Effects of supercritical CO2 on matrix permeability of unconventional formations

Abstract:

There has been a growing interest in stimulation of unconventional formations with supercritical CO₂ (scCO₂), a non-aqueous fluid with a viscosity 10-20 times less than water. Injection of scCO₂ is associated with two main chemo/mechanical processes - dissolution of carbonates and adsorption and swelling of clay minerals and/or organic matter. The former typically increases permeability (but can weaken the rock matrix), while the latter could result in a decline in permeability. Therefore, interaction with scCO₂ might decrease or increase matrix permeability depending on the dominance of each of these processes in formations with different lithologies.

In this study, samples from Wolfcamp and Eagle Ford were used to first estimate permeability using argon gas (as a non-sorbing pore fluid). Then, we let each sample to interact with scCO₂ for three to four days and its permeability was measured. Lastly, another cycle of permeability test was conducted using argon gas. This sequence of permeability tests enabled us to evaluate the effects of scCO₂ on matrix permeability by comparing the pre- and post-CO₂ argon permeabilities. A four-order-of-magnitude variation was observed in initial argon permeabilities prior to introducing scCO₂. Initial permeabilities varied from less than 1 nanodarcy to more than 1 microdarcy. In general, the lowest permeability samples had the highest carbonate content. The scCO₂ permeability is more stress-dependent compared to argon permeability (with a few exceptions), indicating the softening of the samples after exposure to scCO₂. There is more than one order of magnitude difference in pre- and post-CO₂ argon permeabilities for carbonate-rich samples. The adsorption-related decrease in permeability with scCO₂ depended on the amount of “clay+TOC”. The adsorption-related decrease in permeability was fully reversible when scCO₂ was removed from the samples. While the siliceous samples showed moderate change in their post-CO₂ argon permeability. Mineralogical differences can, in-part, explain the contrast between samples. In the carbonate-rich samples, it is more likely that dissolution of carbonate minerals (mainly calcite) is the dominant process occurring with the injection of scCO₂, while dissolution does not significantly affect the siliceous samples.