Theories linking fluid pressure to earthquakes are based on the concept that increases in pore pressure, caused by fluid injection, reduces the effective normal stress across existing or potential natural fracture/faults surfaces. The correlation between the pore pressure increase and the pre-existing in-situ stress conditions, prior to injection, plays a major role in fault/fracture sealing. At two sites in Colorado, earthquakes appear to be located entirely along pre-existing faults. The tectonic conditions in both of these locations implied very unstable faults. Additionally, a concern specific to the oil and industry is fault activation resulting from compaction which can occur during production.

When it becomes necessary to determine whether fluid injection in a disposal zone might affect neighboring faults, ADVANTEK conducts a series of analyses to determine the affect of increased pore pressure on the fault and determines how any injection fluid might migrate along it. Planned monitoring during injection using seismic surveillance provides an early warning of any adverse fault movement.

ADVANTEK INTERNATIONAL expertly conducts these analyses to evaluate the fault activation risk in order to determine the impacts and consequences should the risk occur.

Faults are a common component of the trapping mechanisms controlling hydrocarbon accumulations. They are rarely completely impermeable boundaries; rather, they juxtapose formations with varying amounts of permeability. The fault’s sealing capacity is a relative expression of the extent to which these formations can communicate across the fault plane, exchanging pressure and or fluids. Transmissivity across faults changes as a reservoir’s properties change and can have significant influence on reservoir behavior during hydrocarbon production or injection. Fault seal is, therefore, a major exploration and production uncertainty.
Reservoir compaction may induce relative displacements along a fault that cause severe localized strain and deformation. The offset of formations may be either a relatively sharp discontinuity or a wide transition zone. The more concentrated the transition zone, the more likely these deformations will shear wellbores which cross the fault, or cause wellbore collapse and failure.

The effective normal stress and shear stress acting on the formations across a fault may be used to analyze fault activation and fault slip. Tectonic loads may be determined from Log data analysis or laboratory test results. Since deformation and strains induced from production or injection can be computed at any location in the earth, the induced normal stress and shear stress changes on a fault are computed in the developed numerical model from Hooke’s law or from any specific rock constitutive model. The risk of activating or slippage of the fault plane or bedding planes can then be assessed using a fault-slip criterion, for example, the Mohr-Coulomb criterion.

Production can also cause significant formation compaction and surface subsidence. Peripheral faults may be activated or experience effective aperture or transmissibility changes. Predicting fault integrity and sealing capacity are integral parts of reservoir depletion and injection management, and are necessary to maximize recovery and to mitigate failure risks associated with production and injection.

Geomechanical modeling utilizes spatially oriented reservoir data from reservoir simulations (such as the reservoir grid blocks and the pressure data). Faults are approximated by a plane between grid blocks and the resulting normal and shear stress changes on the fault plane due to production and/or injection can be calculated. These stresses are used to predict relative fault movement and transmissivity between formations across the fault.

Experience	Solutions/Tools
Geomechanical Evaluation Seal Integrity Evaluation	@Log @Comp
Geographical Environments	Contact
Caspian Basin Offshore Angola	Mr. Karim Zaki Ph. 713.532.7630 Email: Karim@ADVANTEKInternational.com