1 research outputs found
Selection of Shale Preparation Protocol and Outgas Procedures for Applications in Low-Pressure Analysis
The low-pressure
gas adsorption (LPGA) method for estimation of
pore capacities, pore size distributions, and total surface area using
adsorption–desorption isotherms is selected as an effective
technique in pore characterization. A recent application of this method
is to understand the complex and heterogeneous nature of shales across
the globe. The LPGA experiments were conducted on shale samples from
Barnett and Eagle Ford formations in the United States using CO<sub>2</sub> for micropores of 0.3–1.5 nm in diameter and N<sub>2</sub> and Ar as the adsorbates to focus on micropores from 1.5
to 2.0 nm and the lower range of mesopores above 2.0–27 nm
in diameter. It was hypothesized that a significant error in estimations
could occur due to inconsistencies in the shale outgas temperatures.
It was observed that lower pore capacities result from lower outgas
temperatures, and higher pore capacities result from increasing outgas
temperatures. It is hypothesized that lower outgas temperatures fail
to completely eliminate adsorbed moisture and adsorbed low-molecular
weight hydrocarbon species from shale pores, which leaves the pores
partially filled and as such result in lower values of pore capacity.
By increasing the outgassing temperature, the adsorbed species in
the pores are completely removed, yielding higher pore capacities.
The cutoff temperature of 250 °C during outgassing for regeneration
of “clean” shale pores was arrived at by analyzing the
LPGA results of samples without any outgassing and samples outgassed
at 60, 110, and 250 °C. The 250 °C maximum outgas temperature
is intended to maximize the results of LPGA while minimizing structural
changes to shales. Mass stabilization as shown by thermogravimetric
analysis and magnetic suspension balance measurements support the
assertion that the shale is not fundamentally altered by processes
such as kerogen cracking until a temperature higher than 250 °C
is reached. The kerogen had approximately 3.0% weight loss at 110
°C, with an additional 1.3% loss between 110 and 250 °C.
Likewise, the desorption experiments carried out on clay at 110 °C
were approximately 1.3%, with an additional 0.5% loss between 110
and 250 °C. On the basis of the interpretation of pore size distributions
using the LPGA method, it was concluded that accurate shale characterization
is achieved when the analysis is limited to results from relative
pressures (<i>P</i>/<i>P</i><sub>o</sub>) less
than or equal to 0.90. At higher relative pressures, the sizes of
the adsorbate-occupied pores cannot be distinguished