Date of Award
12-2024
Document Type
Thesis
Degree Name
Master of Science (MS)
Department
Environmental Engineering and Earth Science
Committee Chair/Advisor
Lawrence Murdoch
Committee Member
Leonid Germanovich
Committee Member
Ronald Falta
Abstract
Climate change is a critical global problem caused by increasing CO2 concentrations in the atmosphere, and rising sea-level is recognized as a major factor behind increased flood risk worldwide. It has been proposed that injecting biomass into the subsurface can address both problems by storing carbon in geologic formations to reduce CO2 levels and raising ground elevations to reduce flood risk. This process, known as Carbon SIRGE, is implemented by injecting wood, or other biomass, as a proppant into horizontal hydraulic fractures. Wood injected into anoxic conditions in the subsurface is essentially inert, creating a long-lasting sink for carbon. Each hydraulic fracture will raise the elevation of the ground surface slightly, but significant uplift can be achieved by repeating the injection process. The Carbon SIRGE concept was evaluated in the field by injecting sawdust into 10 shallow boreholes (2.4m depth) multiple times over one month. The ground surface was surveyed after each injection and the surface displacements created broad gentle domes.
The results of the field test demonstrated that it was feasible to inject wood particles many times into the same borehole to store carbon and raise ground elevation. These results are encouraging, but the slope in the field test would disrupt some surface structures so modifications could be required for the process to be useful for some applications of flood protection. The objective of this project is to develop methods for designing and monitoring the injection process so the resulting slope is less than values that would disrupt surface structures. Such methods would be a first step toward applications for flood protection. The research approach is to evaluate uplift gradient that could be tolerated by different categories of land use and surface structures, and then develop and calibrate mechanical models to evaluate the conditions required to meet those tolerances.
Uplift gradient, (also called differential settlement, angular distortion, slope, differential displacement) is used in building codes, design specifications, and guidelines for most built structures. They all can tolerate some degree of uplift gradient, but the degree of tolerance varies with structure type. Many guidelines for buildings specify that angular distortion should be less than 1/200 to 1/500. For roads the limiting factor is the cross-slope which must be below 2%.
A finite element model of the injection and displacement process during Carbon SIRGE was developed to calculate the factors affecting uplift gradient and to evaluate the feasibility of achieving designs that could meet the tolerance requirements. These models have the capability to simulate the displacement of multiple injections from multiple injection locations to create a singular uplifted zone. The models also use a conceptual model developed during this work that accurately represents the behavior of multiple injections into a singular borehole and the random behavior associated with the location of the center of the fracture within the subsurface in relation to the injection borehole.
This model was validated by predicting the results from the field test, and then modifying the parameters to evaluate other scenarios. One design considers raising elevations of a two-lane road using borings that are located off the shoulder of the road. Other example designs include an array of borings that fit an urban landscape to accommodate raising areas of a city. Specifically, the simulation was run using a real neighborhood in Charleston, SC and the complexity of its road and houses as the geometry. This required multiple different injection categories and hundreds of injection locations. The simulations indicate that it is feasible to create meter-scale uplift with gradients within the low tolerance range over an array of borings by adjusting the distance between boreholes, array configuration, and depth of injection and other factors.
Recommended Citation
Gordon, Max Joseph, "Creating Useful Uplift with Horizontal Hydraulic Fractures" (2024). All Theses. 4409.
https://open.clemson.edu/all_theses/4409