XSite Background

XSite is a powerful three-dimensional hydraulic fracturing numerical simulation program based on the Lattice and Synthetic Rock Mass (SRM) methods.

The Lattice Method

  • Based on the Distinct Element Method (DEM), with particles and contacts replaced by nodes and springs, respectively
  • Nodes with masses are arranged quasi-randomly, connected by normal and shear springs, which can fail in brittle manner (i.e., micro-cracks)
  • Micro-cracks may coalesce to form macro fractures with a propagation criterion based on the fracture toughness
  • Spring elastic/strength parameters calibrated automatically from fracture toughness and unconfined compressive and tensile strengths
  • Pre-existing joints represented by the smooth-joint model that accurately predicts slip and opening/closing of joints
  • Thousands of pre-existing joints (DFN) can be included

Where joint (discontinuity) planes cut springs, the angle of the plane is respected (not the spring orientation). Thus, shear and normal compliances for the joint are used instead. In addition, slip and opening/closing of joint elements are modeled. Sliding on joint planes is independent of the local orientations of component springs.

  • Uses a central difference explicit solution scheme, which is well-suited for simulation of highly nonlinear behavior, such as fracture slip and the opening/closing of joints

  • The lattice method is efficient because it pre-calculates geometrical and interaction data and uses simplified equations of motion.

  • It has been developed for stiff, brittle rock in which (a) failure occurs at small strain and (b) failure is by tensile rupture (e.g., fracture of rock bridges, hydro-fracturing, and blast damage).

Synthetic Rock Mass (SRM)

  • A mechanics-based approach representing the dominant mechanisms of deformation and damage of fractured reservoirs
  • Explicitly defines a discrete fracture network (DFN) within a modeled rock matrix
  • Both the intact rock and the joints can be mechanically characterized by standard laboratory tests
  • Not necessary to rely on empirical relations to estimate the rock mass properties and to account for the size effect (i.e., from the tested sample size to the scale of interest in the model)