Hydro-mechanical modelling of multiphase flow in naturally fractured coalbed using a multiscale approach

This paper presents a multiscale model developed for the modelling of multiphase
flow in fractured reservoirs, particularly in coal seams during coalbed methane recovery
(or carbon dioxide storage). The variation of gas content in the matrix induces a
shrinkage/swelling of the material which is likely to affect the fractures and thus the
permeability of the rock. In addition, the permeability is also sensitive to the effective
stress evolution. In order to simulate the reservoir production taking into account the
cleat-scale phenomena on their specific length, a cleat-scale model is used for the modelling
of the Representative Elementary Volume (REV at microscale). This cleat-scale
model is integrated in a multiscale approach using the finite element square method.
It consists to localize the macroscale deformation to the microscale by applying appropriate
boundaries, then resolve the boundary value problem on the microscale with
finite elements, then homogenize the microscale stresses with appropriate averages to
compute macroscopic quantities, and finally resolve the boundary value problem on
the macroscale with finite elements. This approach has the advantage that it does not
require to write some constitutive laws at the macroscale but only at the REV-scale.
The model is developed and implemented in a finite element code and then used for
reservoir modelling. A synthetic case is first considered to demonstrate the ability of
the model to obtain some bell-shape production curves as expected for unconventional
reservoirs. Finally, a history matching exercise is carried out.