Hydraulic Fracturing

Hydraulic fracture propagation in a jointed rock mass under anisotropic in situ stress conditions.

Hydraulic fracturing (hydrofracturing or fracking) is a technique in which a fluid-solid mixture is injected into the wellbore at high pressures to induce fractures into the surrounding rock. This process is key to the extraction of hydrocarbons sequestered in tight rock formation (e.g., shales), allowing for a cost effective extraction of natural gas and petroleum from unconventional reservoirs. Further engineering applications of hydraulic fracturing include: energy extraction from geothermal reservoirs, rock mass pre-conditioning and/or de-stressing in mining, and measurement of underground in situ stress state.

However, the process imposes environmental risks including groundwater contamination, fresh water depletion, induced seismic activities, and migration of gas and fracking chemicals to the surface. Therefore, the use of fracking has undergone substantial technical, social, and political scrutiny. To date, however, no commercial, numerically predictive tool exists to simulate the entire process of hydraulic fracturing from the initial intact state to the pressurization that generates the fractures. Therefore, the design and optimization continues to utilize empirical approaches. Due to these technological limitations, the environmental risks of the process cannot be predicted prior to performing the operations.

Modelling of hydraulic fracturing is problematic due to the high degree of non-linearity of the processes characterizing its mechanics. Among them, pre-existing rock mass fractures can be involved in several mechanisms, including slippage on natural joints, self-propping of fractures undergoing shear displacement, and propagation and coalescence of cracks due to stress concentrations.

Conventional continuum-based numerical techniques (e.g., FEM) typically assume that cracks can grow only in tension and in the direction parallel to the maximum principal stress. These methods are, therefore, limited in their capacity to properly handle the physics of fractured systems. Although discrete element method (DEM) models can overcome some of the limitation of continuum models, one of the fundamental mechanisms of hydraulic fracturing, i.e. fracture initiation and growth, cannot be explicitly considered, as the fracture geometry must be assumed a priori.

Geomechanica’s Irazu simulation software allows analyzing the mutual interaction between pressure-driven and natural fractures. All the following processes can be captured simultaneously:

  • Fluid pressure driven fracture propagation
  • Mode I and Mode II intact material breakage
  • Shearing along newly-created and pre-existing discontinuities
  • Fracture branching and coalescence
  • Rock mass heterogeneity and anisotropy
  • Discrete fracture network
  • Synthetic micro-seismicity
  • In situ stress anisotropy
Key features
  • Predicting how hydraulic fracturing will impact pre-existing geological structures prior to performing the operations (this will reduce the risk of triggering unwanted seismic events and damaging the cap rock)
  • Assessing the environmental, social, and economical risks involved in hydraulic fracturing
  • Designing optimized hydraulic fracturing operations based on simulation results

Simulation highlights
  • Elastic models including isotropic and transversely isotropic
  • Isotropic and anisotropic strength models
  • Model heterogeneity with arbitrary number of material types
  • Discrete Fracture Network (DFN) capability with frictional or cohesive discontinuities
  • Arbitrary in situ stress conditions (isotropic, anisotropic, gravitational)
  • Hydrostatic pump model with dynamic coupling between rock mass deformation and fluid compressibility
  • Time-varying injection flow rate
  • Adaptive fluid pressure boundary conditions
  • Micro-seismic monitoring function

Simulated injection pressure response for a constant flow rate hydraulic fracture propagation.