Gas shale; relationships between permeability and intrinsic composition

The quantitative mineralogical data, TOC and porosity were used to increase understanding there relationships to permeability, and permeability anisotropy of gas shale based on lab measurements. Four different productive gas shales were used to study these relationships. There is a general increase in permeability with increase gas-felling porosity. At the same time, all samples, regardless of the flow direction, show a nonlinear reduction in permeability with increase of effective pressure (up to 3 orders of magnitude), with large variations from sample to sample and flow direction. This is more consistent with the microcracks model than with the capillary tube one. This reduction in permeability is found to follow a cubic k- law and explained by preferential flow through pore likecracks. There is a good relationship between permeability and mineralogical composition of the studied samples where the quartz-rich samples are more permeable than carbonate–rich samples, and permeability increases with increasing quartz content. Also permeability shows an increase when pyrite content is higher. On the other hand, permeability shows a decrease with increase carbonate and clay minerals especially illite. While the microstructure studies show a lot of microand nano-pores within the TOC, there is no clear relationship between TOC and permeability or porosity. Further intensive studies to establish these relationships are needed.