Geological Heterogeneity in the Niger Delta; A Case for Additional Recovery Through a Combined Effort of Geologically Targeted Infill Drilling, Stimulation, and Gas-Lift Installation

Abstract

Abstract The Niger Delta, located in the seasonally flooded southern part of Nigeria, produces predominantly from the tertiery deltaic sands of Agbada formations at depths between 5,000 ft ss and 12,000 ft ss [1523 to 3655]. Its productive sands are made up of cyclic sequence of transgressive marine and fluvial deposits. Its fields contain multi-reservoir systems with sands that are poorly connected and high permeability heterogeneity. Earliest studies1 of the Niger Delta geology have shown that the productive rocks of the Western Niger Delta were thicker and were deposited much earlier than the Eastern rocks. Thus, while the source rocks of the Western oil lies entirely in the paralic shales of the Agbada formations, those of the Eastern oil lie partly on the shales of the Agbada formation and partly on the shales of the underlaying homogenous Akata formation. This study has observed that a majority of the giant oil fields in both the Eastern and Western Niger Delta fall within a narrow "bowlike" band that cuts across the depositional or structural sequence. This narrow band accounts for more than 70% of Nigeria’s proven oil reserves. In addition, a number of structural similarities have been observed in a majority of fields within this band. A number of Oil Mining Leases (0ML) are contained in this narrow band. One of such leases, 0ML 57, located in the Western Niger Delta and made up of four geologically similar fields namely Agigbo Obodo, Okpoko and Upomami is of great interest. The reservoir geometry of the fields is highly complex and consist of stack barrier bars, cut across by a number of distributary channels, dip-oriented facies and a number of growth faults. The original oil in place for 0ML 57 was estimated at 120 MM stb (19 MM m3) of relatively viscous crude. Aquifer support is strong and parts of the individual reservoirs are underlain by bottom water. The productive history of the fields show that about 6 - 8% of the original oil in place have so far been recovered in each of the fields. Coupled with the above low oil production is high water cut which tended to suggest premature water coning and cusping characteristic of most fields/reservoirs in the band. Implementation (study)2 of the geologically targeted in-fill drilling yielded less than desired results. In the Eastern Niger Delta, the Agbada field is one of the fields located in 0ML 17 which are contained in the above band. Its reservoirs have similar geologic characteristics as the reservoir in the fields of 0ML 57, with its major reservoirs Agbada D 5.20 having an original oil in place of about 160 MM stb (25.4 MM st m3) of relatively viscous oil. Twelve out of its seventeen drainage points were closed-in essentially due to high water-cuts. Oil production rate declined by about 80% from 8000 bopd in 1970 to 1640 in 1990. About 11% of STOI IP has so far been recovered in twenty years of its productive history even with strong aquifer support. A detailed reservoir simulation study3 on Agbada D5.20 sand aided by 3-D seismic reinterpretation identified all the poorly invaded and unswept areas which provided target locations for infill drilling. The study indicated that higher oil rewards (ultimate recovery up to about 50% STOI IP) can be realised from a combined effects of geological infill drilling, stimulation, and gas-lift installations. The above results are applicable to more than fifty fields contained in our "bowlike" band including the fields in 0ML 57.