Aperture Modelling for the Flow-Based Determination of Fracture-Matrix Ensemble Saturation Functions for Naturally Fractured Reservoirs

Abstract

DFN (fracture-only) and DFM (fracture and rock matrix) modelling is a rapidly growing field. While more and more geometrically realistic models get published, fracture aperture is often treated as single-valued or as set-by-set constant parameter. However, this is incompatible with field observations indicating variable apertures, lognormal or multimodal aperture distributions, and-or partial sealed fractures in naturally fractured hydrocarbon reservoirs. This presentation explores how realistic aperture variations across multiple sets of intersecting fractures can be modelled taking into account geometry (orientation, length versus frequency distributions, abutting relationships), mechanical rock properties, in situ stress, and pore pressure. New algorithms are used to account for fracture dilatation, asperity gliding, asperity crushing, and dissolution-precipitation. They are used in concert to produce physically realistic aperture models. These techniques are already part of a fracture modeling and upscaling workflow that has been applied in the field, and flow simulation highlights the first-order control that the ensuing variable apertures exert on permeability, anisotropy and flow localisation. The key remaining challenge, however, is the modeling of mechanical interactions between fractures and rock fragments. As an important aspect of this, here we address whether far-field-stress-based fracture aperture computations are applicable to rock fragmented by multiple fracture sets.