Self-sealing of fractures in indurated claystones measured by water and gas flow

Abstract

Self-sealing of fractures in the indurated Callovo-Oxfordian (COX) and Opalinus (OPA) claystones, which are considered as host rocks for disposal of radioactive waste, was investigated on artificially fractured samples. The samples were extracted from four lithological facies relatively rich in clay mineral, carbonate and quartz, respectively. The self-sealing of fractures was measured by fracture closure, water permeability variation, gas penetration, and recovery of gas-induced pathways. Most of the fractured samples exhibited a dramatic reduction in water permeability to low levels that is close to that of intact rock, depending on their mineralogical composition, fracture intensity, confining stress, and load duration. The self-sealing capacity of the clay-rich samples is higher than that of the carbonate-rich and sandy ones. Significant effects of sample size and fracture intensity were identified. The sealed fractures become gas-tight for certain injection pressures. However, the measured gas breakthrough pressures are still lower than the confining stresses. The gas-induced pathways can recover when contacting water. These important findings imply that fractures in such indurated claystones can effectively recover to hinder water transport but allow gas release under relatively low pressures without compromising the rock integrity.