Heterogeneity Modeling and Heterogeneity-Based Upscaling for Reservoir Characterization and Simulation

Abstract

Summary Reservoir heterogeneity is a key factor in modeling reservoir performance. Heterogeneity measures can be calculated for a given permeability field but are not sufficiently straightforward to reverse the process. Detailed heterogeneity can be built into a fine-scale model but can be lost during upscaling to a coarse scale, no matter which method is chosen from simple averaging to flow-based (FB). In this paper, we propose a method of heterogeneity modeling and heterogeneity-based upscaling with the aim of solving these problems. Unlike the traditional geostatistical method used to generate a permeability field that is not directly linked to a desired heterogeneity coefficient, the proposed method creates a heterogeneous permeability field directly using a Lorenz coefficient. Using a given Lorenz coefficient as an input, the expected heterogeneous permeability field can be generated via the proposed steps and equations. Using the proposed heterogeneity-based upscaling method, the Lorenz coefficient and curve, defined from the fine-scale model, can be preserved with minimum heterogeneity loss after upscaling such that gas/oil ratio (GOR) can be matched between the fine- and coarse-scale models without using pseudofunctions. The proposed method has been applied successfully in modeling a giant carbonate oil field in the Caspian Sea consisting of a matrix-dominated platform and a fracture/karst-dominated rim. Due to the field’s complex geology and high H2S content, a dual-porosity/dual-permeability compositional model has been created to model compositional flow within/between matrix and fracture/karst initialized with an abnormally high reservoir pressure. The field surveillance data show that reservoir heterogeneity is a key component for the field reservoir characterization and simulation. The Lorenz coefficient and curves can be estimated from the cores and logs, but the challenge is how to preserve the characteristics of the Lorenz coefficient and curves during modeling, upscaling, history matching, and uncertainty analysis (UA). Application of the new method has demonstrated its ability to overcome this challenge and has significantly improved the quality of the field’s reservoir modeling, upscaling, history matching, and UA. The fine-scale model Lorenz coefficient and curves were directly applied to calculate the coarse-scale permeability. The range of the Lorenz coefficient and curves estimated from cores and logs were used to generate a range of heterogeneous permeability fields for UA. Regional Lorenz coefficients were adjusted based on the mismatches of GOR, and new heterogeneous permeability fields were generated to improve the history-matching quality.