Visualising miscible and immiscible fluid displacement in fractured Coquina rock by means of High-Speed Neutron tomography

Mechanical deformation features like fractures add complexity to the carbonate rock pore system and can significantly control/alter the fluid movement within these reservoir rocks. Thus, the evaluation of the hydraulic behaviour of carbonate rocks becomes very challenging. In this study, we investigate the impact of lab-induced mechanical deformation on both miscible and immiscible fluid displacement within a 38 mm diameter Coquina-type limestone. High Speed Neutron Tomography (HSNT) was acquired at the Institut Laue Langevin (ILL) and Helmholtz-Zentrum Berlin (HZB) to capture in 3D the movement of fluids within the sample before and after deformation. Tomographies were
taken every 2 min and had a spatial resolution of 185 μm. A miscible fluid flow experiment (water (H2O) displacing heavy water (D2O)) was initially
conducted in the intact sample. Then the sample was dried and deformed under triaxial compression at 2.5 MPa confining pressure. An almost vertical fracture with an aperture smaller than 0.5 mm developed along the sample’s length. Miscible and immiscible fluid flow experiments were carried out afterwards on the deformed sample. During the miscible fluid flow experiment, H2O was injected at a constant rate of 0.1 ml/min into the previously D2O saturated sample. During the immiscible fluid flow experiment, D2O was injected at a constant rate of 0.05 ml/min into the previously Decane saturated sample.
Thanks to different sensitivity of neutrons to Hydrogen and its isotopes, good contrast was obtained between D2O and Hydrogen rich fluids (H2O/ Decane) in HSNT images. This enabled us to track the advancement of the displacing fluid during flow experiments. HSNT image analysis allowed for the 3D fluid speed distribution, the capture of the resulting flow patterns and their spatial correlation with the lab-induced fracture. Processes like fracture-matrix diffusion and near-fracture capillary-end effect were also resolved by HSNT images. Our results show that the flow pattern during miscible fluid displacement is controlled mainly by the textural variation within the intact sample. In the laboratory deformed sample, the miscible fluid displacement is dominated by advective transport within the fracture while spatially variable diffusion from fracture into matrix is also observed. During immiscible fluid displacement within the same deformed sample, the wetting phase (D2O)
is quickly imbibed by the matrix while the capillary discontinuity caused by the fracture results in the accumulation of the wetting phase at the matrix-fracture interface (due to capillary end effect) and a delay in movement of the displacing phase into the fracture.