Geological Characterization Of Naturally Fractured Reservoirs Using Multiple Point Geostatistics

Abstract

Abstract The spatial distribution of fractures in a reservoir affects the displacement of fluids and the prediction of future performance. Realistic characterization of fractured reservoirs requires quantification and classification of fracture patterns on the basis of the underlying geological characteristics and developing reservoir modeling algorithms that can integrate connectivity based (multiple point) statistics related to fracture patterns. A methodology for summarizing the characteristics of fracture networks based on multiple point connectivity functions is presented. The paper also presents a stochastic simulation methodology for constraining the target reservoir model to the connectivity characteristics derived from analog models and to all other available reservoir specific data in the form of well information and seismic areal proportion maps. Introduction A natural fracture is a planar discontinuity in reservoir rock due to deformation or physical diagenesis1. Fractures may have either a positive or negative effect on fluid flow depending on whether they are open or sealed due to mineralization. For the purposes of this paper, a fractured reservoir is defined as a reservoir in which naturally occurring fractures are predicted to have a significant effect on fluid flow either in the form of increased permeability and/or porosity or increased permeability anisotropy. Natural fracture patterns are frequently interpreted on the basis of laboratory-derived fracture patterns corresponding to models of paleo-stress fields and strain distribution in the reservoir at the time of fracture2. Stearns and Friedman3 proposed a genetic classification of fracture systems based on stress/strain conditions in laboratory samples and features observed in outcrops and sub-surface settings. Based on their work, fractures are generically classified into: Shear Fractures - exhibit a sense of displacement parallel to the fracture plane. Shear fractures form when the stresses in the three principal directions are all compressive. They form at an acute angle to the maximum principal stress direction and at an obtuse angle to the minimum compressive stress direction. Extension Fractures - exhibit a sense of displacement perpendicular to and away from the fracture plane. They form perpendicular to the minimum stress direction. They too result when the stresses in the three principal directions are compressive and can occur in conjunction with shear fractures. Tension Fractures - Exhibit a sense of displacement perpendicular to and away from the fracture plane. However, in order to form a tension fracture, at least one of the principal stresses has to be tensile. Since rocks exhibit significantly reduced strength in tension tests, the frequency of fractures under tensile stress conditions is more.