Mechanical and Geochemical Assessment of Hydraulic Fracturing Proppants Exposed to Carbon Dioxide and Hydrogen Sulfide

The Energy & Environmental Research Center (EERC) has conducted a study to determine the mechanical and geochemical changes experienced by proppants as a result of exposure to CO₂ and H₂S in simulated reservoir conditions. This study was conducted under a cooperative agreement with the U.S. Department of Energy's National Energy Technology Laboratory with the goal of investigating factors affecting production in the Bakken Formation of North Dakota.

As hydraulic fracturing is critical to successfully producing tight oil, proppant longevity is also critical to maintain fracture conductivity to the wellbore. Long-term conditions such as reservoir souring and the potential for CO₂ enhanced oil recovery (EOR) within the Bakken were considered in the study to examine the effects on proppant strength. The possibility exists for proppant degradation due to the large amount of fluids that travel through fractures, the high surface area of the proppant within the fractures, and the already taxing subsurface conditions.

In order to understand the potential degradation of proppant compressive strength, samples were acquired that represent common proppants, including both natural and synthetic materials. Samples were subjected to simulated downhole conditions and exposure to supercritical CO₂ and acid gas containing 85% CO₂ and 15% H₂S. Effects of the presence or absence of brine was also examined. Samples removed from the reactor were subjected to one-dimensional confined compressive strength testing and sieve analysis to examine the amount and size of damaged material compared to baseline, as-received material.

Preliminary results indicate that all proppants show varying degrees of strength loss postreaction, and work is being performed to determine whether the causation is a product of brine interaction, reactor pressurization, or reactor depressurization, or is indeed due to mineralogical gas/proppant interactions. The results of this study will aid in the selection of optimal proppant materials for long-term or multiple-use performance in unconventional oil shale and gas shale production wells, as well as potential for retrofitting a well for injection purposes. The work has further applications with the potential use of hydraulic fracturing for increasing the injectivity of CO₂ relative to carbon storage.