Summary Using focused-ion-beam (FIB)/scanning-electron-microscope (SEM) imaging technology, a series of 2D and 3D submicroscale investigations revealed a finely dispersed porous organic (kerogen) material embedded within an inorganic matrix. The organic material has pores and capillaries having characteristic lengths typically less than 100 nm. A significant portion of total gas in place appears to be associated with interconnected large nanopores within the organic material. Thermodynamics (phase behavior) of fluids in these pores is quite different; gas residing in a small pore or capillary is rarefied under the influence of organic pore walls and shows a different density profile. This raises serious questions related to gas-in-place calculations: Under reservoir conditions, what fraction of the pore volume of the organic material can be considered available as free gas, and what fraction is taken up by the adsorbed phase? How accurately is the shale-gas storage capacity estimated using the conventional volumetric methods? And finally, do average densities exist for the free and the adsorbed phases? We combine the Langmuir adsorption isotherm with the volumetrics for free gas and formulate a new gas-in-place equation accounting for the pore space taken up by the sorbed phase. The method yields a total-gas-in-place prediction. Molecular dynamics simulations involving methane in small carbon slit-pores of varying size and temperature predict density profiles across the pores and show that (a) the adsorbed methane forms a 0.38-nm monolayer phase and (b) the adsorbed-phase density is 1.8–2.5 times larger than that of bulk methane. These findings could be a more important consideration with larger hydrocarbons and suggest that a significant adjustment is necessary in volume calculations, especially for gas shales high in total organic content. Finally, using typical values for the parameters, calculations show a 10–25% decrease in total gas-storage capacity compared with that using the conventional approach. The role of sorbed gas is more important than previously thought. The new methodology is recommended for estimating shale gas in place.

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To understand the deliverability of gas shales, one needs to understand the controls on porosity and permeability. The microstructure of shale is defined in part by the grain size which is typically less than 5 μm and composition. Gas shales unlike other lithologies contain significant quantities of organic matter in various stages of maturation. The geometry and nature of the mineralogical components and the organics would be easy to describe if the objects could be identified optically. Most investigations into shale microstructure have relied on technologies such as Scanning Electron Microscopy (SEM), X-ray imaging, Transmission Electron Microscopy (TEM) or Scanning Acoustic Microscopy (SAM). Each has advantages and limitations. Our focus here is limited to SEM studies we performed on gas shales. Our journey began with generic imaging of broken surfaces and has progress to imaging of ion-milled surfaces in dual beam SEM. The latest technology has provided three dimension images with maximum resolution of 4-5nm pores. Porosity is found on the microscale in organics, between grains, in pyrite framboids, fossils, within minerals and in the form of microcracks. The majority of pores in some shales are located in the organics. Other shales show the porosity to be largely associated with minerals. SEM resolved pore dimensions agree well with "as received" NMR measurement. However, high pressure mercury injection measurements suggest that the paths connecting these pores are even smaller. Three dimensional reconstruction of sequentially ion-milled surfaces using a dual beam SEM provides controls on the volumetric distribution of pores and organics and their connectivity. Initial and limited analyses indicate that the shales investigated are dominated by smaller pores. Keep in mind that any such study only samples an extremely small portion of any reservoir and that while generalizations are tempting, statistical studies are required to establish the universality of such observations.