Practical Flow Simulation Method for A Naturally Fractured Reservoir: A Field Study

Abstract

This paper presents the methodology, implementation and results of dynamic modeling of a naturally fractured carbonate reservoir, which consists of fracture swarm system and high matrix permeability. The focus of this paper is on the description of the dynamic single media model that was used to history match the historical production. The challenges in properly quantifying the separate effects of matrix and fracture within the framework of single media model become the major objective of the study. Brief description of the static model that consists of development of matrix and fracture models, as well as the method to integrate them, are also presented. Both matrix and fracture systems play an important role in the production mechanism of the reservoir. History matching for twenty years of production was successfully done in a single media model through an iterative process between static and dynamic models to ensure the consistency between the two models. Different sets of relative permeability curves (for matrix and fracture systems) were generated to properly simulate the fluid movement in the reservoir. The results indicate an excellent history match that is also followed by the success in the blind tests of newly drilled wells. Significant improvement was obtained compared to the previous model in simulating water production that comes from the aquifer to the wells through a complex fracture network. The results were achieved through various sensitivity analyses of fracture conductivity and by fine-tuning of relative permeability curves that are based on the eight rock types defined for the reservoir. In conclusion, the single media model used in this study can successfully simulate the behavior of a naturally fractured reservoir. The model has shown that it can produce excellent results in a very practical way. The methodology described in this paper is suitable for other naturally fractured reservoirs, especially in the Middle East Area where significant difference between core-derived permeability and the well-test derived permeability exists. Introduction This paper presents the methodology, implementation, and results of dynamic modeling of a naturally fractured carbonate reservoir of an oil field in the Middle East. The study is part of the Reservoir Management and Long Term Development Plan (LTDP) of the field. The overall objective of the study is to obtain a representative model that can be used to improve the reliability of the predictions of the future reservoir performance. The specific objective is to match the 20 years production history and to predict field future performance using three reservoir simulation models that represent the Low (pessimistic), Medium (most-likely) and High (optimistic) cases. The dynamic model (or flow simulation model) is based on a static model that was generated through a detailed integrated reservoir characterization study.1,2,3,4,5,6 The study included the integration of various data/information from geology, geophysics and engineering disciplines using a stochastic approach. The static model was developed in a fine-scale grid system of 4.2 million cells that has been upscaled into a coarse-scale grid system of 181,000 cells. The upscaling was performed through a grid optimization process utilizing the streamline simulation in order to maintain the high level of heterogeneity of the reservoir as much as possible.5 The main characteristic of this reservoir is the presence of a complex fracture network within the highly heterogeneous matrix system. Additionally, the reservoir is characterized by the presence of Sucrosic Dolomite rock (mainly in the South part of the middle reservoir unit) that significantly enhances matrix permeability further. Matrix permeability ranges from 0.01 mD to 12 Darcy with the average in excess of 100 mD. The fractures are described as fracture corridor system (or fracture swarms) that show two major trends, namely N40E and N70E. Fracture swarms/sub-seismic faults are structural features extending laterally over several hundreds meters and sometimes over several kilometers. This network was assumed to be fully extended in the vertical direction. It is assumed that there are very few fractures outside the fracture swarms.