Geomechanics is important from two main perspectives: cap rock integrity and enhancement of production performance. Both of these areas rely on concepts from Geological Engineering.
A significant caprock failure occurred on the Joslyn SAGD property in 2006, and it had a wide impact on the approval process for future SAGD projects. Two reports were released by the Alberta Government:
- “Total E&P Canada Ltd., Surface Steam Release of May 18, 2006, Joslyn Creek SAGD Thermal Operation, ERCB Staff Review and Analysis, February 11th, 2010″
- “Summary of Investigations into the Joslyn May 18th, 2006 Steam Release, Total E&P Canada Ltd.”
The latter report is very large, and a number of potential mechanisms are postulated without definitive resolution.
Thermal expansion of reservoir oil sands is an important aspect of SAGD operations. Influx of heat in the reservoir causes the oil sands to move vertically and laterally, transferring strain and deformation to the caprock and underburden strata. The total thermal expansion in the oil sands is comprised of sand grains and pore fluids expansion.
Pore Fluid Expansion
The thermal expansion of the pore fluids exceeds that of the rock matrix thereby causing an increase in reservoir pressure. Dissipation of increased pore pressure is a function of reservoir permeability and bitumen viscosity. Increasing lateral stresses in the heated zone causes rock dilation in the reservoir sands and shearing along the shoulders of the steam chamber’s region of influence. Heave develops in the overlying strata and can extend to the ground surface, though partially attenuated by the overburden.
The ERCB currently requires that caprock integrity be addressed in applying for a license for a commerical project.
Enhancement of Production Performance
The effects of geomechanics is based on:
- Extensive documentation in the literature
- Laboratory results
- Numerical simulations
- Field observations
Geomechanics is not currently included in standard petroleum engineering programs. This is a classic area where cross-disciplinary understanding is critical, much like the link between reservoir simulation and geology.
Oils sands are a frictional material, i.e., they derive their mechanical strength from the frictional resistance of the sand grains. Because they are uncemented by calcareous or siliceous adhesions at sand grain contact points, they are extremely weak when unsupported by a confining stress. The bitumen, being a highly viscous fluid, is unable to provide any strength other than under rapid loading.
Effective stress is a central concept to both rock and soil mechanics. Effective stress is the portion of the total stress in excess of the fluid pressure. An explanation of effective stress can be learned from a vacuum-packed “brick” of coffee. The grounds are a granular material and the vacuum packing provides a confining stress of atmospheric pressure. Most people can stand on one of these bricks. There will be some deformation with a person standing on it. However, if one punctures the package, the “brick” will immediately collapse into a loose pile of ground coffee that cannot support any weight without deforming completely. This is the essence of confining stress.
Methods were developed in civil engineering to quantify the strength of unconsolidated samples. These tests can be run in two fundamentally different ways. In the first method, pore fluids are allowed to escape, which is termed a drained test. In the second method, the pore fluids are not allowed to dissipate. This is termed an undrained test. This has a profound effect on the behavior of the tested materials. In an undrained test, pore pressures will initially support the majority of the incremental load. In a drained test, sufficient time is allowed for the pore pressures to dissipate during loading. The speed of drainage is a function of permeability.
Generally results are expressed using a Mohr-Coulomb model to determine when shear failure may occur. When the oil sands shear, the rock dilates and this changes permeability. Triaxial tests can be modified to simultaneously measure fluid flow through the failing specimen. In this manner, permeability can be related to stresses and axial and volumetric strains. Obtaining samples that are undisturbed can be very difficult.