Modelling Gas Injection for Enhanced Oil Recovery in Deepwater Fields

Abstract

Abstract Enhanced oil recovery using miscible and partially miscible gas injection processes have been utilized extensively and have been successfully applied in a significant number of mainly onshore reservoirs. Miscible and partially miscible gas injection is the most mature technology of all the EOR processes. The fact that the incremental recoveries may be significant makes it very attractive to assess and potentially deploy gas injection technology and overcome the challenges in offshore deep water settings. In this paper, we present a technique which uses established methods that simulate gas flood performance. The methods used in the study allow the use of a pseudo-miscible-black oil method in full field reservoir simulation models for initial scoping purposes. This avoids computationally expensive flash calculations resulting in a fast method to develop production profiles which allow the testing of the economic robustness of the projects. While simulations using the Todd and Longstaff (T-L) pseudo-black oil method are strictly for first contact miscible (FCM) processes, in essence, most of the published field applications for this pseudo-miscible technique have been for multi-contact miscible (MCM) processes. The T-L pseudo miscible method utilizes a mixing parameter, ωo that ranges from zero to one, simulating completely immiscible and first contact miscible floods respectively. A literature review indicated that models matched using field production rates yield mixing parameters within a range of 0.6–0.8, while models matched against only a corresponding compositional model yield mixing parameters closer to 1.0, a condition which assumes no fingering in a sub-grid block level (Bronchalo, et al, 2004). In order to evaluate the potential of a miscible gas flood, it was required to generate reliable production forecasts. This study was embarked upon in order to establish that simulation generated forecasts could be validated. This study utilized homogenous 2D models (box models), heterogeneous sector models with fine gridding and coarsely gridded full scale models to evaluate the impact of the mixing parameter at different scales. A fully compositional simulation was used to benchmark the results all of the pseudo-miscible modelling exercise. For the simple box models, it was found that thermodynamic effects were dominant in the determination of the mixing parameter. Below the minimum miscibility pressure (MMP), a lower mixing parameter value was required to achieve a match with a corresponding compositional model. As a result of testing the various pressure conditions which may exist in the reservoir a pressure-dependent mixing parameter has been able to be defined. For the purposes of this study a specific geologic facie was simulated using small grid cells was in both a fully compositional and a pseudo-black oil simulator. The mixing parameter as a function of pressure that resulted from the 2D models was reduced by the theoretical limit derived from Fayers (1992) was sufficient to produce a match in the fine scale model. This method also resulted in a sufficient match in the coarse scale simulated grid that models the entire reservoir.