MEASURING CONTACT ANGLE DYNAMICS ON ROCK FRACTURE FACES

Determining contact angles of fluids on rock fracture surfaces is important for modeling oil and gas extraction from fractured unconventional reservoirs as well as leakage of stored fluids from deep well repositories. Contact angles of wetting fluids are typically measured on polished flat surfaces. Unfortunately, these surfaces are not representative of natural rock fracture faces, which often display varying scales of roughness. Here we apply a modified Wilhelmy plate method for determining the effective equilibrium contact angle from the height of capillary rise of water in the presence of air on a vertical fracture surface. Cylindrical core samples (5.08 cm long x 2.54 cm diameter) of 6 low porosity rock types (4 granites, a sandstone, and a shale) were investigated. Mode I fractures were created using the Brazilian method. The cores then separated into 2 semi-cylindrical halves, each with an exposed fracture face. Data on menisci dynamics were collected using neutron radiography at ORNL’s Neutron Imaging Facility (CG-1D, HFIR). Fracture faces of each rock type were oriented parallel to the neutron beam. Images from the neutron radiography were acquired every 0.03 s after a water reservoir contacted the base of the fracture face. The pixel size was 100 µm. The images were normalized to the initial dry condition in ImageJ so that the vertical movement of the meniscus was clearly visible. The change in height of the meniscus was determined through time in each normalized neutron radiograph through mean-variance Change Point analysis in R. The resulting height measurements of the meniscus were then fitted using a non-linear regression model to determine the equilibrium height. This value was inserted into the standard expression for capillary rise at a vertical surface in order to determine an effective equilibrium contact angle for each fracture face.