Matrix Heterogeneity Effects on Gas Transport and Adsorption in Coalbed and Shale Gas Reservoirs.pdf

Matrix Heterogeneity Effects on Gas Transport
and Adsorption in Coalbed and Shale Gas Reservoirs
Ebrahim Fathi I. Yücel Akkutlu
Abstract In coalbeds and shales, gas transport and storage are important for accurate
prediction of production rates and for the consideration of subsurface greenhouse gas seques-
tration. They involve coupled fluid phenomena in porous medium including viscous flow,
diffusive transport, and adsorption. Standard approach to describe gas–matrix interactions is
deterministic and neglects the effects of local spatial heterogeneities in porosity and material
content of the matrix. In this study, adopting weak-noise and mean-field approximations and
using a statistical approach in spectral domain, matrix heterogeneity effects are investigated
in the presence of non-equilibrium adsorption with random partition coefficient. It is found
that the local heterogeneities can generate non-trivial transport and ic effects which
retard gas release from the matrix and influence the ultimate gas recovery adversely. Macro-
transport shows 1 [1 N 1 N ] dependence on the Peclet number, and persists at
the diffusive ultra-low permeability limit. Macro-ics is directly related to Thiele modu-
lus by the following expression: N 1 2N . It leads to trapping of gas in the adsorbed
phase during its release from the matrix, and to an adsorption threshold during the gas uptake
by the matrix. Both effects are proportional to the initially available adsorbed gas amount and
es more pronounced with the increasing variance of the porosity field. Consequently,
a new upscaled deterministic gas mass balance is proposed for practical purposes. Numerical
results are presented showing free and adsorbed gas distributions and fractional gas sorp-
tion curves for unipore coal matrix exhibiting Gaussian porosity distribution. This study is
a unique approach for our further understanding of the coalbeds and gas shales, and it is
important for the development of sound nu