Shale and tight sand permeability is not constant, but varies significantly, and to some degree independently, with variations in confining and pore pressures. Permeabilities of such rocks are low (on the order of 10-17 m2 (10 µD) or less).
Changing the way effective pressure is applied, e.g. increasing it by increasing the externally applied pressure at constant pore pressure, or increasing it by decreasing the pore pressure at constant confining pressure, squeezes the pore space by different amounts, and therefore changes the permeability potentially by different amounts. Drawdown of gas pressure during production will cause permeability to decrease, and seriously to impact upon reservoir yield.
Through our extensive testing at Reo Tight Rocks (RTR) we have established that a range of shales and tight sands display a common generic pattern of behaviour, so that we can optimally determine the pressure sensitivity of permeability in order to be able to make realistic and reliable measurements on core samples that will help predict reservoir performance.
The traditional GRI (Gas Research Institute) method provides no information about pressure-sensitivity and we do not recommend its use. The steady-state flow testing method is easy to perform but is only well suited to high permeability materials in order to ensure reasonable test durations.
For low permeability materials, pressure sensitive permeablity can be measured using the oscillating pore pressure method (Mckernan et al. 2014; Bernabé et al. 2006; Faulkner and Rutter, 1998; Fischer and Paterson, 1992; Kranz et al. 1990) and/or the pulse transient decay method (Brace, 1968; Kwon et al. 2001; Cui et al. 2009). These tests can be performed over the full range of reservoir pressure and temperature conditions.
Oscillating pore pressure test: - A low amplitude, sinusoidal pressure signal of fixed-frequency is applied via the upstream pore fluid reservoir. The attenuated and phase-shifted downstream pressure signal is recorded. From these data the permeability and sample storativity can be derived.
Pulse transient decay method: - The simplest implementation is to maintain upstream pressure constant and to record the downstream response to the sudden application of a small pressure step in the upstream pore fluid reservoir.
Well-layered materials, such as shale, are normally transversely isotropic. However, there may be up to 3 orders of magnitude variation in permeability between bedding parallel and bedding perpendicular orientations.
At low gas pore pressure (<7 MPa) measured permeabilities are higher than expected. This is attributed to the fact that the mean free path of the gas is larger than the diameter of the pore throat, resulting in gas slippage along the pore walls. This phenomenon is known as the ‘Klinkenberg effect’.
Partial water saturation
Normally, permeability will be measured in the as-received state of parial saturation. At high saturations a capillary inlet pressure may be observed. Higher permeability may be observed when the sample has been oven dried at 60 oC.
Influence on non-hydrostatic stress
In nature, rocks are generally subject to non-hydrostatic stress states. These can arise simply through the Poisson ratio effect during burial. We have an ongoing program of investigating the influence of non-hydrostatic stress on permeability of tight rocks.
- Mckernan, R.E., E.H. Rutter, J. Mecklenburgh, K.G. Taylor, S.J. Covey-Crump. Influence of Effective Pressure on Mudstone Matrix Permeability: Implications for Shale Gas Production. Proceedings of SPE/EAGE conference on unconventional reservoirs, Vienna. SPE 14UNCV-167762-MS. SPE: 2014: 1-13. eScholarID:220407
- Rutter, E.H., R. McKernan, J. Mecklenburgh, S.E. May. Permeability of stress-sensitive formations: its importance for shale gas reservoir simulation and evaluation. Petro-Industry News. 2013 September; 44-45. eScholarID:205885
- Faulkner, D.R., Rutter, E.H. The effect of temperature, the nature of the pore fluid, and subyield differential stress on the permeability of phyllosilicate-rich fault gouge. Journal of Geophysical Research-Solid Earth. 2003; 108(B5): 2227. eScholarID:1a8248 | DOI:10.1029/2001JB001581