Self-sealing capacity of argillaceous rocks: Review of laboratory results obtained from the SELFRAC project

Abstract

This paper describes laboratory tests that have been carried out, in the framework of the EC-funded SELFRAC project, on a soft (Boom Clay) and on an indurated clay (Opalinus Clay) to determine their respective sealing properties. In order to distinguish the respective influence of the mechanical and chemical components of the sealing process, two types of tests have been designed. The first test type consisted in testing the influence of the injected fluid chemistry on the sealing capacity of the clay. An artificial planar fracture has been produced through the axis of a cylindrical sample prior to its installation in a permeameter cell. The evolution of the samples hydraulic conductivity has been monitored. We used micro focus X-ray computer tomography (μCT) as an imaging technique to visualise the sealing process of the fracture. We observed a total recovery of the hydraulic properties for Boom Clay, and only a limited recovery of these properties for the Opalinus Clay. The mechanical properties of Boom Clay could however not be totally recovered. The second test type consisted in testing combined mechanical and chemical sealing. The principle was to drill a hole through the axis of a cylindrical clay sample and to submit it to an increasing confining pressure, while the evolution of the clay hydraulic conductivity was monitored by applying an axial hydraulic gradient. Mean pore water pressure and mean effective stress were kept balanced throughout the test. For Boom Clay, no influence of the hole dimension on the progressive recovery of the hydraulic properties in function of the applied confining pressure could be detected. The final hydraulic conductivity measurement showed a total recovery of the hydraulic properties. However, the decompression of the sample during the dismantling operation provoked the appearance of a central fissure throughout the sample in a direction parallel to the bedding plane orientation. The μCT tomography imaging technique detected the presence of a central zone of lower density. For Opalinus Clay, the hole dimension tested has a clear influence on the sealing process of the clay. The two samples tested showed a distinct behaviour. While no sealing could be established in the case of the sample with the largest hole diameter, a slow sealing process could be observed in the sample with the smallest initial hole diameter. Nevertheless, it must be noted that the undisturbed permeability conditions of Opalinus Clay could never be restored at the timescale of the laboratory experiment.