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Horizontal Permeability Anisotropy in Intact Tight Reservoir and Caprock Samples Caused by In-Situ Stress Anisotropy

Armitage, Peter; Worden, Richard H.; Faulkner, Dan; Blake, Oshaine

CO2 injection into saline aquifers and depleted hydrocarbon reservoirs inevitably alters the in-situ stress conditions, thus altering fluid flow properties of the storage rocks. InSAR surface uplift data have shown a non-circular uplift pattern above the zone area of injection at In Salah. (Bissell et al., 2011) have explained and modelled this with a northwest/southeast fracture zone along the direction of the largest principal stress, leading to preferential fluid flow along the direction of the fault (Rutqvist et al., 2010). Here we present the results of an experimental programme showing how the pre-existing, in-situ horizontal stress anisotropy leads to a horizontal permeability anisotropy of two, helping to explain some of this effect currently modelled as preferential flow along fractures.

Our results show that increasing the differential stress on intact Krechba rocks can increase their permeability by up to two orders of magnitude before failure. As the Krechba field has a significant difference in magnitude of the principal horizontal stresses, it would be expected that there would be a horizontal permeability anisotropy caused by this in intact rock, in addition to the enhanced fluid flow along the inferred fault. This has been demonstrated in reservoir models (Bissell et al, 2011) where an anisotropic horizontal permeability field was required to match the measured pressure changes. Reservoir simulation work (Bissell et al., 2011) showed the horizontal anisotropy ratio to lie between 10 and 30.

We have found a horizontal permeability anisotropy ratio of two in intact samples of Krechba rock, caused by horizontal stress anisotropy. Hence the anisotropy observed must be the result of not only the in-situ stress field, but an anisotropic crack network. This will contribute to the non-circular distribution of CO2 currently explained by faulting. If the rock was unfractured, we would still expect to see non-circular distribution of CO2, though to a much lesser degree.

 

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