Controls of natural fractures on the texture of hydraulic fractures in rock

摘要

Hydraulic fracturing plays an important role in the exploitation of oil, shale gas and coal seam gas resources-all of which contain natural fractures. We systematically explore the role of the pre-existing texture of such natural fractures on the form of the resulting stimulated reservoir volume (SRV). A blocky discrete element model (DEM) coupled with fluid flow is used to explore this response. Numerical predictions for the evolution of fluid pressure and fracture width at the well are compared with first-order analytical approximations of the zero-toughness solution (FMO). We then construct four typical joint system models separately comprising orthogonal, staggered, diagonal and randomly oriented joints and conduct the virtual hydraulic fracturing simulations via DEM. This defines the influence of structure on breakdown pressure and fracture propagation and allows the analysis of the main factors that influence behavior and resulting SRV. Results for the four forms of jointed rock mass show that: (1) the aggregate/mean extension direction of the fractures is always along the direction of the maximum principal stress but significant deviations may result from the pre-existing fractures; (2) there are negative correlations between the maximum fracture aperture with both Poisson ratio and elastic modulus, but the breakdown pressure is only weakly correlated with Poisson ratio and elastic modulus; (3) an increase in injection rate results in a broader fracture process zone extending orthogonal to the principal fracture, and the breakdown pressure and interior fracture aperture also increase; (4) an increase in fluid viscosity makes the fractures more difficult to extend, and the breakdown pressure and interior fracture aperture both increase accordingly.