Date of Award

Spring 1-1-2017

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Civil, Environmental & Architectural Engineering

First Advisor

Harihar Rajaram

Second Advisor

Roseanna M. Neupauer

Third Advisor

Satish Karra

Fourth Advisor

Owen A. Sherwood

Fifth Advisor

Joseph N. Ryan

Abstract

Faulty oil and gas wells, with compromised structural integrity, serve as a pathway for hydrocarbons between the deep and shallow subsurface. In some cases, fugitive gas escapes the wellbore and contaminates the surrounding groundwater. We gather and analyze records of surface casing pressure (SfCP) from the Colorado Oil and Gas Conservation Commission database to assess the frequency of oil and gas well integrity loss in Colorado. We develop a physically meaningful well-specific rubric to identify oil and gas wells that pose a greater risk for groundwater contamination. Wellbore deviation and subsurface geology are the biggest factors that contribute to oil and gas well integrity loss. Thus, newer unconventional wells with deviated wellbores are more prone to integrity loss than older vertical wells. However, improved well construction practices in the region for newer wells have reduced their risk of groundwater contamination. To further investigate the mechanisms that control stray gas migration, we develop a model of methane leakage along a wellbore that simulates the buildup of SfCP. Our model is the first to consider methane leakage through both a closed annulus, fully cased along its length, and open annulus, left open to the surrounding formation beneath the surface casing. Our results explain SfCP observations for wells constructed on the Colorado Front Range and show that the consequences of integrity loss are greater for wells with an open annulus than wells with a closed annulus. We also provide the framework for future modeling studies that will couple our model of SfCP buildup with a porous media code to fully simulate stray gas migration away from a faulty well.

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