Study Under X-Ray Tomography of the Impact of Self-Sealing Process on the Permeability of the Callovo-Oxfordian Claystone.

To analyze the self-sealing process in the Callovo-Oxfordian claystone, self-sealing tests were performed on initially fractured samples under different temperatures with water and gas injection. Cylindrical samples oriented in parallel and perpendicularly to the bedding plane with an artificial initial fracture were used in a triaxial compression cell transparent to X-rays. Water and gas permeability were measured and the evolution of cracks volume was analyzed from X-ray tomography 3D images to characterize the self-sealing process. All tests performed at 20 °C with water injection showed a rapid drop in permeability at the beginning followed by a progressive decrease and a stabilization after one month. The permeability of fractured samples decreases significantly after self-sealing but is still higher (by 2 orders of magnitude) than the permeability of healthy claystone. Otherwise, the less calcite the sample contains (i.e., the more clayey it is), the faster the crack self-seals. The smaller the opening of the initial crack is, the faster the water permeability decreases and the crack closes. No significant influence of the sample orientation on the self-sealing kinetic was identified at this stage. It seems that high temperature has a slight retarding effect on the self-sealing process. For the test with water and gas injection, the injection of gas delays the decrease of water permeability and the self-sealing process, which is probably due to the crack desaturation induced by gas injection. Highlights: Self-sealing tests were performed on initially fractured samples of Callovo-Oxfordian claystone with different temperatures, orientations and fluids (water, gas). Self-sealing process was analyzed thanks to permeability measurements and crack volume estimation from X-ray tomography 3D images. The self-sealing process is fast at the beginning of the test and stabilizes after about one month and the initial permeability of the healthy rock is partially restored. The less calcite the sample contains (the more clayey it is) and the smaller the opening of the initial crack is, the faster the crack self-seals. Temperature and gas seem to have a delay effect on the self-sealing process. There is no significant influence of the sample orientation. [ABSTRACT FROM AUTHOR]