3rd Quarter 2014 Featured Article

Hydraulic-Geomechanical Coupling in Fluid Injection Projects

Reinjecting fluid byproducts into deeper reservoirs is a frequently considered fluid disposal method. The suitability of a site for brine injection is assessed through several criteria, including chemical compatibility, injectivity potential, and minimizing drilling costs. Recently, there has been increased interest in considering the potential reactivation of discontinuities at different scales due to fluid injection to avoid unwanted seismic events and prevent the development of hydraulic paths to protected aquifers through reactivated features.

The rationale behind this interest is the complex hydraulic-geomechanic, and potentially thermal, response of the rock environment to fluid injection. As the fluid pressure increases around the well and the pressure-front propagates away from the well over time, the effective stress in the rock falls. As a result, it is possible that some pre-existing fractures become unstable or reactivate. Fracture properties are modified by:

  • Fracture opening due to normal stress release
  • Shear dilation
  • Fracture inflation (hydrojacking)

In addition to these fracture-scale processes, the microstructure—surface roughness, or Joint Roughness Coefficient (JRC)—and geomechanical properties (stiffness, joint compressive strength, etc.) of the fractures also change throughout fluid injection. Any of these processes may modify fracture hydraulic properties (e.g., transmissivity and storativity), which causes a coupled mechanism between hydraulic and geomechanical processes.

The coupled nature of fluid injection can be assessed by using the macro capabilities in FracMan software. FracMan MAFIC solver simulates the hydraulics of fluid injection, while hydraulic properties of the underlying discrete fracture network (DFN) are updated after each individual time-step. The initial input parameters are:

  • Fracture aperture
  • Young’s modulus
  • JRC
  • Joint Compressive Strength (JCS)
  • Stress matrix
  • DFN geometry (orientation, size, intensity)
  • Hydraulic boundary conditions

The primary property recalculated after time-step is aperture; however, fracture transmissivity and storativity may be related to aperture. The aperture evolution during injection is approximated by the experimental work of Asadollahi et al., 2010; Bandis et al., 1983; Barton et al., 1985, which relate fracture aperture to evolving normal stress over the fracture surfaces as a result of changing pore pressure, changing shear properties, friction angle, dilation angle, normal stiffness. Using this approach allows the hydraulic and geomechanic parameterization of the fracture system to dynamically evolve during numerical simulation, which provides more realistic simulation results.

Pressure and derivative plot of an injection test. Bottom right inset shows the Barton’s strength criteria with stress components and fractures. As a fracture gets above the line of strength criteria, its aperture, hence transmissivity, increases (bottom left inset), resulting in an inflexion point on the derivative curve.

Pressure and derivative plot of an injection test. Bottom right inset shows the Barton’s strength criteria with stress components and fractures. As a fracture gets above the line of strength criteria, its aperture, hence transmissivity, increases (bottom left inset), resulting in an inflexion point on the derivative curve.