INFLUENCES OF NETWORK MATURITY AND MEGABLOCK STRUCTURE ON FLUID FLOW AND SOLUTE TRANSPORT PROPERTIES OF VOLUMETRIC CONTRACTION FRACTURE NETWORKS

Volumetric contraction of an initially metastable solid creates well-connected fracture network systems with symmetric patterns of polygons. Fluid flow and solute transport properties of volumetric contraction networks have received far less attention than fracture networks formed by tectonic stresses. This study numerically generates volumetric contraction networks in a 250 m × 250 m domain with realistic geometries consistent with natural systems. A discrete fracture network solver is used to simulate fluid flow and track particle trajectories for 250 statistically equivalent network realizations for each parameterized network structure to systematically investigate influences of network maturity and higher transmissivity mega column joints on fluid flow and solute transport. Maturity of volumetric contraction networks refers to network formation beginning with initial cooling cracks and evolving towards well-connected, hexagonal fracture patterns with 120-degree fracture triple junctions. Mega column joints in these network types are a relic of the initial cooling cracks and serve as preferential flow features that lead to greater volumetric flow and faster breakthroughs. Networks with constant transmissivity are first used to ascertain the degree of dispersion introduced by geometric heterogeneity in network patterns, with subsequent simulations incorporating variable transmissivity. Increases in heterogeneity are reflected in ensemble particle breakthroughs and irregular symmetry in particle plumes. Plume snapshots are utilized for qualitative analysis of the ensemble particle transport behavior along with quantitative analysis of breakthrough curves and particle statistics. Particles in networks with mega column joints tend to exhibit dual peaks characteristic of timescales of rapid transport through mega columns and slow transport in smaller fractures. The dual-peak breakthroughs are replaced by a single peak and heavier late time tails when heterogeneity in transmissivity is assigned to the smaller fractures.