Fault Damage Zones-Observations, Dynamic Modeling and Implications on Fluid Flow

Johri, Madhur; Zoback, Mark; Dunham, Eric M.; Hennings, Peter

We present a methodology that accounts for the effect of permeability heterogeneity and anisotropy introduced by fault damage zones on the flow characteristics of fractured reservoirs. In this study, we carry out an integrated observational and modeling study of sub-surface fault damage zones in a gas field and adjacent to the San Andreas Fault in central California. We compare observations (from image logs) with theoretically-predicted damage zones from dynamic rupture propagation modeling. Flow simulations are then performed on reservoir models containing damage zones to illustrate the impact of the damage zones on fluid flow.

Despite the geologic differences, damage zones in both the gas field and the arkosic sandstone (adjacent the San Andreas Fault) have a number of similar characteristics. The decay of fracture density with distance can be described by a power law F(r)=F0 r-n in the 50-80 m wide damage zones. F0 (Fault constant) is the fracture density 1 meter from the fault. It ranges from 6-30 fractures/m. The rate of decay n ranges from 0.4-1.

Damage zone modeling utilizes two-dimensional plane-strain dynamic rupture models with strong rate-weakening fault friction and off-fault Drucker-Prager plasticity. The number of induced fractures is calculated by assuming that the dilatational plastic strain is manifested as discrete fracture planes. Theoretical results obtained post model calibration with field observations suggests that the damage zones are approximately 60-100 meters wide and the fracture density decreases with distance from the fault according to a power law with the rate of decrease approximately 0.8, both predictions in good agreement with our observations and outcrop studies of fault damage zones.

Finally, the damage zone observations and modeling results are used to build a reservoir model of the gas field using a discrete fracture network framework. The model is upscaled, and flow simulations performed (in a dual porosity framework) to highlight the hydraulic impact of damage zones on flow. Flow simulations show that pressure drawdown due to production is significantly larger in the absence of damage zones. This is due to higher flow rates facilitated by damage zones. These results show that considering the presence of damage zones is important in modeling flow and optimizing production.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013