EOR Evaluation Method for Highly Heterogeneous and Complex Reservoirs

Abstract

Giant reservoirs in North of Monagas in Venezuela have been submitted to Waterflooding (since 1993) and Gas injection (since 1998) to support the oil production. currently the oil production is around 840 MBNPD. Those highly heterogeneous reservoirs are deep, with high pressure and temperature and presenting compositional fluid variation with depth. In order to maximize the reserves of these reservoirs the feasibility of applying EOR processes have been analyzed. Several existing methodologies and commercial software packages designed for the evaluation of EOR processes do not offer the adequate support for the decision making in highly heterogeneous and complex reservoirs. An integrated methodology has been proposed to evaluate and select the possible EOR processes in the North of Monagas. The methodology is based on a combination of economical criteria, recovery improvement and risk analysis. The methodology can be structured in three stages. In stage I, multiple analytical methodologies for the pre selection of all applicable processes are integrated. During stage II, the uncertainty associated with each pre selected process is assessed through the areal, vertical and displacement sweep efficiencies evaluation using 1D and 2D numerical simulation models. In Stage III, each process is evaluated at 3D full-field scale and a risk analysis is performed as a function of stage II results. The proposed methodology states a structured way for the evaluation and selection of EOR processes in complex and a heterogeneous reservoir, which integrates conventional analyses, numerical simulation and risk analysis as decision tools. This paper enhances existing methodologies for the evaluation of EOR processes. Introduction North of Monagas fields are one of the largest hydrocarbon accumulations in Venezuela. They consist in a complex structural system, which is characterized by anticlines in sequence with compressive elements such as low angle reverse faults (thrusts) and backthrusts with fault-bend folds (See Figure 1). The stratigraphic column of the producer formations has around 3000 ft of sediment with three important sequences going from superior Cretaceous to middle Miocene having inside a conjugation of transitional and continental marine environments. This generates a high level of heterogeneity vertically and horizontally (See Figures 2 - 3). In addition, those fields have high current pressures (6000 - 8000 psi) and temperatures (280ºF - 310ºF), all this with depths from 12000 to and 18000 ft. Also, compositional variation with depth is observed, finding condensated gas in the structural crest and medium oil down the flank (See Figure 4). A significative asphalthene precipitation is presented in these reservoirs, because of this water injection (1993) and gas injection (1998) projects have been implemented to guarantee pressure maintenance and higher oil recovery. Currently, the implanted exploitation plan has permitted to recover 20% of OOIP and the plan is to reach an average recovery factor of 50%. In order to increase even more the oil recovery factor some studies were started to evaluate new EOR process. Due to the complexity of those fields, several limitations with the application of traditional methodologies were found. Taking this into account, a new methodology was proposed to allow us to introduce more reservoir details in the selection of EOR process. The proposed methodology consists in a work flow that allows visualizing, evaluating and selecting new EOR processes with a special application in complex and heterogeneous reservoirs. This integrates conventional analysis, numerical simulation and risk evaluation. The evaluation process is structured in three stages (See Figure 5). The first one has as objective the EOR processes pre-selection, according to the field characteristics; in this stage is recommended the integration of analytical methodologies previously published. The second stage allows the identification, evaluation and ranking of uncertainties associated to each pre-selection process. The third stage set up the full-field evaluation of each process altogether with risk analysis. The principal result of the proposed methodology is to select the best process through the combination of economical criteria (VPN), recovery improvement (Fr) and risk analysis (s).