Gas Permeability of Shale

Abstract

Permeability is one of the most fundamental properties of any reservoir rock required for modeling hydrocarbon production. However, shale permeability has not yet been understood fully because of the complexities involved in modeling flow through nanoscale throats. In this paper, we analyze the effects of adsorbed layers of methane (CH 4 ) and of gas slippage at pore walls on the flow behavior in individual conduits of simple geometry and in networks of such conduits. The network is based on scanning elecron microscope (SEM) images and a drainage experiment in shale. To represent the effect of adsorbed gas, the effective size of each throat in the network depends on the pressure. The hydraulic conductance of each throat is determined on the basis of the Knudsennumber (K n ) criterion (Knudsen 1909). The combined effects of adsorption and slip depend strongly on pressure and on conduit diameter. The results indicate that laboratory measurements made with N 2 at ambient temperature and 5-MPa pressure, which is typical for a transient pulse-decay (TPD) method, overestimate the gas permeability at early life of production by a factor of four. This ratio increases if the measurement is run at ambient condition because the low pressure enhances the slippage and reduces the thickness of the adsorbed layer. Moreover, the permeability increases nonlinearly as the in-situ pressure decreases during production. This effect contributes to mitigating the decline in the production rate of shale-gas wells. Laboratory data available in the literature for CH 4 permeability at pressures below 7 MPa agree with model predictions of the effect of pressure.

Impact of Petrophysical Properties on Hydraulic Fracturing and Development in Tight Volcanic Gas Reservoirs

“…The pulse-decay permeability measurement method has been widely recognized as a way to identify the ultra-low permeability rocks [28,30,[48][49][50][51][52][53]. Because the conventional permeability measurement is based on recording the flow rate during the experiment, the flow rate through the ultra-low permeability rock is too small to be measured exactly under the precision of present equipment.…”

Gas Flow Behavior of Nanoscale Pores in Shale Gas Reservoirs

“…In order to analyze production dynamics in shale gas reservoirs, an extended dynamic-slippage model was put forward by Clarkson et al [19,20]. Civan et al [21], Sakhaee and Bryant [22] extended Knudsen diffusion into slippage factor to describe gas transport in shales. Anderson et al [23] proposed a model to describe the transport mechanisms, which is related to the experiment empirical coefficients.…”

Impact of Shale-Gas Apparent Permeability on Production: Combined Effects of Non-Darcy Flow/Gas Slippage, Desorption, and Geomechanics

“…In order to propose corrections for Non-Darcy flow over different flow regions in nanopore space, numerous authors (Civan, 2010;Darabi et al, 2012;Florence et al, 2007;Fathi et al, 2012;Javadpour et al, 2007;Javadpour, 2009;Sakhaee-Pour and Bryant, 2012;Singh et al, 2014) have quantified these effects by modifying the slippage factor or determining the apparent permeability as a function of Knudsen number. Swami et al (2012) investigated and compared 10 theoretical and empirical models for quantifying Non-Darcy flow/Gas-Slippage effects in unconventional gas reservoirs; although these different approaches provided different results, they all indicated that the flow rate at nanoscale level is greater than that predicted from Darcy's equation.…”

“…Swami et al (2012) investigated and compared 10 theoretical and empirical models for quantifying Non-Darcy flow/Gas-Slippage effects in unconventional gas reservoirs; although these different approaches provided different results, they all indicated that the flow rate at nanoscale level is greater than that predicted from Darcy's equation. Specifically, compared to other theoretical models investigated in the study, the correlations proposed by Sakhaee-Pour and Bryant (2012) are considered to be most reliable because they are supported by well-designed laboratory experiment.…”