Fluid Flow in a Fractured Reservoir Using a Geomechanically Constrained Fault-Zone-Damage Model for Reservoir Simulation

Abstract

Secondary fractures and faults associated with larger, reservoir scale faults affect both permeability and permeability anisotropy and hence may play an important role in controlling the production behavior of a faulted reservoir. It is well known from geologic studies that there is a concentration of secondary fractures and faults in a damage zone adjacent to larger faults. Because there is usually inadequate data to incorporate damage zone fractures and faults into reservoir simulation models, in this study we utilize the principles of dynamic rupture propagation from earthquake seismology to predict the nature of fractured/damage zones associated with reservoir scale faults. We include geomechanical constraints in our reservoir model and propose a workflow to more routinely incorporate damage zones into reservoir simulation models. The model we propose calculates the extent of the damage zone along the fault plane by estimating the stress perturbation associated with dynamic rupture propagation. Fractures created by the stress pulse accompanying rupture propagation enhance permeability along reservoir scale faults in both the horizontal and vertical directions. We calibrate our modeling with observations from a number of studies and show that dynamic rupture propagation gives a reasonable first order approximation of damage zones in terms of permeability and permeability anisotropy in order to be incorporated into reservoir simulators.

Predicting fault damage zones by modeling dynamic rupture propagation and comparison with field observations

“…A few field studies have documented the influence of damage zones on production. For example, Paul et al [2009] report a study from the CS gas field (in the Timor gap between Australia and Indonesia) where it is only possible to explain large gas production rates by introducing spatially variable permeability anisotropy (in flow simulators) representative of damage zones present in the reservoir. Hennings et al [2012] report a case from the Suban gas field in SE Asia where the production from wells that sample damage zones is several times greater than those which do not traverse through damage zones.…”

“…Fault damage zones may be created by various cumulative processes during or after fault formation, including Andersonian fracturing [Anderson, 1942;Scholz, 2002], early fault tip migration, fault tip linkage, cumulative fault wear with increasing displacement, (which are all quasi-static processes), and damage caused by dynamic rupture events [Rudnicki, 1980;Wilson et al, 2003;Paul et al, 2009]. These models are discussed by Mitchell and Faulkner [2009].…”

“…These models also account for dynamic and inertial effects, which are neglected in the static models. Previously, Paul et al [2009] used principles of dynamic rupture propagation to model damage zones associated with reservoir-scale faults. They used Freund's asymptotic solution of stress perturbations around a propagating rupture tip [Freund, 1979] coupled with Kostrov's solution for a circular crack [Kostrov, 1964] to calculate stresses induced by slip in regions close to a propagating rupture tip.…”

Fractured reservoir modeling: From well data to dynamic flow. Methodology and application to a real case study in Illizi Basin (Algeria)

“…Another important aspect is related to the fracture intensity decay from fault planes to the host rock: outcrop and subsurface studies showed that fracture intensity decrease while increasing distance from the fault plane to the host rock (Mitchell and Faulkner, 2009;Paul et al, 2007;Savage and Brodsky, 2011;Johri et al, 2014). This last topic is detailed in Section 4.3.…”

A scaling law to characterize fault-damage zones at reservoir depths

“…Because the degree of brittle deformation is significantly larger in the fault core (Andersson et al, 1991;Chester et al, 1993), the low fault-core permeability is governed by grain-scale (or smaller) matrix deformation of the fault rock (lowered further by clay gouge development). However, damage zones comprising higher permeability fractures and faults produce bulk permeability anisotropy related to higher permeabilities along the fault plane, thus enhancing fluid flow (Zhang and Sanderson, 1995;Paul et al, 2009). Lockner et al (2000) and Wibberley and Shimamoto (2003) reported experimentally measured permeabilities and showed that the permeability of core samples from the damage zone is several orders higher than that of samples taken from the fault core.…”