Influence of Mineral Veins on Hydraulic Fracture Propagation

Fluids are often injected into tight geothermal reservoirs to create conductive fractures and enhance reservoir productivity. Mineral veins are ubiquitous in reservoir rocks because of the hydrothermal mineralisation process. We hypothesize that vein strength anisotropy and geometrical complexity could introduce complex hydraulic fractures through vein-fracture interactions. To test this hypothesis, we develop an improved hydro-mechanical coupled scheme using the Discrete Element Method to examine hydraulic fracture propagation in veined rocks. The coupled scheme considers incremental updates of domain volume and fluid saturation. We follow topology in root architecture to develop a benchmark model with simplified yet representative vein networks. We find that vein strength significantly influences the fracture pattern of the tested veined models during hydraulic fracturing. Cracks are preferentially reactivated in the calcite (soft) veins, leading to much more complex fracture networks compared with those created in quartz (hard) veins. An increase in stress anisotropy (high-stress contrast) leads to a reduction of the maximum aperture of dominant fractures. Under a high-stress contrast, fluid lag is not evident; the number of reactivated cracks in soft-veined models is also reduced. It is interesting to note that the bifurcation angle of veins has negligible influence on fracture patterns and propagation in the model. We also find that high-stress contrast leads to a higher level of the maximum moment magnitude and a lower b value. This research could provide some insights into reservoir stimulation to enhance production and mitigate seismic hazards.