Comparison of CO2-EOR performance between offshore and onshore classes of reservoirs

Abstract

CO2 has been extensively used in onshore fields, primarily for EOR. However, it has been used less offshore due to limited transportation infrastructure and the lack of secure CO2 supply. Recently, CO2 flooding has been reconsidered in offshore fields for both EOR and storage. The performance of CO2 flooding in the offshore classes of reservoirs, which are characterised by fundamentally dissimilar properties and development characteristics than onshore reservoirs, might be different from the past experience of CO2 flooding observed onshore. Offshore developments are characterised by higher rates of depletion, fewer wells, larger well spacing and higher well rates compared to onshore reservoirs which are characterised by pattern development and shorter well spacings; moreover, the motivation behind CO2 flooding might be different offshore. The aim of this study is to review these differences between CO2 flooding in offshore and onshore classes of reservoirs, exclusively within the context of reservoir engineering. In the first part of this study, different aspects of CO2 flooding are compared between two major provinces i.e. the onshore Permian Basin province located in the United States and the offshore North Sea province. It will be shown that CO2-EOR has many similar characteristics in these two provinces despite the fact that ambient reservoir conditions are fundamentally different between them. Next, flow patterns are compared between these two classes of reservoirs. Flow patterns in each of reservoirs are investigated by deriving the key dimensionless numbers which may characterise CO2 flooding in each of them. It will be shown that CO2 flooding is slightly more gravity dominated in the North Sea class of reservoirs. Additionally, in the absence of gravity effects, flow patterns upon CO2 flooding are expected to be more stable in the North Sea class of reservoirs due to better mobility ratios that characterise the displacement in this province. The fact that the motivation for CO2 flooding is potentially different between these two classes of reservoir may also promote alternate CO2 flooding process designs offshore, which should satisfy both the EOR and storage requirements of CO2 flooding in the offshore class of reservoirs. The second part of this thesis investigates the grid size requirements for modelling miscible processes such as CO2-EOR. A new approach based on measuring heterogeneity induced dispersivities in longitudinal and transverse orientations is introduced and developed. Matching these dispersivities with equivalent numerical dispersion may determine the correct size of grid blocks in a miscible displacement simulation.