Carbonate reservoirs mainly consist of carbonate rocks. Carbonate rocks are part of sedimentary rocks family. The composition of carbonate rocks is CaCO3. Generally, the sedimentation of carbonate rocks is marine or sea water and the rocks are built by shells. Carbonate reservoirs are relatively fragile in which many fractures are created during subsidence or lifting and produce fractured carbonate reservoir. Many oil and gas reservoir are structured by fractured carbonate reservoir. Some major oil reservoir in the world is carbonate rocks.
Natural fracture properties are complex. The fracture length, width, aperture, and orientation are mostly inaccurately known. The fractures are irregularly connected. Usually, fracture properties can be measured by production logging tool, image log, core analysis, and well testing. During well testing, if fractures exist, a large production rate is observed at early time and dropped sharply until a stable rate. Such well flow test is usually done by a well testing companies.
During field development, new data are continuously evaluated analytically and/or numerically. Large fracture can be evaluated analytically while small fracture is best evaluated numerically. There are three numerical models to assess fractured formation:
1. Discrete fracture model.
2. Continuum model.
3. Hybrid model that combines discrete fracture model and continuum model.
Simulators to simulate fluid flow, which is available in the market such as eclipse reservoir simulator, mostly use homogeny fracture properties. In this patent, the simulator uses heterogenic fractures with various lengths, width, aperture, and orientation (Figure 1). It uses a hybrid model. The fluid flow equations of matrix blocks and fractured blocks are obtained separately. Both equations are coupled by the transport index (Figure 2). The numerical calculation is solved continuously through a fracture network generated by the stochastic process. Either the black oil model or compositional model is applicable. Finite difference and boundary element methods are used throughout the numerical processes.
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