Extensive testing and numerical analysis/modeling has been done by others on the coupling of normal stress and permeability associated with natural and man-made horizontal fractures. These researchers have shown that there is a decrease in permeability with increasing normal stress. However, there has been little published research into how permeability changes when loading non-horizontal fractures, and loading unfractured rock into the upper portion of the stress-strain curve. We have uniaxially loaded a sandstone through the upper stress-strain curve while measuring radial permeability. The test is very simple, but is providing very interesting results. We have shown that there are three stages in the permeability evolution. Initially, the permeability decreases, then remains constant in the lower and middle stress-strain curve. In the very upper portion of the stress-strain curve permeability increases markedly. The decrease in permeability is believed to be due to a reduction of pore space. The constant permeability is believed to be due to a subsequent tight packing of grains where the pore space cannot become smaller, yet the stress is not sufficient to initiate cracks. The increased permeability is believed to be due to the coalescence or localization of failure along vertical cracks.
Many researchers have investigated coupling of normal load (stress) and permeability of horizontal/pre-existing fractures: Raven and Gale (1985), Walsh and Brace (1984), Tsang and Witherspoon (1981 and 1983), Oda (1986), Ellsworth and Goodman (1986), Gale (1983 and 1987), Witherspoon et. al (1980), Senseny et. al (1983). They agree, though differing in details, that the cubic law is valid for an open horizontal fracture, and that the permeability of this type of fracture decreases with normal stress. However, research into the coupled effect of fluid flow and progressive fracturing in sandstone caused by uniaxial load has not been seen in the literature. The vertical fracture is the predominant fracture orientation caused by uniaxial loading. Wang and Kemeny (1994) have shown that the lateral fluid flow out of Apache Leap tuff increases throughout the uniaxial stress-strain curve, while axial permeability decreases in the lower stress-strain curve and then increases in the upper portion of the stress-strain curve. For the axial flow case, they concluded that this is probably due to the closure of void channels in the early stages of loading, and the subsequent growth and coalescence of cracks in the upper portions of the loading curve. For the lateral flow case, they concluded that initially pores were being propped open and later cracks were growing and coalescing. While Kemeny and Wang (1994) were able to show changes in permeability with load, their test rock was a very high permeability tuff, and not a typical rock for flow characteristics investigation. We have chosen a sandstone with excellent flow characteristics for this experiment, in order to investigate changes in permeability for the entire stress-strain curve. After a literature search, there does not appear to be any papers written on the subject of uniaxially loading sandstone samples through the upper stress-strain path while measuring permeability.
