumes recorded for the high case, base case, and low case using a deterministic versus stochastic approach are 30.52 MMSTB, 12.46 MMSTB, 4.57 MMSTB, and 18.52 MMSTB, 13.59 MMSTB, and 9.40 MMSTB for reservoir A, 58.87 MMSTB, 10.94 MMSTB, 1.51 MMSTB, and 25.56 MMSTB, 14.59 MMSTB and 7.63 MMSTB for reservoir I. While the base case was similar for both methods (stochastic and deterministic), there is a huge difference in the low and high-case hydrocarbon volumes recorded in both methods. This change could be attributed to the reservoir bulk volume with (>85%) with little contribution from oil saturation and porosity. Cross plot analysis confirms that bulk volume is the main control of the estimated stock tank original oil in place (STOIIP). Hence, a slight alteration in bulk volume will significantly affect the estimated STOIIP. It is recommended that bulk volume be given most attention when conducting reservoir simulation as this will increase simulation time, reduce simulation cost, and provide more accurate simulation results.