Long-term permeability evolution of shale seal rocks with argon and scCO2

Abstract

We conducted a suite of experiments to evaluate the long-term permeability evolution of shale rocks under constant effective stress, before, during, and after interaction with supercritical carbon dioxide (scCO₂). To do so, we measured the time-dependent evolution of argon permeabilities for the pre- and post-reacted samples (i.e., before and after long-term interaction with scCO₂). In addition, we obtained permeability evolution during long-term interaction with scCO₂. The samples showed either relatively constant permeabilities or a moderate decrease during pre-reaction long-term argon tests. The permeability evolution during long-term CO₂ tests showed continuous increase, continuous decrease, or cycles of increase/decrease in permeability. The long-term response of the samples to CO₂ included phenomena such as (i) salt precipitation, (ii) swelling-induced cracks, and (iii) carbonate dissolution. While it is obvious that salt precipitation and swelling-induced cracks decrease and increase the permeability, respectively, the sample response to carbonate dissolution proved to be more complex and may increase or decrease the permeability. The permeability evolution during post-reaction long-term argon injection is also affected by the contribution of each of these three phenomena, during long-term interaction with CO₂. We observe increase, decrease, and constant permeability evolution during post-reaction argon tests. Our experiments reveal that the initial permeability of the samples plays a significant role on the long-term permeability response of shales in the presence argon and scCO₂ fluids. This study shows that when shales are hydraulically fractured with CO₂ their initial permeability has a more significant role than their permeability evolution over time.