Date of Award

Spring 1-1-2019

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

First Advisor

Shemin Ge

Second Advisor

Anne F. Sheehan

Third Advisor

Kristy F. Tiampo

Fourth Advisor

David A. Budd

Fifth Advisor

Jonathan W. Godt

Abstract

Injection-induced seismicity caused by wastewater injection is a continuing problem for the central and eastern United States. Mitigation of induced earthquakes often focuses on operational parameters like injection rate. While pore pressure increase has been the main mechanism invoked in injection-induced seismicity, other mechanisms like Coulomb static stress transfer may play a role. In this dissertation, I examine the mechanisms of injection –induced earthquakes in relation to mitigation.

I investigate the role of aggregate injection rate, the combined injection rate of multiple wells, by modeling pore pressure increase caused by 22 wastewater disposal wells injecting into the same disposal zone within 30 km of seismicity in Greeley, Colorado. I find that the wells 15 – 30 km from the seismicity contribute approximately 44% of pore pressure increase at the location of the earthquakes. Therefore, aggregate injection rate and well spacing is important when planning mitigation strategies.

I also derive a simple relation between pore pressure change and surface deformation that can be used to constrain hydraulic parameters of confined aquifers to a first-order. This relation can estimate expected surface deformation associated with pore pressure model results, which can then be compared to observed surface deformation using geodetic techniques. I validate this relation by constraining the storativity of an aquifer in Texas that experienced uplift associated with wastewater disposal.

Finally, I investigate the role of small magnitude earthquakes in induced seismicity. I use generic models to test if small magnitude earthquakes can cumulatively transfer, through earthquake interactions, stress of a magnitude comparable to pore pressure increase from wastewater injection. I find that the stress caused by earthquake interactions (Coulomb static stress transfer) is comparable in magnitude to pore pressure increase. However, the area influenced by the increased stress is much smaller than in pore pressure diffusion. This means that earthquake interactions may induce more earthquakes though over a smaller area than pore pressure increase. If earthquake interactions induce additional events, reduction in injection rate or even shutting down a well may not mitigate seismicity. Therefore, earthquake interactions should be taken into account when planning mitigation, especially the timing of mitigation measures.

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