PETROPHYSICAL EVALUATION OF “AYO” OILFIELD, OFFSHORE NIGER DELTA, NIGERIA

Abstract

Porosity, Permeability and water saturation are the main petrophysical properties of a reservoir rock, and they play a vital role in the characterization of hydrocarbon reservoirs at all stages. This research is about the determination and evaluation of the petrophysical properties of “AYO” oilfield, offshore Niger Delta, Nigeria from a suite of wireline logs obtained from four wells (W1, W2, W3 and W4) drilled in the oilfield. The suite of wireline logs used, consist of Gamma Ray log (GR), Resistivity log (RES), Neutron log (NT), and Density log (DEN). A volume of 3D Seismic data was employed to map the geological structures present in the oilfield, and it showed that the trapping mechanism present in the field is a fault assisted closure. Petrel® Seismic to Simulation Software by Schlumberger (2009 Edition) was used to process the wireline and seismic data for this research work. Two hydrocarbon bearing reservoirs were mapped across the four wells and are designated as reservoir one (R-1) and reservoir two (R-2) respectively. The first Reservoir (R-1) cuts across the four wells at a depth range of 8283.33ft (2524.76m) to 9903.13 ft (3018.47 m), while the second Reservoir (R-2) is penetrated by the four wells at a depth which ranges between 8512.07 ft (2594.48 m) and 10383.30 ft (3164.83 m). Thus, R-1 has an average thickness of 1619.80 ft (493.72 m) while R-2 has an average thickness of 1871.23 ft (570.35 m). The wireline logs (the Neutron and Density cross plots) generally indicates the occurrence of hydrocarbon (both oil and gas) in R-1 while R-2 is devoid of oil, that is, the latter has only gas. The porosity model shows that R-1 has an average total porosity of 23% and an average effective porosity of 22%. Similarly, R-2 has a total porosity of 22% and an average effective porosity of 21%. These values indicate that the pore spaces within the reservoirs are well connected. The permeability model shows that R-1 has an average permeability of 6577 mD, while that of R-2 has an average permeability of 5101 mD. These indicate that the pore spaces within the reservoirs are interconnected and fluids can move properly within them. A vii probabilistic method employed for the water saturation model establishes three cases (Base-Case, Low-Case and High-Case). The Base-Case for R-1 has an average water saturation of 46%. This implies that the reservoir is 54% saturated with hydrocarbon, 1% being oil while 53% being gas. The Low-Case for R-1 has an average water saturation of 73%, indicating that the reservoir is 27% saturated with hydrocarbon with 2% being oil and 25% being gas. The High-Case shows that R-1 has an average water saturation of 32%, an indication that the reservoir is 68% saturated with hydrocarbon with 2% oil and 66% gas. The water saturation model for Base-Case in R-2 has an average water saturation of 48%, indicating that the reservoir is 52% saturated with gas. At the Low- Case of R-2 there is an average water saturation of 67%, indicating that the reservoir is 33% saturated with gas. For the High-Case, R-2 has an average water saturation of 34%, indicating that the reservoir is 66% saturated with gas. Petrophysical evaluation of AYO Oilfield classifies it as very good reservoirs. The reserve estimation for R-1 ranges between 143 - 298MSTB (Million Stock Tank Barrels) for oil and 2935181- 7233930 MSCF (Million Standard Cubic Feet) for gas. R-2 has an estimated reserve which ranges between 3663715 – 7372695 MSCF (Million Standard Cubic Feet) of gas. This research has provided insight to new locations where wells could be drilled for further exploration and production of hydrocarbon in the oilfield. It also provided Information on the trapping system present in the oilfield being structurally supported (fault assisted closures).