The Physicochemical and Systems Level Implications of Using CO2 as a Working Fluid in Hydraulically Fractured Shale Gas Wells

The rapid growth of natural gas production from unconventional gas shale formations over the past decade has changed the US energy landscape dramatically. This growth has been enabled by directional horizontal drilling and hydraulic fracturing technologies, which allow engineers to access larger portions of a host rock and improve its permeability. The most effective hydraulic fracturing operations use millions of gallons of water at each wellhead. Even though much of this water is returned and must be treated, the majority remains trapped within the target formation. The fate of these large volumes of water and its impact on gas production are still poorly understood. This work seeks to answer critical questions concerning the fate of water in unconventional shale wells and to understand how the design of these fluids might be engineered to improve production, reduce wastewater generation, or both. The role of fluid-solid interfacial properties in these processes is studied in depth and the possible role for non-aqueous alternatives, specifically carbon dioxide is explored. Finally, the system-level environmental impacts of switching to waterless fracturing operations are quantified using a life cycle assessment framework.
To better understand the fate of water during fracturing operations, a Marcellus shale core sample from the Marcellus Shale Energy and Environment Laboratory (MSEEL) experimental well in West Virginia was characterized to understand its pore structure, minerology, and interfacial characteristics. Its wettability to synthetic fracturing fluid and CH4 was measured at high temperatures and pressures. These results were then used to simulate water imbibition and gas production flow dynamics in a hydraulically fractured shale well using the TOUGH2 code. The results suggest that water imbibition increases 125% as contact angle decreases from 85 to 5 degrees, while the water produced/imbibed fraction decreases 73%. Using the measured interfacial properties, gas production rates were simulated for the MSEEL well with good agreement between field data and simulation results. Exploring different fracture spacing scenarios in the model and comparing results with field data reveals information about fracture area otherwise not available. These findings provide new insight into the fate of water in fracturing operations and could inform improved design of hydraulic fracturing fluids.
While our experiments and modeling suggest that the properties of fracturing fluids had a small impact on natural gas production, most estimates suggest that only 15-25% of the gas originally in these well is actually produced. This inefficiency could be tied to the geomechanics of fractures or it could be tied to the presence of natural gas liquids, which are common in many regions of the Marcellus and other shale plays. To help elucidate the role that capillary forces might have in regions with higher proportions of natural gas liquids (NGL’s), experiments and modeling were performed on the MSEEL core using different fluid pairs. Contact angles for slickwater in propane and a propane/methane mixture were measured confirming shale likely becomes hydrophobic as NGL concentration and pressure increase, impacting capillary forces. As a non-aqueous alternative, CO2 is miscible with CH4 which reduced the role of capillary forces in mass transport. CO2 is also though to increase fracture complexity, which could increase the fracture area and gas production rates. Model simulations with CO2 confirmed fracture complexity increases gas production 133% in addition to an increase of 33% if NGL’s are present. Results further show CO2 fracturing has the potential to sequester up to 1 x 107 kg CO2 per well. These data are assimilated to provide the first geospatially explicit data of CO2 fracturing and storage potential for the Marcellus region.
To quantify the impact of hydraulic fracturing on the environment, water resources, and energy consumption, a Life Cycle Analysis (LCA) model was developed to analyze key impacts for a typical gas well in the Marcellus shale. This analysis compared the impacts of fracking for three scenarios: (1) a base-case well fracked with water-based (slickwater) fluids, (2) a base-case well fracked with CO2 using current data, and (3) a forward looking CO2 outlook scenario using parameters which assume key advances in these processes. The impact on energy (MJ), greenhouse gas emissions (GHG in CO2 equivalents), and water consumption (m3) was measured for each scenario for the functional unit of lifetime energy production (GJ natural gas). LCA results show CO2 based fluids have the potential to reduce GHG emissions by 400% and water consumption by 80% compared with conventional water-based fluids. However, these are offset by a 44% increase in net energy consumption, pointing to the need to reduce CO2 transport and processing energy requirements while pursuing improved processes to increase natural gas recovery.