FLUID FLOW AND SOLUTE TRANSPORT PROPERTIES OF WELL-CONNECTED FRACTURE NETWORKS IN COLUMNAR BASALT

Fluid flow and solute transport properties for fracture networks formed by tectonic stress have been extensively studied, yet networks formed by thermal cooling and volumetric contraction have received far less attention, particularly for columnar patterns of joints. Columnar jointing creates symmetric patterns of polygons bounded by a well-connected fracture network system. Individual fractures migrate as the network matures trending towards hexagons with triple junctions comprised of three fractures separated by 120 degrees. This pattern is present in mud cracks, permafrost, rocks, and on other planets. This study involves the numerical generation of mature columnar fracture networks with realistic geometries consistent with outcrop observations. A discrete fracture network solver is used to simulate fluid flow and track particle trajectories for 250 columnar basalt network realizations generated in a 250 m × 250 m domain. Five different fracture transmissivity distributions are applied to the networks: constant, lognormal with log variances of 0.50, 0.75, and 1.0, and two approaches using outcrop training images to identify and assign higher transmissivity to mega column joints. Mega column joints are a relic of the initial cooling cracks and have a tend to cause preferential flow within columnar basalt net. Fluid flow solutions reflects the strong inter-connectedness of the network where flow is more concentrated in higher transmissivity fractures aligned with the hydraulic gradient. Solute transport properties of the columnar networks are assessed using particle breakthroughs at the downgradient boundary and snapshots of plume evolution in space. Transport in networks with constant transmissivity are primarily influenced by geometric heterogeneity of the system, whereas the other simulations include heterogeneity in both the network geometry and transmissivity. Increases in heterogeneity are reflected in broader distributions of ensemble particle breakthrough, irregular symmetry in particle plumes, and more pronounced early and late time particle arrivals.