The Influence of Offset and Gouge on Permeability in Macro-Fractured Basalt

Abstract

Abstract

Sheared fractures are ubiquitous and commonly filled with gouge-like materials. And the existence of fracture and infillings influences physical and transport properties of rock significantly. Thus, it's essential to study the influence of offset and fracture-fill on permeability of macro-fractures. Here, first, we did steady-state flow permeability measurements on both unfilled and gouge-filled (three thicknesses of gouge) mated macro-fractured sample of Seljadur basalt, an intrusive basalt with no visible cracks and exceptionally low initial permeability.at effective pressures up to 60MPa, respectively, as baseline measurement. Second, introducing offsets to sample and repeat above measurements, to investigate the influence of offset and gouge on unmated fracture. Then after tests, the thickness (porosity) and particle size distribution (PSD) changes were determined to explore permeability reduction mechansim of gouge layer during tests.

The results showed that once “mated” fracture dislocated, its permeability would increase significantly but after several offsets, the permeability under higher effective pressure would become steady. And once the fracture was fully filled with gouge, no matter it had offset or not, its permeability, gouge thickness and grain size changes of the same thickness gouge would be steady. And as gouge thickness increased, the thickness changes and permeability of the fracture went up while PSD changes became less, but when the thickness of gouge was similar to the grain size of gouge, the thickness and PSD changes might be larger than others. And for partly gouge-filled fracture, permeability is larger than that of fully gouge-filled fracture, but smaller than that of no gouge and it also showed an anistrophy of permeability as we changed the direction of fluid flow.

Overall, offset will make fracture unmatch, which definitely will increase spaces for water flow. And gouge will prevent the contact of fracture surfaces and fill its aperture. Because it will suffer from similar changes in porosity and PSD changes under similar conditions, once fully filled, offsets of fracture will have little influence on its permeablity. But if partly filled, due to the unevenness of fracture aperture and movablity of gouge grains under pore fluid pressure, it's easy to imagine that it has a larger permeability than that of fully-filled while smaller than that of no gouge, and has a permeability difference as the fluid flow direction changes.

AAPG Datapages/Search and Discovery Article #90260 © 2016 AAPG/SEG International Conference & Exhibition, Cancun, Mexico, September 6-9, 2016