VARIABILITY OF PORE ACCESSIBILITY TO GAS IN CLOSELY-SPACED BARNETT SHALE SAMPLES

Small-angle neutron scattering (SANS) and ultra-small angle neutron scattering (USANS) with contrast matching techniques were used to investigate size distribution and gas accessibility in pores in an approximately 10.6 cm long Mississippian Barnett Shale core. SANS and USANS measurements record scattering from all pores, both open and closed, in the size range ~10 µm–10 nm. The techniques can also be used to determine the phase (organic or mineral composition) that contains pores and the number of pores as a function of size. By injecting CD₄ gas at contrast matching pressure it is possible to distinguish which pores are accessible, or open, to gas and which ones are not.

We measured the variability in the fraction of accessible pores in three (approximately 16 mm in diameter and 0.5 mm in thickness) wafers cut from the core. One of the wafers was obtained from near the top of the butt core and the other two were obtained from within ~2 mm of one another from the middle of the core. SANS and USANS results show that the larger pores (~10 µm–100 nm) in the sample from the top of the core were more accessible (80-90%) to gas than the smaller pores (100–10 nm). In contrast, the largest pores in the samples taken within 2 mm of one another were only ~50-60% accessible to gas.

Results of this study show that samples taken from within centimeters of one another can differ in their pore structure and accessibility, even if they have similar bulk organic and mineral compositions. Other techniques that utilize small samples may have similar issues of subsample representation. Although cutting-edge techniques offer results that allow us to answer fundamental questions on the structure, connectivity, and accessibility of pores that contain hydrocarbons in unconventional reservoirs, any conclusions based on small samples, no matter what micro-technology is utilized, must be used with caution and should not be used to characterize reservoirs at play or resource levels.