Evaluation of Nanoscale Accessible Pore Structures for Improved Prediction of Gas Production Potential in Chinese Marine Shales

Abstract

The Lower Cambrian Niutitang and Lower Silurian Longmaxi shales in the Upper Yangtze Platform (UYP) are the most promising strata for shale gas exploration in China. Knowledge of the nanoscale pore structure may improve the prediction of the gas production potential in Chinese marine shales. A systematic investigation of the pore accessibility and its impact on methane adsorption capacity has been conducted on shale samples using various techniques including geochemical and mineralogical analyses, field-emission scanning electron microscopy (FE-SEM), small-angle neutron scattering (SANS), helium porosimetry, and methane adsorption. The results show that organic matter (OM) pores with various shapes dominate the pore systems of these shales. OM tended to mix with clay minerals and converted to organoclay complexes, developing plentiful micro- and mesopores. A unified fit model with two pore structures, fractal pores and finite pores, was used to model the SANS data to characterize the pore structure of the shales. Both mass and surface fractals are identified for each pore structure. The total porosity estimated by the Porod invariant method ranges between 2.35 and 16.40%, of which the porosity for finite pores ranges between 0.35 and 6.36%, and the porosity for the fractal pores ranges between 2.07 and 8.51%. The fraction of open pores was evaluated by comparing the porosities estimated by He porosimetry and SANS. We find that the fraction of open pores is higher than 64% for most of these shales. Correlation analyses suggest that clay and total organic carbon (TOC) have opposite effects on pore structure and methane adsorption capacity. Samples with higher clay contents have higher pore accessibility and lower total porosity, surface area, and maximum methane adsorption, whereas samples with higher TOC content show the inverse relationships. The high percentage of open pores may reduce methane adsorption capacity in these shales, whereas low pore accessibility may reduce methane production at specific pressure differences. Thus, both TOC and pore accessibility may be essential controlling factors in methane production from shale gas reservoirs.

Supercritical Methane Adsorption on Shale over Wide Pressure and Temperature Ranges: Implications for Gas-in-Place Estimation

“…According to petrophysical test results, the shale physical model can be simplified into two parts: mineralogical components and pore space (Figure b). Mineralogical components include brittle minerals (quartz, feldspar, carbonate minerals, and pyrite), clay minerals, and dispersed organic matter, ,, in which organic matter and clay minerals are regarded as the main carriers for shale gas adsorption. ,,, Pore space covers the pores and fractures developed within and between various minerals. ,,, Some pores are isolated, which the fluids (formation water, liquid hydrocarbon, and gas) cannot access. , In the accessible pore space, with the subtraction of the fraction occupied by formation water and oil, the remaining is the available space for shale gas storage in the form of adsorbed gas and free gas.…”

Quantifying Fluid‐Wettable Effective Pore Space in the Utica and Bakken Oil Shale Formations

“…For each sample, the measured total porosity (7.47–10.5%) is larger than the oil‐ or water‐accessible porosity (2.01–5.14%), implying a significant volume of closed or unwetted pore space. This low pore connectivity is consistent with the results of Wood's metal impregnation followed by laser ablation‐ICP‐MS for the Barnett Shale (Hu et al, 2015; Zhao et al, 2020), MIP analyses showing decreasing edge‐accessible porosity with increasing sample size for the Wolfcamp Shale (Hu, 2018), nano‐CT analyses for the Longmaxi Shale (Y. F. Sun et al, 2018), as well as a combined MIP, FE‐SEM, SANS, helium porosimetry, and high‐pressure methane adsorption study of the Niutitang and Longmaxi Formations (Wang et al, 2018, 2020).…”

Technical Aspects of the Pore Structure in Shale Measured by Small-Angle and Ultrasmall-Angle Neutron Scattering: A Mini Review

“…SANS/USANS measure all of the pores in shale within the size range determined by the instrument configuration. This offers an opportunity to estimate the fraction of open (or closed) pores in conjunction with the fluid invasion data. ,− However, one should keep in mind that the porosity data derived from different instruments used to estimate the open porosity need to come from the same pore size range. The accessibility of pores in shale to water or methane has been more frequently studied by contrast variation SANS/USANS. ,,,,,,,− The SLDs of water and D 2 O are −0.56 × 10 –6 and 6.36 × 10 –6 Å –2 , respectively.…”