Use of Horizontal Drilling for CO2 Sequestration in Low Storage Capacity Aquifers: from Impossible to Feasible

Abstract

Abstract The United States Department of Energy (DOE) Carbon Storage Assurance Facility Enterprise (CarbonSAFE) focuses on developing geological storage sites that can accommodate more than 50 million metric tons of Carbon Dioxide (CO2) over 25 years period. Few formations can accept this volume of CO2 through one classic vertical injection well. Multiple injection wells are usually needed to handle the targeted CO2 volume, with well spacing of several miles to avoid any pressure interference between the injectors. Nebraska is among the largest ethanol-producing states in the USA, with 25 ethanol plants that produce more than 17 million metric tons of ethanol per year. These plants produce a significant volume of CO2 as a typical ethanol plant produces around 150,000 metric tons of CO2 annually. Several techniques have been proposed to capture and sequestrate the emitted CO2, including mineral carbonation and carbon geological storage. Among these techniques, carbon geological storage is the most feasible option, especially sequestration in deep saline aquifers because of the larger volume that can be stored underground, and lower cost compared to the other techniques. Most of the ethanol plants are located on the eastern side of the state, while geological evaluation suggests that thick aquifers that can handle the large volume of CO2 are located in the southwest area of the state. Due to the high cost of building more than 100 miles of pipeline to transport the CO2 from the source to the injection point (pipeline costs around one million dollars per mile), thin aquifers have been identified locally near the plants to receive the generated CO2 volume. However, conducting CO2 injection operations through multiple scattered wells will increase the anticipated cost, including pore space rights, well drilling cost, land acquisition, CO2 transportation between sites, multiple injection systems and high-pressure pumps, labor, and injection monitoring. Drilling horizontal wells can maximize the volume of CO2 that can be injected in a single well at lower injection pressure than a vertical well. The long horizontal section will expose a larger formation volume and increase the surface area available for CO2 to flow through. St. Peter formation has been identified as one of the thin candidate formations to inject CO2 in the eastern part of Nebraska. The injection modeling conducted in this study shows that a single horizontal well with a lateral of 2,000 to 3,000 ft can replace at least three classic vertical injection wells.