A Combined Geological, Geophysical and Rock Mechanics Approach to Naturally Fractured Reservoir Characterization and Its Applications

In this paper, we present a combined geological, geophysical and rock mechanics approach to natural fractured reservoir characterization. The local structure entropy analysis on 3D seismic data is used to detect distributions of fault and subfault systems. The curvature attribute along with modeled strain and stress field, constrained with the log data measuring Pand S-wave velocities and rock density and the inverted elastic modulus from pre-stack seismic data, reveal effects of the geological structure, bed thickness and lithology on fracturability of the reservoir layer. These analyses quantify the relationships between the geologic factors and rock fracturability and describe physically the weighting factors for geologic parameters in controlling the rock fracturing. The comparison of the seismic azimuthal analysis results to these of geological and rock mechanics modeling provides an opportunity to verify whether the seismic anisotropy derived from seismic data is caused by structure related natural fracture patterns or by other mechanisms. The consistency among different techniques provides the confidence in the interpretation of the distribution of fractures induced by structures. If azimuthal seismic attribute data can be combined, the application of this procedure results in the development of the fracture connectivity anisotropy by considering relationships between the present and palaeostress fields. In addition, the scale depend analysis technique in this approach can improve the ability to identify the distribution of fractures with multiple length scales. In this paper, case studies are used to illustrate applications of these technologies and their efficiency. Introduction Fractures are a crucial factor controlling the well performance in reservoirs with low permeability. Fractured reservoirs are highly heterogeneous. A practical approach in modeling of naturally fractured reservoir is from geology to flow simulation, which consists of two parts, characterization of fracture geometry and fracture network dynamic behavior. The time-independent static data is mainly used for characterizing fracture distribution. Dependent on scale at which fracture network is observed, these data include wireline logs, conventional cores, sub-seismic investigation (outcrop and structure data) and seismic data. The spatial distributions of fractures and their geometry at the field scale derived from static modeling plays important role in the reservoir modeling. Studies show that structure, lithology, bed thickness, porosity, and other geologic factors control the fracture intensity. The complex interaction between all geologic factors needs to be considered during the static modeling in order to make these models to be expected input to the reservoir models. The rock properties of a reservoir layer depend primarily on the depositional process. Tectonic events act on the reservoir layer can lead to the spatial heterogeneous structures which resulted from the heterogeneously distributed mechanical properties. The fracturability of a reservoir layer is not only influenced by mechanical properties, which are controlled by the lithology of reservoir rocks such as shale content, matrix porosity, and carbonate contents etc, but also influenced by the tectonic events that the reservoir rocks have undergone. To reduce the uncertainties in fractured reservoir modeling, integrating all available data from geology, geophysics, petrophysics and engineering is necessary. Since the geological heterogeneity significantly controls fracture distributions, the statistic assumptions between two or more wells are hard to be made. Borehole data and analysis of core data provide us the direct observation of fractures. The fracture detecting can benefit from the 3D seismic data and observed geological data. Seismic data can be used to get better geologic models and make geo-mechanical modeling physically efficient and geologically meaningful. The discontinuity analysis or a coherence-type attribute is often used to interpret the faults and fractures at the seismic scale. The local discontinuity related seismic attribute is computed from the seismic data set comparing each trace with surrounding traces. An analysis cube can be selected by the interpreter, according to the type of geological feature that is of interested. The curvature analysis at the top of the reservoir can be performed and a robust curvature map can be extracted from SPE 90275 A Combined Geological, Geophysical and Rock Mechanics Approach to Naturally Fractured Reservoir Characterization and Its Applications Feng Shen and Shuiquan Li, SPE, EP Tech