The production of natural gas has become increasingly important in the U.S.A. due to the development of hydraulic fracturing techniques which significantly increase the permeability and fracture network of black shales. The pore structure of shale is a controlling factor for hydrocarbon storage and gas migration. Here we investigate the porosity of the Union Springs (Shamokin) Member of the Marcellus Formation from a core drilled in Centre County, Pennsylvania (U.S.A.), using ultra small-angle neutron scattering (USANS), small-angle neutron scattering (SANS), focused ion beam scanning electron microscope (FIB-SEM) and nitrogen gas adsorption. The scattering of neutrons by Marcellus shale depends on sample orientation: for thin sections cut in the plane of bedding, the scattering pattern is isotropic, while for thin sections cut perpendicular to bedding, the scattering pattern is anisotropic. The FIB-SEM observations allow attribution of the anisotropic scattering patterns to elongated pores predominantly associated with clay. The apparent porosities calculated from scattering data from the bedding plane sections are lower than those calculated from sections cut perpendicular to bedding. A preliminary method for estimating total porosity from the measurements made on the two orientations is presented. This method is in good agreement with nitrogen adsorption for both porosity and specific surface area measurement. Neutron scattering combined with FIB-SEM reveals that the dominant nano-sized pores in organic-poor, clay-rich shale samples are wateraccessible, sheet-like pores within clay aggregates. In contrast, bubble-like, organophilic pores in kerogen dominate organic-rich samples. Developing a better understanding of the distribution of the water-accessible pores will promote more accurate models of water-mineral interactions during hydrofracturing.

Shale is an increasingly important source of natural gas in the United States. The gas is held in fine pores that need to be accessed by horizontal drilling and hydrofracturing techniques. Understanding the nature of the pores may provide clues to making gas extraction more efficient. We have investigated two Mississippian Barnett Shale samples, combining small-angle neutron scattering (SANS) and ultrasmall-angle neutron scattering (USANS) to determine the pore size distribution of the shale over the size range 10 nm to 10 μm. By adding deuterated methane (CD4) and, separately, deuterated water (D2O) to the shale, we have identified the fraction of pores that are accessible to these compounds over this size range. The total pore size distribution is essentially identical for the two samples. At pore sizes >250 nm, >85% of the pores in both samples are accessible to both CD4 and D2O. However, differences in accessibility to CD4 are observed in the smaller pore sizes (∼25 nm). In one sample, CD4 penetrated the smallest pores as effectively as it did the larger ones. In the other sample, less than 70% of the smallest pores (<25 nm) were accessible to CD4, but they were still largely penetrable by water, suggesting that small-scale heterogeneities in methane accessibility occur in the shale samples even though the total porosity does not differ. An additional study investigating the dependence of scattered intensity with pressure of CD4 allows for an accurate estimation of the pressure at which the scattered intensity is at a minimum. This study provides information about the composition of the material immediately surrounding the pores. Most of the accessible (open) pores in the 25 nm size range can be associated with either mineral matter or high reflectance organic material. However, a complementary scanning electron microscopy investigation shows that most of the pores in these shale samples are contained in the organic components. The neutron scattering results indicate that the pores are not equally proportioned in the different constituents within the shale. There is some indication from the SANS results that the composition of the pore-containing material varies with pore size; the pore size distribution associated with mineral matter is different from that associated with organic phases.

No abstract