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Relative permeability in shale gas
Permeability is one of the fundamental properties of any reservoir rock required for modeling hydrocarbon production. However, shale permeability is not yet fully understood because of the complexities involved in modeling flow through nanoscale throats. 
History of shale in hydrocarbon production
Historically, shale was thought to perform two key functions: act as a seal for conventional reservoirs and as a source rock for hydrocarbons. Recently, several shale formations have also proven to be major self-sourcing hydrocarbon reservoirs. Liquid production from numerous shale reservoirs confirmed shale as an important source of hydrocarbons and spurred a worldwide assessment of the production potential of shale. Although the Gas Research Institute (GRI) process using crushed samples allows the relatively quick measurement of porosity and permeability, the assessment of relative permeabilities is not possible. Recent improvements in SEM sample preparation, selection, and imaging along with the use of pore scale flow simulators make it possible to compute meaningful relative permeability curves for shale rock. 
Debate over productivity of shale reservoirs
Shale has provoked a great deal of research recently due to the considerable volume of natural gas stored in the reservoirs. Natural gas produced from shale provides a substantial fraction of US gas production, with some reports of as much as half by 2020. However, there are ongoing debates about how much gas these types of reservoirs can produce. 
Problems with accurately measuring shale permeability
The accurate measurement of the shale permeability is challenging because it is small, around 10-21 m2 (nanoDarcies). The challenge presents itself in both experimental and theoretical investigations. For instance, constant pressure steady-state flow measurement is not used for shale since it requires considerable time. Consequently, other approaches such as transient pulse decay (TPD) and crushed rock are commonly employed. These methods reduce the time required for measurement by changing the boundary conditions and sample size. The test is altered from steady state to transient in TPD by changing the constant pressure boundary conditions to pressures that vary with time. Therefore, it is conducted more quickly as sustained flow at steady state is not attempted. In the crushed rock method, the rock is broken into small pieces so the transient flow of gas out of the pieces is faster.
Parameters of gas-flow measurement
Studies of gas flow through shale have invoked different sets of parameters. Because many of the voids in shale are within organic material, methane molecules can adsorb to an appreciable extent. Additionally, the volume and productivity of adsorbed methane has direct implications for reserves and has been widely discussed. Conversely, the impact of adsorbed layers on permeability has not been widely explored. However, because the voids are so small, the adsorbed layer can also affect the transport properties of the shale.
- Sakhaee-pour, A., Bryant, S.L. 2011. Gas Permeability of Shale. Presented at the SPE Annual Technical Conference and Exhibition, Denver, Colorado, USA, 30 October-2 November. SPE-146944-MS. http://dx.doi.org/10.2118/146944-MS.
- Cantisano, M.T., Restrepo, D.P., Cespedes, S., et al. 2013. Relative Permeability in a Shale Formation in Colombia Using Digital Rock Physics. Presented at the Unconventional Resources Technology Conference, 12-14 August, Denver, Colorado, USA. SPE-168681-MS. http://dx.doi.org/10.1190/URTEC2013-092.
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