Shale permeability

Diffusional transfers of potassium and silicon between sandstones and shales may be sufficient to accomplish feldspar dissolution, illitization, and quartz cementation (Thyne, 2001 Thyne et al, 2001). Losses of the magnitude observed for detrital carbonates in shales exceed the capacity of diffusion-mediated transfer. Large-scale advection seems required, although our understanding of shale permeabilities seems to preclude this (Bjprlykke, 1989, 1993 and Lynch, 1997). The possibility of convection driven by salinity heterogeneity within thick shale sequences has been demonstrated by Sharp et al (2001), who note that more information for rock properties and fluid compositions within deep basinal shales is needed before the generality of their results can be assessed. 

Best, M.E. and Katsube, T.J. 1995. Shale permeability and its significance in hydrocarbon exploration. The Leading Edge, 14 165-170. 

Pressure and stress reversion below the overpressured zone because of increased shale permeability... 

Revil and Cathles (1999) published a systematic analysis of shale permeability and found a high correlation between permeability and porosity. England et al. (1987) derived a relationship between permeability (in md) and porosity (fraction) for shales and mudstones... 

For laminated shaly sand, a dramatic decrease of permeability in the vertical direction can be expected because the flow is COTitroUed by the low shale permeability, whereas in the horizontal direction, the magnitude of permeability is still controlled by the sand fraction for moderate shale content also. This creates permeability anisotropy. 

Katsube, T.J., Connell, S., 1998. Shale permeability characteristics. In Current Research 1998-E. 

Katsube, T.J., Issler, D.R., Cox, W.C., 1998. Shale permeability and its relation to pore-size distribution. In Current Research 1998-E. Geological Survey of Canada, Ottawa, pp. 51-57. Katz, A.J., Thompson, A.H., 1986. Quantitative prediction of permeability in pmous rock. Phys. Rev. B 34, 8179-8181. 

Bioturbation, due to the burrowing action of organisms, may connect sand layers otherwise separated by clay laminae, thus enhancing vertical permeability. On the other hand, bioturbation may homogenise a layered reservoir resulting in an unproducible sandy shale. 

If a sand line (0% shale) and a shale line (100% shale) are defined on the gamma ray log, a cut-off limit of 50% shale can be used to differentiate the reservoir from non-reservoir intervals. This type of cut-off is often used in preliminary log evaluations and is based on the assumption that reservoir permeability is destroyed once a rock contains more than 50% shale. 

Alixant, J-L., Real-time Effective Stress Evaluation in Shale Pore Pressure and Permeability Estimation, Ph.D. dissertation, Louisiana State University, p. 210, December 1989. 

Properties and extraction processes Tight-formation gas is natural gas trapped in low-porosity (7 to 12%), low-permeability reservoirs with an average in-situ permeability of less than 0.1 millidarcy (mD), regardless of the type of the reservoir rock tight gas usually comprises gas from tight sands (i.e., from sandstone or limestone reservoirs) and shale gas. Sometimes tight gas also comprises natural gas from coal and deep gas from reservoirs below 4500 m. Shale gas is produced from reservoirs predominantly composed of shale rather than from more conventional sandstone or limestone reservoirs a particularity of shale gas is that gas shales are often... 

This indicates that a general geothermal pattern has been established in the total column and that rapidly circulating warmer water has only local effects on the clay mineralogy. The mineralogy of these different types of semi-permeable rocks corresponds, on a depth-temperature basis, very closely with that found in pelitic shale rocks of other studies. It is likely therefore that high permeability gives a noticeably different set of chemical parameters (intensive variables) to a rock whereas medium to low permeability can be assimilated to a "closed" system where rock and fluid are effectively part of the same physicochemical unit. 

Because of the obvious complexity of the controlling factors that may be involved, we shall not attempt at present to estimate their functional magnitude. The existence of interconnecting channels was postulated from scanning electron micrographs of bioleached shale surface (II) and is supported by evidence from the present study. The increase of porosity by removing the carbonate mineral with dilute acid would presumably improve the permeability of certain chemical compounds into and out of the remaining shale structure. 

Comparing these airflow intervals with those permeable zones induced by conventional hydraulic fracturing indicated that explosive fracturing created additional communication paths to wells 2 and 5 at the 73-ft level. However, the injection capacity of well 3 was reduced 64%. This may have resulted from too wide a dispersion of the liquid explosive, so that the shot was not strong enough to lift and fracture the overburden rock permanently, or the fractures may have been plugged by fine oil-shale particles or mud. 

In general, hydraulic fracturing with sand propping provided adequate void space for emplacement of the NG1 in these explosive-fracturing tests in the oil shale. Explosive fracturing caused significant increases in fracture permeability when a sufficient NG1 charge was detonated. 

Broadly distributed, lean hydrate deposits Related to dispersed, low concentrations CH4 generated near hydrate deposit Low-permeability flow Frequently are deposited in shales Forming flow is slow frequently diffusive Form deeper in occurrence zone GHOZ Represent the majority of hydrate Frequently modeled... 

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