Measuring fracture aperture and water saturation distributions using computed tomography and its application to modeling geomechanical impact on fluid flow in fractures [electronic resource]
Keywords: Energy Resources Engineering.

Understanding multiphase fluid flow behavior in fractured porous media is crucial for developing fractured hydrocarbon reservoirs, implementing hydraulic fracturing, and predicting potential leakage in gas and CO2 storage. Since fracture aperture distributions are stress-dependent, the flow behaviors are also stress-dependent. The goal of this research is to investigate stress-dependent fluid flow properties in rock fractures. To carry out this work, fast and accurate methods to measure the stress-dependent fracture aperture and water saturation during the core-flooding experiments are needed. X-ray CT imaging is attractive for measuring fracture apertures and water saturations because it can be combined with dynamic flow experiments. In this dissertation we develop a set of methods for measuring fracture aperture and water saturation distributions and quantify the errors associated with these methods. Furthermore, we investigate the stress-dependency of fluid flow in fractures using the methods. In the first part of this dissertation, we develop a full set of methods for calculating fracture aperture and water saturation distributions using X-ray CT scanning. For fracture aperture measurements, the method relates fracture aperture with missing CT attenuation, scanner voxel size, and CT numbers of the rock matrix. The validity of the model is established by comparing apertures calculated with the conventional calibration-based method, evaluating model predictability at different scanner voxel sizes, comparing with calibration coefficients in the literature from a number of experiments with different rocks and X-ray scanners and comparing aperture measurements for air-filled and water-filled fractures. The results suggest that the method provides reliable aperture measurements. The method also avoids the need for time-consuming calibration and accounts for rock property heterogeneities. The missing CT attenuation concept is then applied to multiphase scans (scans where the fracture is partially filled with water and gas) to develop a method for measuring the water saturation in rock fractures surrounded by porous rocks. We analyze the new method by comparing water saturation measurements with two previously developed methods. The new method includes the missing CT attenuation changes for different matrix porosities and water saturations and thus it is more broadly applicable to a wide range of conditions. Finally, we quantify the systematic and random errors related to the fracture aperture and water saturation measurements. Error analysis shows the method provides an aperture measurement error of 22 microns with 5 repeated scans, which is less than one-twentieth of the voxel size. The analysis demonstrates that averaging of replicate scans highly improves the detection accuracy. Comparisons between aperture measurements made in air and water-filled fractures show that the dry scan is the most recommended method due to its lower errors. Water saturation measurements for individual pixel exhibit a high degree of error (40%). However, the average water saturation in the fracture can be measured with an accuracy of around 10% by averaging 5 repeated scans. The fracture aperture and water saturation measurement methods are particularly valuable for combining dynamic core flooding experiments with simultaneous fracture aperture and water saturation measurements. Using concurrent CT scanning and core flooding, we improve understanding of stress-dependent fluid flow in fractures. The stress-dependent permeability, capillary pressure and relative permeability in rock fractures are separately explored. From the stress-dependent permeability data, changes in permeability are attributed to three factors: changes in mean aperture, changes in roughness, and changes in contact area. We find that stress-dependent permeability and hysteretic behavior is influenced by both aperture and roughness changes. For a small aperture fracture tested here, changes in roughness dominate the permeability response to stress changes. The Modified Cubic Law (Witherspoon et al., 1980), Walsh (1981), Zimmerman et al. (1992) and Sisavath et al. (2003) models are compared with the experimental data and results show that all four models do not result in sufficiently large permeability variation. Additionally none of the previous models quantifies the relative contribution of aperture and roughness to permeability change. A new empirical model is proposed based on the Modified Cubic Law (Witherspoon et al., 1980) that provides a better match to the experimental data and accounts for both stress-dependent aperture and roughness. Using laboratory measurements of fracture aperture distributions under various conditions of effective stress, invasion percolation simulations are employed to model the capillary pressure curves as a function of the effective stress. The stress-dependent aperture distribution data demonstrates that increasing stress has two effects: (1) the mean aperture will decrease; (2) the variance of aperture distribution will increase. The mean aperture decrease will increase entry pressure. As the variance of aperture distribution increases with stress, the plateau area of the capillary pressure curve tends to grow steeper, indicating that capillary behavior changes from more fracture-like to more porous media-like. For the stress-dependency of relative permeability in rock fractures, previous studies provide contradictory evidence of the influence of increasing stress on the relative permeability of fractures. Some studies suggest that irreducible water saturation increases, while others show the reverse. In an attempt to resolve these differences, laboratory core flooding experiments are applied to measure the relative permeability of nitrogen-water mixtures in a fracture under various states of effective stress. Simultaneous X-ray CT measurements are made of aperture and water saturation distributions in the fracture. Two effective stress levels, 2.07 MPa and 5.52 MPa, are applied to investigate the stress-dependency. For both states of stress, the measurements show that the relative permeability to gas is very low until a critical saturation is reached. As gas saturation increases beyond the critical value, relative permeability to gas increases quickly while water becomes essentially immobile. Results also demonstrate that increasing stress lowers the irreducible water saturation and the end-point non-wetting phase relative permeability when the experiments are conducted at the same flow rate. Using invasion percolation theory with the fracture aperture maps made at the two different effective stresses, capillary pressure curves are calculated and used to explain changes in phase interference at different stress levels. Finally, the preferential flow paths are analyzed at both stress levels. We use this analysis to reconcile flow regimes observed in earlier studies, and conclude that the differences between them can be explained by the relative importance of viscous and capillary forces. Specifically, if the experiments are designed to keep the capillary number constant, the irreducible water saturation increases with increasing confining stress. If the experiments are conducted at the same flow rate, higher confining stress decreases the irreducible water saturation, as was observed in these experiments. The analysis and data presented here also suggest that small increases in the water saturation of a fracture may dramatically reduce gas flow rates. This may present an additional and unexplored explanation for rapid production decline of gas wells in fractured reservoirs.