Understanding Reservoir & System Uncertainties to Enable CO2 EOR

Abstract

Abstract When implementing any Enhanced Oil Recovery (EOR) or tertiary recovery project it is critical to understand the key variables that influence the success of the scheme – reservoir parameters and fluid properties – and the behaviour of the existing development scheme. The additional components of miscible fluid fronts in heterogeneous carbonates, sources of miscible injectant and the systems needed to manage a miscible EOR development will generate additional uncertainty to that already present during primary and secondary recovery. This paper describes the reservoir on which evaluation of CO2 miscible EOR has been carried out, the reasons it is a candidate for CO2 EOR and identifies the key uncertainties that will have a major impact on the behaviour of a CO2 development scheme; namely fluid distribution, reservoir structure, compositional variation and reservoir property distribution. A key element of the paper is describing how the uncertainties are recognised and managed by the modelling philosophy, given the relatively immature status of the reservoir development and the limited historical data available. The paper describes the interpretation of incremental benefits of CO2 EOR over existing development schemes. A small component of the paper discusses options of appraising CO2 EOR further, the approach and need to pilot injection patterns and concepts of phasing development of CO2 into an existing pattern development. Uncertainty range will reduce over time and the paper describes how work emphasised the importance to recognise when this range is acceptably small to make decisions that involve larger capital investment, such as new development options involving additional wells and facilities. Although the majority of work done in this study was on subsurface elements, this paper briefly touches on infrastructure needed for CO2 EOR and implications of the scheme on existing facilities. Also considered are the external influences that impact EOR value, such as the source of CO2, price of fluid streams entering and leaving the system, and potential for the scheme to be a CO2 storage option once EOR benefit is exhausted. At present in Abu Dhabi, CO2 or Miscible Injectant (MI) EOR has been recognised as the EOR development of choice by a number of different studies. The fluid types and pressure/temperature regimes in Abu Dhabi reservoirs mean that miscible flooding is likely to be the most appropriate mechanism to further increase recovery factors. Using CO2 also has the benefit of demonstrating CO2 capture and likely storage at a time when the UAE is seeking to curb carbon emissions. Improving the understanding of how CO2 can play a role in Abu Dhabi reservoirs is therefore highly topical.