Imaging Tracer Transport and In-Situ Fracture Aperture Changes Due to Shear Displacement with X-Ray Computerized Tomography

The hydromechanical coupling between flow and stress controls fracture and fault permeability. While the literature report on stress dependency of fracture aperture and permeability, fewer studies have considered shearing, with even fewer having the ability to image and map aperture changes. We designed and built a shear-displacement, X-ray transparent core-holder to study the transport of a tracer in fractured rocks that undergo shearing. The so-called Calibration-Free Missing Attenuation method [Huo et al., 2016] was applied to quantify and map the distribution of local fracture apertures with a spatial resolution of (0.25 x 0.25) mm². Simultaneously, pulse tracer injection experiments were carried out while acquiring X-ray CT scans to obtain time-series images of transport through the rough-walled fracture. The experiments were performed with Westerly granite and Carrara marble samples (5 cm x 10 cm) to emulate geothermal reservoir rocks at confining pressures of 1.5 MPa with displacement up to 5.75 mm. Solutions of deionized water mixed with KCl (saturating fluid) and KI (tracer fluid) were used due to their contrasting CT numbers.

We observe that shearing increases the fracture aperture with displacement, thereby increasing the fracture volume and reducing the surface contact area of the fracture walls that creates flow. The magnitude of aperture change is controlled by the initial roughness. To study the heterogeneous flow properties within the fracture, numerical simulations using the 2-D local-cubic-law approximation coupled with the advection-diffusion equation are performed using the fracture aperture maps. The numerical models are calibrated by comparing calculated effluent concentration with the measured concentrations from the pulse tracer injection tests. direct imaging of flow through the fracture and numerical simulations are qualitatively similar. Studying aperture changes associated with displacement aids interpretation of hydraulic property changes due to (induced-) seismicity, which can be applied to a range of systems due to the scalability of fractures.

Huo, D., Pini, R., & Benson, S. M., 2016. A calibration-free approach for measuring fracture aperture distributions using X-ray computed tomography. Geosphere, 12-2, 558-571.