A USANS/SANS Study of the Accessibility of Pores in the Barnett Shale to Methane and Water

Abstract

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.

High-Pressure Methane Adsorption and Characterization of Pores in Posidonia Shales and Isolated Kerogens

“…Within the phyllosilicates group, illiterich mixed layer illite-smectite is the most prominent component, followed by kaolinite, illite, muscovite and chlorite. In addition, there is a moderate content of quartz (8)(9)(10)(11)(12)(13)(14)(15)(16) wt.%) and minor contents of pyrite (4-9 wt.%), feldspars (1-5 wt.%) and dolomite (0.3-6.4 wt.%).…”

“…Ultra small angle neutron scattering and small angle neutron scattering techniques are also useful for the determination of pore connectivity in shales. [8][9][10] Sorbed gas in shale is in equilibrium with the homogeneous bulk gas phase ("free gas") in larger pores. Sorption capacity depends on pressure and temperature, and structural characteristics, such as (micro-) pore volume and organic matter type, maturity and content.…”

Supercritical Fluids in Confined Geometries

“…Understanding the interrelation between the pore morphology, sorption characteristics, and potential gas storage capacities of organic-rich shales is important because of the discovered significant gas contents of shale gas reservoirs. The developed new technologies of hydrocarbon extraction from low-permeability, unconventional-type reservoirs utilizing hydrofracturing and horizontal drilling stimulated a significant increase in research in this area [81].…”

“…Recently initiated SANS/USANS studies demonstrated high effectiveness of these techniques for characterizing the morphology of dry and wet shales, and providing information on their accessible and closed porosity as a function of pore size (Fig. 10.38, [81][82][83]). …”

Observations of pore systems of natural siliciclastic mudstones

“…Desbois et al, 2009;Heath et al, 2011;Klaver et al, 2012;Clarkson et al, 2012Clarkson et al, , 2013Ruppert et al, 2013;Yang et al, 2014), e.g. in pressure shadows or at the interfaces of clay aggregates.…”