In partial fulfillment of the requirements for the degree of

Doctor of Philosophy in Ocean Science and Engineering

In the School of Civil and Environmental Engineering

Savannah L. Howard

Will defend her dissertation

FUNDAMENTAL HYDROGEOMECHANICAL PROPERTIES OF CARBONATE FORMATIONS: IMPLICATIONS FOR OFFSHORE COMPRESSED AIR ENERGY STORAGE

May 5th, 2026 at 8 AM

Ford ES&T, Room 3243 (The Ocean Room)

Teams: https://teams.microsoft.com/meet/252549462205277?p=eDS8aBGnt00SviVjBt

Meeting ID: 252 549 462 205 277

Passcode: hx7Vz7zX

Thesis Advisors:

Dr. Sheng Dai

School of Civil and Environmental Engineering

Georgia Institute of Technology

Committee Members:

Dr. Philipp Braun

Navier Laboratory

Ecole des Ponts, Paris Tech (ENPC)

Dr. Kevin Haas

School of Civil and Environmental Engineering

Georgia Institute of Technology

Dr. Joseph Montoya

School of Biological Sciences

Georgia Institute of Technology

Dr. Zhigang Peng

School of Earth and Atmospheric Sciences

Georgia Institute of Technology

Summary:

The U.S. energy grid faces a critical storage challenge: renewable sources such as wind and solar generate electricity intermittently, while demand peaks occur independently of generation. Among available grid-scale storage technologies, Compressed Air Energy Storage (CAES) — which stores energy by compressing air underground and releases it through turbines on demand — offers the lowest levelized cost of storage of any current technology, surpassing batteries, pumped hydro, hydrogen, and thermal systems. Existing CAES plants (Huntorf, Germany; McIntosh, Alabama) use salt caverns, which are geographically restricted. Porous carbonate formations, distributed across the southeastern U.S. coastal zones where offshore wind and wave energy resources are abundant, offer a far more geographically widespread alternative — but their suitability as CAES reservoirs under realistic subsurface conditions has never been systematically characterized.

This dissertation presents a comprehensive laboratory characterization of three distinct carbonate formations - St. Maximin, Miami, and Hemingway. Experiments characterized the full suite of properties relevant to CAES viability, including mineralogy (XRD), density, total and accessible porosity, micro-CT pore imaging, permeability, ultrasonic wave speeds and elastic moduli, unconfined compressive strength and failure mode (via acoustic emissions), water retention curves, creep behavior, and cyclic loading response. Energy storage density was calculated directly from measured pore and mechanical properties and benchmarked against the two operating CAES plants. The results highlight that pore geometry and connectivity must be characterized jointly with porosity for meaningful CAES modeling; water retention and evaporation govern energy storage density; creep is saturation-dependent and operationally significant; and cyclic pressurization does not cause rapid structural and strength deterioration, but cumulative strain can lead to failure in long-term operation. Calculated energy storage densities for offshore carbonate CAES are competitive with, and in high-porosity cases exceed, those of existing salt cavern plants (~7.5–18 MJ/m³), demonstrating the technical feasibility of porous carbonate formations as CAES reservoirs. The work establishes the first systematic experimental dataset for air–water saturated carbonate reservoir media under CAES-relevant conditions, filling a critical gap in a field previously dominated by oil–water and CO₂ storage studies. Findings directly inform site selection criteria, reservoir performance modeling, and operational safety assessments for future offshore CAES development in the southeastern U.S. — a region that combines abundant offshore renewable energy resources with extensive shallow carbonate geology.