The Influence of Matrix Diffusion from Production Rates of Gas Shales: Results from Experimental and Numerical Analyses

Amanda M. Bustin, Xiaojun Cui, and Robert M. Bustin
UBC, Vancouver, BC, Canada

The heterogeneity and complexity of gas shales cause substantial and often inexplicable variability in the production histories of gas wells. In order to improve our understanding of the main factors controlling the production of gas shale reservoirs, we developed a 2-D numerical model using experimental and field data from a variety of important gas shales. The results of initial, constant parameter, numerical simulations showed that for a wide range of relative permeability, matrix diffusion and fracture spacing, the productivity of a gas shale reservoir is mainly dependent on matrix diffusion rates. Estimates of diffusion rates are normally derived from canister desorption, which are dependent on the particle size of the tested samples and are often not conducted at reservoir temperatures and never at in situ stress. Based on a series of adsorption/desorption and diffusion/flow experiments under triaxial stress (reservoir) conditions we obtain more accurate diffusion rates and show that the rate of gas released from the matrix and fracture network is strongly stress dependent. The stress sensitivity of flow rates through shales is directly correlatable to their mineraology and fabric, with clay-rich shales being both more compressible and stress sensitive than most biogenic silica-rich shales. The diffusion rates and stress dependent fracture permeability data when integrated into the numerical simulator can be tested against measured production histories leading to more accurate production forecasts of new reservoirs and optimisation of field design.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas