Investigation of Pore Structure and Sorption Behavior for Unconventional Gas Reservoir Rocks

Restricted (Penn State Only)
Author:
Zhang, Rui
Graduate Program:
Energy and Mineral Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
October 04, 2018
Committee Members:
  • Shimin Liu, Dissertation Advisor
  • Shimin Liu, Committee Chair
  • Derek Elsworth, Committee Member
  • Hamid Emami-Meybodi, Committee Member
  • Demian Saffer, Outside Member
  • Lilin He, Special Member
Keywords:
  • pore structure
  • adsorption
  • small-angle neutron scattering
  • coalbed methane
  • shale gas
  • unconventional natural gas
Abstract:
Pore structure and gas sorption behavior are essential to quantify the adsorption capability and diffusivity of unconventional formation rocks. Both adsorption capability and diffusivity are fundamental reservoir properties for evaluating the gas production and carbon sequestration potentials of unconventional gas reservoirs including coalbed methane (CBM) and shale gas reservoirs. In this dissertation, small-angle neutron scattering (SANS) and complementary techniques, including small-angle X-ray scattering (SAXS) and low-pressure N2 adsorption (LPNA), are applied to characterize the rock matrix’s pore structure and gas adsorption behavior for two coal and two shale samples. The interconnectivity of pores not only determines the dynamics of gas transport in the coal matrix, but also influences its mechanical strength. Higher-rank coal with relatively higher total organic carbon (TOC) content has lower pore accessibility in smaller pores than does lower-rank coal with relatively lower TOC content, and vice versa, indicating that organic-matter pores tend to be disconnected from one another. There is a power-law correlation between pore accessibility and pore size that is confirmed in both the experimental and theoretical results. The theoretical model is derived based on the power-law scattering theory. Generally speaking, pore accessibility increases with increasing pore size for the tested samples. The anisotropic pore network is shown to affect gas transport and storage in unconventional gas reservoirs. The coal bedding direction has a smaller surface area but comparable pore volume and porosity to the direction perpendicular to the bedding. The shale bedding direction has a slightly lower surface area but higher pore volume and porosity compared to the direction perpendicular to the bedding. The pore properties’ values are smaller for the coal than the shale. The fraction of accessible pores follows the trend: C_ac of powder sample > C_ac along the bedding direction > C_ac along the vertical direction between the tested coal and shale. The coal has less C_ac than the shale, indicating that organic-matter pores may have lower connectivity than mineral-matter pores. Based on the aforementioned pore structure analyses, we argue that gas may tend to be transported along the bedding direction whereas it tends to be stored along the vertical direction for both coal and shale. However, gas transport is much harder in coal due to the extremely tight structure. Moreover, the fractal dimension, known as the degree of self-similarity or irregularity, is an important parameter to quantitatively characterize gas storage capacity and gas transport properties in the pores of the rock matrix. Surface fractal dimension D_s of inaccessible pores is greater than that of total pores according to the SANS results for all four tested samples. The D_s of accessible pores estimated by N2 desorption is greater than that of N2 adsorption for each linear section of each tested sample. Based on the in-situ SANS results, D_s slightly decreases for San Juan coal with continuous argon injection. D_s decreases with increasing methane and CO2 pressure for samples with relatively higher D_s. However, D_s significantly increases when CO2 enters the liquid phase for samples with relatively lower D_s. Stability results show that D_s hardly changes after methane and argon penetrations for all these samples, with the exception of Marcellus outcrop shale. In addition, gas density in rock nanopores is influenced by both the compressive storage governed by the gas equation of state (EOS) and the adsorptive storage governed by the Gibbs energy-driven adsorption. Estimating and modeling methods of gas density, as well as the estimating method of adsorbed gas volume fraction, are proposed to quantitatively characterize pore size-dependent densification and adsorption in the accessible nanopores of coal and shale. Gas densification is more significant for small nanopores compared to large ones for both coal and shale, where CO2 has a higher density than methane. The adsorbed phase densities of methane and CO2 are directly estimated. The adsorbed phase density of coal may linearly increase with increasing pressure. There may be a Langmuir type correlation between the adsorbed phase density and pressure for shale. For both coal and shale, pore-filling adsorption is shown to occur in small nanopores, while multi-layer adsorption occurs in large nanopores. Therefore, gas densification and adsorption in rock nanopores may be pore size-, pressure-, gas type- and rock type-dependent.