CHARACTERIZING PORE-SIZE DISTRIBUTION IN ORGANIC-RICH SHALES WITH N2 ADSORPTION

Pore-size distribution is a key attribute for characterizing pore networks, pore connectivity, and pore volume of organic-rich mudstones. A series of N₂ adsorption experiments were conducted on samples of the organic-rich Barnett shale at -196 °C to characterize the distribution of pores across a range of crushed particle sizes.

Two samples, having contrasting thermal maturities, were collected from the Ft. Worth Basin, Texas. A highly mature sample (Ro=2.01%) was collected from a subsurface well at a depth of 7191 ft in the center of the basin; TOC is 6.6%. The second, more immature sample (Ro=0.58%), was collected from the shallower western margin of the basin (depth: 1250 ft; TOC: 7.88%). Samples were crushed and sieved into fractions of 830-270 micron, 270-150 micron, 150-53 micron and < 53 micron particle size populations. N₂ adsorption was carried out on all aliquots for both samples. Pores were imaged using scanning electron microscopy to obtain independent evidence of pore sizes and pore-size distribution. Isolated kerogen samples were also examined to compare pore sizes and distribution within mineral and organic components.

Results show that, for both samples, smaller sizes of crushed particles are associated with greater surface area and larger pore volume. This indicates that, at the scale of the finest particles (< 53 micron), the pore network has been more extensively exposed, resulting in an increase in measured total surface area and pore volume. The low maturity sample exhibits a bimodal distribution of pore sizes in all particle size ranges. By contrast kerogen from this sample shows a unimodal pore size range (20-80 nm) suggesting that pores in this size range are associated with organic materials while smaller nanopores (<6 nm) are associated with mineral components. In contrast, the high thermal maturity sample displays a bimodal pore size distribution in all particle sizes as well as in the kerogen, indicating that nanopores <6 nm occur not only in mineral components, but also in organic material. SEM images of pore size distribution in clay minerals and kerogen are consistent with these observations. This initial examination demonstrates that combined N₂ adsorption and SEM pore studies can provide key information for better understanding pore connectivity in organic-rich shales.