Civil Engineering ETDs

Publication Date

Spring 2-6-2018


Wellbore integrity is a principal concern for the safe and efficient underground storage of hydrocarbons, disposal of various wastes and in oil and gas production. Poor wellbore integrity can lead to leakage and loss of extracted or stored material, and the potential for contamination of groundwater. The microannuli, which is the space between the steel casing and the cement sheath has been identified as a common and critical leakage pathway. There are no data of the actual size or character of microannuli; however, it is possible to estimate the hydraulic aperture of the microannulus from flow measurements at the wellhead. This information, while providing some insight, could be misleading since it represents the microannuli as uniform annular gap along the wellbore. We created wellbore-shape samples with microannuli between the steel casing and cement and measured gas flow through them to obtain their hydraulic aperture and their permeability. We injected the microannuli with epoxy after the gas flow measurements, sliced them into five circumferential sections per specimen and measured the microannulus along the steel casing using microphotography. We measured the aperture sizes and the contact between the steel and the cement, creating profiles of the microannulus for each circumferential section. The aperture sizes were fit to different statistical distributions. The capillary entry pressure for CO2 displacing brine in the microannulus was estimated from the measured aperture size. We found that the capillary entry pressures estimated from the actual aperture sizes were generally much greater than that estimated from the hydraulic aperture of the entire specimen, resulting in a wide range of values, usually far from the interpretation obtained from the flow test. The measured aperture sizes were also used to evaluate the possible repair of the microannulus by estimating the penetration of cementitious materials into the microannulus; we found that the repair is unlikely to be effective for microannuli with a hydraulic aperture less than 50 µm. We conducted a convergence study to estimate how many circumferential sections were necessary to analyze to obtain a representative value of permeability and hydraulic aperture for a specimen.


Microannuli, wellbore integrity, characterization

Document Type




Degree Name

Civil Engineering

Level of Degree


Department Name

Civil Engineering

First Committee Member (Chair)

John C. Stormont

Second Committee Member

Mahmoud R. Taha

Third Committee Member

Edward Matteo