Forties CO2 IOR Evaluation Integrating Finite Difference and Streamline Simulation Techniques

Abstract

In September 2000 the Forties Field celebrated 25 years of production. Recovery up to this point is approximately 60% of the original oil in place. The field contains undersaturated oil and is being developed under waterflood. A screening study of Increased Oil Recovery (IOR) options highlighted CO2 injection as technically feasible, suggesting an additional recovery of in the range 5–10% of the initial oil in place (STOIIP), subject to further work on the investment economics of the project and corporate sanction. This paper describes the study of the CO2 injection scheme with a focus on the reservoir simulation workflow. Various techniques for evaluating the full field benefits of IOR schemes can be used including the Todd and Longstaff approach1 or a coarsely gridded conventional compositional reservoir simulation model. The evaluation presented in this paper builds upon a technique that incorporates detailed conventional simulation results into a full field streamline front-tracking simulation. This method was originally developed by Arco and has been successfully applied in several Alaskan oil fields2. The technique captures the complex physics of the IOR process through fine scale, 3D, compositional, finite difference simulations of ‘type’ sections of the reservoir (the original method used 2D simulations). Results from these simulations are then used to calibrate ‘recovery’ curves that capture characteristics of oil mobilisation and returned solvent volumes as a function of gas injected. The calibrated curves representing gas injection response are then applied as tracers using streamline front tracking simulation to scale up to full field response. Introduction The Forties field, discovered in 1970, is located 170 km NE of Aberdeen, UK, in blocks 21/10 and 22/6a. The field is a relatively simple four-way dip structure. The oil is trapped in a stacked turbidite sandstone sequence. The field consists of four main channel sand complexes: the Main, Alpha/Bravo, Echo and Charlie. The Charlie sand is isolated from the other channels. On either side of the Charlie Channel is an area of more sheet-like sands referred to as the Charlie wing. Forties is currently producing around 60 mstb/d, approximately 10% of its plateau production rate. The field is being produced under waterflood. STOIIP at discovery was 4.2 billion bbls and remaining reserves are currently some 137 mmbbls of oil, giving a recovery factor of approximately 61%. The Forties IOR screening project is one of a group of such projects forming an integrated approach to Forties depletion, to reduce costs and improve production. Together they have the potential to improve the recovery factor by 10% of STOIIP. We believe that gas displacement IOR can contribute to more than half of this. The study of the Forties field IOR response to CO2 injection requires the use of reservoir simulation: CO2 injection results in a highly complex multi-contact miscible displacement process. Presently the most complete capture of the complex physical behaviour of the CO2 injection process is achieved by using a Compositional Finite Difference Simulator (CFDS) with small-sized grid blocks to enable the required multiple gas and oil contacts to be modelled. A full field large grid block model could be used to determine the IOR benefit but, without the necessary multiple oil and gas contacts between injectors and producers, the response of the model will probably be inaccurate.