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 CO2 for micropores of 0.3-1.5 nm in diameter and N-2 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 degrees 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 degrees C. The 250 degrees 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 degrees C is reached. The kerogen had approximately 3.0% weight loss at 110 degrees C, with an additional 1.3% loss between 110 and 250 degrees C. Likewise, the desorption experiments carried out on clay at 110 degrees C were approximately 1.3%, with an additional 0.5% loss between 110 and 250 degrees 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 (P/P-o) less than or equal to 0.90. At higher relative pressures, the sizes of the adsorbate-occupied pores cannot be distinguished.