Water-alternating-gas enhanced oil recovery optimisation for greater CO2 storage and oil recovery in a mature turbidite reservoir : case study of the Niger Delta

Abstract

The alternating injection of water and CO2 gas (WAG) is applied to improve hydrocarbon production left behind by primary and secondary recovery techniques, securely stored the injected CO2 in the subsurface, and increase the economic value of the turbidite reservoirs. However, the numerical simulations and optimisation studies of WAG projects in these reservoirs are plagued with inherent geological and other associated uncertainties that result from petrophysical, geological, and geophysical assumptions, as well as measurement errors, limited sampling, insufficient data, etc. Also, the prediction and optimisation of the economic value of the WAG project would require that relevant uncertainties in oil and gas prices and other associated uncertainties are considered. In this thesis, the Markowitz classical optimisation theory was applied for the robust optimisation of WAG injection in a Niger-Delta turbidite reservoirs under geological and economic uncertainties. This was coupled with a Non-Dominated Sorting Algorithm to obtain the engineering control parameters that optimise the Net Present Value (NPV) of the WAG project. To reduce the computational cost associated with the optimisation routine, we applied approximation methods to construct suitable surrogate models of selected reservoir outputs. An assessment of the technical feasibility of CO2EOR processes in the Niger Delta was carried out. Then, relevant geological uncertainties that plague CO2 EOR processes were described and quantified. We also developed and described a new economic model for predicting the economic value and quantifying economic uncertainties during CO2 EOR processes in the Niger Delta. Finally, the robust optimisation of the WAG project under geological and economic uncertainties was carried out using suitable data-driven approximation models. There are four significant contributions relating to WAG simulation and optimisation in reservoirs in the Niger-Delta hydrocarbon basin that are presented in this thesis. Firstly, the technical feasibility of CO2 EOR and storage in reservoirs of the hydrocarbon basin is presented. Secondly, an improved understanding of micro-scale and macro-scale physical instabilities/ heterogeneities such as the interplay of viscous fingering and permeability channelling in fine-scale grids is presented. These effects are also upscaled from the fine-scale grids to more realistic coarse grids. Thirdly, a new economic model for quantifying the economic value of CO2 EOR and storage processes in the Niger-Delta is presented. Finally, the applicability of data-driven proxy models for the robust optimisation of WAG processes under geological and economic uncertainties is presented.