Carbonate formations are usually highly fractured. The fractures exist at all length scales ranging from microscopic fissures to kilometer-size structures. Consequently, it is vitally important to rigorously characterize the multi-scale fracture system and to accurately model the fluid flow in such highly fractured porous media. In this paper, we will present an integrated workflow for characterization of natural fracture system and accurate numerical modeling of fluid flow in fractured rocks. We have then applied the workflow in a field-scale reservoir simulation study, and the simulation results demonstrate significant advantages of the proposed workflow over the conventional dual-porosity/dual-permeability modeling approach.
The fracture characterization usually involves the integration of multiple data sources including core analysis, image logs and seismic interpretations etc., at different scales. The fracture mappings are thus required to perform at different length scales and resolutions. In this study, we present a systematic approach that characterizes the actual fracture distribution with seismic and log interpretations. Firstly, the small-scale fractures are interpreted using image logs and seismic anisotropy analysis with a co-Kriging and stochastic modeling approach. The small-scale fractures are initially constructed in an explicit fashion, and are then incorporated in the matrix medium using a flow-based upscaling procedure. Secondly, the large-scale fractures are mapped out from post-stack seismic data with ant-tracking and maximum curvature analysis. The reservoir is then gridded into unstructured tetrahedral or prism meshes to fully resolve these large fractures. This discrete fracture model (DFM) is employed in a reservoir simulation study based on such grid system.
We apply the procedure for simulation study and history match on a portion of a gas reservoir which is highly fractured in Longwangmiao formation in Southwest China. Currently, there are 7 horizontal and 2 vertical wells in operation in this block and some large-scale fractures are connected to edge water causing early water breakthrough. We partition the reservoir block into about 0.6 million cells, in which 153 large-scale and thousands of small-scale fractures are fully resolved. The simulation study is carried out and history match is successfully achieved using an extended Ensemble Karman Filter (EnKF) algorithm. Due to the fast edge water invasion, both gas and water rates exhibit sharp changes in a short time period. The proposed simulation procedure is able to efficiently capture such extreme flow dynamics and achieve good history match. The simulation results clearly demonstrate the major advantages in efficient multi-scale fracture characterization and numerical simulation in the proposed workflow.