New insights on fracture roughness and wall mismatch in carbonate reservoir rocks

Abstract

Roughness and mismatch of fracture surfaces (walls) are key hydromechanical rock properties that influence (or control) how the rock masses slide and the ways in which fluids permeate the structure. Fracture roughness has been studied for decades from outcrop fractures and experimental analogues because it is difficult to detect and describe roughness of reservoir rock fractures in the deep subsurface. Here, we present new insights about roughness and wall mismatch of opening-mode fractures (hereinafter fractures) in deep carbonate reservoir rocks using a combination of new measuring techniques. The fractures are described from core and modern electrical borehole imaging data that were obtained from extended-reach drilling into the Arab oil reservoir rocks in Eastern Arabia. The fractures, both unfilled and calcite-filled fractures, exhibit moderate and high roughness, ranging between 5 and 15 on the joint roughness coefficient scale. Most fractures exhibit matching walls, permitting use of the cubic law for fluid-flow modeling. Some unfilled fractures exhibit mismatching walls that preserve their apertures under compression of the current stress field in Eastern Arabia. Roughness and mismatch of fracture walls were found to reflect the fabrics of host carbonates and hence are potentially predictable from analyses of carbonate lithological facies (lithofacies).