Article pubs.acs.org/EF
A USANS/SANS Study of the Accessibility of Pores in the Barnett Shale to Methane and Water Leslie F. Ruppert,*,†,∇ Richard Sakurovs,‡,∇ Tomasz P. Blach,§,∇ Lilin He,∥,∇ Yuri B. Melnichenko,∥,∇ David F. R. Mildner,⊥,∇ and Leo Alcantar-Lopez#,∇ †
United States Geological Survey, Eastern Energy Resource Science Center, MS 956, National Center, Reston, Virginia 20192, United States ‡ CSIRO Energy Technology, CSIRO Riverside Life Sciences Centre, North Ryde 2113 NSW, Australia § Institute for Future Environments, Queensland University of Technology, Gardens Point Campus, Brisbane Q4000, Australia ∥ Biology and Soft Matter Division, Neutron Scattering Directorate, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6393, United States ⊥ Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-6102, United States # Chesapeake Energy Corporation, 6100 North Western Avenue, Oklahoma City, Oklahoma 73118, United States 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 ( 0.02 Å−1, indicating that about 80−85% of both large and small pores in this sample is accessible to methane. In contrast, ϕAC is roughly constant, at about 0.8−0.85, over the SANS range (Q = 3 × 10−5 to 3 × 10−3 Å−1) for sample 152, but it decreases at larger Q to about 0.65 Å−1 at Q = 0.02 Å−1. This suggests that ∼85% of larger (>100 nm) pores is accessible but the smaller (12− 100 nm) pores are less accessible to methane, with the poorest accessibility of ∼65% occurring near 25 nm. At greater Q values (not shown), condensation effects prevent an accurate determination of the accessible porosity for the smallest pores in both samples. Even though the two samples have similar pore size distributions, they differ in their accessibility to gas. Statistical uncertainties in the USANS scattered intensity are negligible. The uncertainties in the SANS region increase with Q as the measured intensity drops off, and the total uncertainties of ϕAC are less than 10% even at Q values as high as 0.01 Å−1. Because a considerable proportion of the fine pores cannot be readily penetrated by methane, and since some methane in shale may be stored in fine pores, it could be difficult to extract methane from the finest pores by normal extraction processes. I(Q) varies as a function of CD4 pressure over the Q range in the SANS measurements (shown in Figure 2). In the low Q region, the intensity at first drops rapidly as pressure increases and then levels off. Since the scattering intensity in accessible pores is proportional to the square of the difference in SLD values, the relationship between I(Q) and gas density should be of a parabolic form, with a minimum at the zero contrast gas density. If the fraction of closed pores, ϕAC, and the zero contrast density, ρzc, are treated as independent variables, we can measure the scattering intensity at a number of pressures and then use a least-squares fit to obtain both values. This can lead to a more accurate estimation of the SLD of the material in
Figure 2. Variation of SANS patterns from sample 152 when pressurized CD4 is added. Pressure in the cell was increased in 1000 psi steps through 8000 psi and in a single 2000 psi step to 10 000 psi. The intensity levels off at 8000 psi, which is the zero contrast density, and does not decrease further at higher pressures; thus, the observed scattering at the zero contrast density indicates that there are closed pores inaccessible to methane.
greater than in a vacuum. This trend has been observed previously using CD4 in coal samples,15 although not to the extent seen in these shale samples. If the observed increase in scattering intensity is caused by uniform condensation of CD4 on pore walls and within micropores, the densities required to explain such scattering would be higher (∼1 g/cm3) than those reported for liquid CD4 (∼0.5 g/cm3). It is therefore more likely that the increased scattering intensity is the result of clustering of CD4 molecules in micropores that introduce pressure-dependent SLD variations on a small linear scale (12.5 nm (Q < 0.02, Figures 2 and 3). This may suggest that any layer of high-
Figure 3. Combined USANS/SANS data for samples 72 and 152 under vacuum and at 10 000 psi CD4 show that pores in both samples are penetrated by deuterated methane and the effect is uniform over a wide Q range. There appears to be density clustering of CD4 molecules in pores
