Effect of Methane Adsorption on Stress-Dependent Porosity and Permeability in Shale Gas Reservoirs

Abstract

Although stress-dependent porosity and permeability have been applied to majority of models to calculate effective porosity and permeability, effect of adsorbed gas specified as pore volume occupied by adsorbate is not properly accounted. However, nanoscale porosity and permeability of shale rocks would be significantly reduced, since the pore volume is occupied by layers of adsorbed gas molecules.In this paper, we combine stress and adsorption effects to establish a new correlation of shale porosity and permeability. We calculate the stress-dependent pore volume with a constant pore compressibility and obtain the volume of adsorbate from doing regression analysis for density of adsorbed gas by Dubinin-Astakhov (D-A) equation under the supercritical condition. Lab experiments for methane adsorption in shale are measured by five real core samples. The data of Gibbs adsorption obtained by experiments is converted to absolute adsorption so that it could be used to generate the correlation of stress-dependent porosity and permeability including consideration of adsorption effect.The new generated correlation illustrates that the trend of changes in porosity and permeability with pore pressure is quite similar to the trend calculated by Shi-Durucan (S-D) model in application to coalbed methane, which indicates that porosity and permeability decrease with the increase of pore pressure. This tendency is the inverse of changes in stress-dependent porosity and permeability in relation to pore pressure, which demonstrates that adsorption has a larger influence on pore volume than stress, because more gas attempts to adsorb on surface area of matrix as pore pressure increases. The new correlation is added into a numerical model in field-scale, which successfully matches production data from a horizontal well with multistage hydraulic fractures in Barnett shale reservoir. The simulation results point out that without considering the impact of pore volume occupied by adsorbed gas, cumulative gas production would be greatly overestimated during economical producing period in shale gas reservoirs.Thus, overestimation of porosity and permeability in shale might be avoided by applying the new correlation considering both stress and adsorption effects. The correlation incorporated into numerical simulators could provide accurate estimation and evaluation of gas production.

Gas sorption and non-Darcy flow in shale reservoirs

“…The sorbed methane layer density was suggested to be 0.34 g/cm 3 by molecular dynamic modeling. Pang et al (2016) experimentally measured the sorption in five shale samples and successfully matched the data using the Dubinin-Astakhov model. The sorption effect on porosity and permeability was carefully studied in their work.…”

“…q 0 a is suggested to be around 0.34 g/cm 3 (Ambrose et al 2010). The experimental data used here from Pang et al (2016) are given in Table 2.…”

“…(6)) to fit the experimental data using the least squares method. The same values of q values are chosen and confirmed by matching experimental data from Pang et al (2016). The sum of the fitting errors for five cores is used as the objective function.…”

Experimental study of core size effect on CH4 huff-n-puff enhanced oil recovery in liquid-rich shale reservoirs

“…Due to the limitation of the huff-n-puff experiments that the core plug's weight is needed to be measured after each gas huff-n-puff cycle, the in situ measurements with three principle stresses cannot be implemented during these experiments. When considered the tectonic regimes, the grains are assumed to be incompressible, while the porosity and permeability are stress-dependent (Mckee et al, 1988;Pang et al, 2016). The total volume of core plug can be expressed as:…”

A Numerical Study on the Diversion Mechanisms of Fracture Networks in Tight Reservoirs with Frictional Natural Fractures

“…With the technological progress in petroleum industries, petroleum engineers are increasingly concerned with the exploration and development of tight reservoirs in recent years. Due to the ultra-low matrix permeability, hydraulic fracturing is a key technology for enhancing the recovery of tight hydrocarbon reservoirs [1][2][3][4][5][6][7][8][9][10][11]. Activation of preexisting natural fractures (NFs) during fracturing treatment is favorable for creating complex fracture networks.…”