A Pore by Any Other Name Would Be as Small: the Importance of Meso- and Microporosity in Shale Gas Capacity
In organic-rich shales, a significant portion of the total porosity is within the size ranges of 50 to 2 nm (mesopores) and < 2 nm (micropores). Using the U.S. Barnett and Haynesville shales and the Canadian Buckinghorse and Shaftesbury shales as examples, we identify the pore size distribution, TOC content (1-20%), mineralogy (quartz between 30-80%), methane sorption capacity (< 8cm3/g at reservoir pressure) and with nano-imagining techniques, evaluated the importance of the meso- and microporosity in controlling the methane capacity. Samples were ion-milled and gold-coated to obtain FE-SEM images of pore spaces at the meso-/micropore boundary for the first time. A positive relationship exists between microporosity and methane sorption capacity of a shale as microporosity is primarily associated with organic matter. Microporosity is also created during organic maturation. Meso- and microporosity in gas shales have two significant impacts: 1) they are a major contributor to the surface area for sorption of methane molecules and 2) they increase the total porosity of the sample and hence the free gas component. When describing gas shale porosity, we recommend abandoning the usage of the ill-defined term, nanopore, and instead use the terms macro-, meso- and micropores that are defined by the International Union of Pure and Applied Chemistry (IUPAC) as the latter terms have defined size limits.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009