XSite Features

XSite is a special-purpose software package designed to meet the needs of hydraulic fracturing simulation. Main features available with XSite include:

  • No assumptions regarding the fracture shape or trajectory
  • Fracture propagation in inhomogeneous and naturally fractured rock masses
  • Interactions between hydraulic fractures, joints, and between stages and wells (including the stress shadow effect and hydraulic connectivity) are accounted for
  • In situ stresses may be specified
  • Arbitrary number of wellbores and injection points
  • Synthetic microseismicity is generated as the fractures slip or propagate, providing a method for model calibration with field observations
  • Non-steady fluid flow within joints and the intact rock
  • Proppant transport and placement
  • Thermal analysis
    • Heat conduction (and thermo-mechanical effects) in the rock and heat exchange with the fluid in fractures can be simulated
    • Approximated heat advection by injected fluid

The model simulates open-hole, sliding-sleeve hydraulic stimulation along the horizontal section of the borehole. Five stages are included in the model. The Sh,min in 50-m high reservoir is 30 MPa. The verical stress and SH,max are 42 MPa. The stress barrier between the reservoir and underburden and overburden is approximately 2 MPa. The horizontal segment of the borehole is oriented in the direction of Sh,min.  For the sake of simplicity, the DFN was not explicitly represented in this model. For each stage, slick water was injected for 33 minutes at a rate of 11m3/min.

Hydro-mechanical Coupling and Flow

  • Fluid flow and mechanical simulations can be done separately or coupled
  • The fluid flow is approximated by a flow through a network of pipes that connect fluid elements, located at the centers of either broken springs or springs intersected by pre-existing joints
  • The flow pipe network is dynamic and automatically updated by connecting newly formed micro-cracks to the existing flow network
  • Non-steady fluid flow is modeled within the joint network and intact rock, maintaining continuity of fluid mass and pressures between joints and the rock matrix
  • Effective stress calculations are carried out
  • Fracture permeability depends on aperture, or on the deformation of the solid model
  • Fluid pressure affects both deformation and the strength of the solid model
  • The deformation of the solid model affects the fluid pressures (i.e., fluid pressure changes under undrained conditions)
  • Includes logic for simulation of proppant transport and placement
  • Newtonian and power-law fluid flow available
  • Non-Darcy effects and pressure drop at perforation clusters are possible
  • Represents leakoff explicitly, as flow into DFN and porous medium flow into rock matrix, or as Carter leakoff

Microseismicity

  • Synthetic microseismic events are formed in the model by new micro-crack (tensile bond breaks) and slip on existing joints (assuming all slips are seismic)
  • Event magnitudes associated with micro-cracks and joint slips are calculated differently
  • Since springs are brittle and their breakage always results in local instability, it is possible to estimate radiated seismic energy based on transient change in strain energy in the vicinity of the broken spring or micro-crack
  • In the case of slippage across natural fractures, the energy released by the slip events is calculated based on the slip area, slip magnitude, and elastic properties of the surrounding medium
  • This allows ranking of different microseismic events (both bond breakage and joint slips) occurring during simulation based on their magnitude
  • Events that are related spatially and temporally are clustered together

XSite model of a multi-stage horizontal completion with five regularly spaced injection ports (plan view). A set of generic rock properties was used with an existing fracture network (not shown) orientated at 60° to σHmax for illustration. Acoustic emissions are shown colored by magnitude associated with micro-cracks (left) and with slip along existing joints (right).