A Study of Fracturing and Heat Transfer in Enhanced Geothermal Systems


EGI Research Assistant Seth Craig Receives Ph.D.

EGI Research Assistant Seth Craig recently defended his dissertation, titled A Study of Fracturing and Heat Transfer in Enhanced Geothermal Systems, in an open presentation at the EGI offices in Salt Lake City. Seth’s extremely well received presentation was attended by EGI staff and fellow students as well as Seth’s advisors, mentors, and research colleagues.

Seth has worked as a Research Assistant at EGI since 2009. Working closely with Dr. Joseph Moore and Dr. John McLennan, Seth contributed to expanding the scientific knowledge surrounding Enhanced Geothermal Systems as contributors to energy systems and application to energy technology. Specifically, Seth has been involved in active research on the Raft River Enhanced Geothermal System Thermal Stimulation project. He has presented elements of his research at the Stanford Geothermal Workshop and Geothermal Resource Council Proceedings.

Seth received a M.Sc. in Mechanical Engineering from the University of Utah and a B.Sc. from Southern Polytechnic State University. He was a Fellow of the Nuclear Regulatory Commission from 2007-2008 and in 2009 published “Enhanced Heat Transfer From Geothermal Systems Using Pressure Cycling” for the Geothermal Resource Council Proceedings. In 2014 Seth published “An Experimental Study of Thermal and Hydraulic Geothermal Reservoir Stimulation of Brittle Impermeable Material” for the Stanford Workshop on Geothermal Reservoir Engineering Proceedings.

Seth’s Ph.D. committee members are Dr. Kent S. Udell (Chair), Dr. Eric Pardyjak, Dr. Kuan Chen, Dr. Joseph Moore, and Dr. John McLennan.

The Dissertation abstract appears below. For further details, the Dissertation Executive Summary is available on LinkedIn at https://www.linkedin.com/pub/seth-craig/35/85/8b6. Seth welcomes interaction and feedback via his LinkedIn account.

Dissertation: A Study of Fracturing and Heat Transfer in Enhanced Geothermal Systems

ABSTRACT:

Enhanced Geothermal Systems (EGS) have the potential to tap vast amounts of energy. In order to improve EGS functionality an in depth experimental and computational study of the heat transfer and fracture mechanics of bench top geothermal rock simulations was performed. These experiments contribute to the understanding of hydraulic and thermal fracturing as well as the effects of different heat transfer modes that can be used for heat mining. The work was conducted as follows:

  • A comparison of fluid flow and vaporization, heat transfer in a fracture in simulated hot dry rock was performed.
  • A plane strain experimental examination of hydraulic and thermal fracturing was conducted to validate theoretical results and study the fracture morphologies.
  • Thermal fracturing of cement paste, acrylic, and granite was examined computationally to understand the role of flaw orientation on resultant fracture geometry in a wellbore.

Proof of concept experiments were performed to evaluate the heat mining potential of a new and innovative way to operate an Enhanced Geothermal System. By injecting water into hot dry rock, allowing it to thermally equilibrate and then dropping the pressure, steam can be produced at a large rate of heat transfer from the rock. This process has a distinct advantage of only needing one well to function. It was found that the steam generation has around 10 times higher heat transfer rates than that of low Peclet number single phase constant flow, which would be found in the reservoir away from the preferential flow pathways.

Experimental work was performed to evaluate the fracture morphology from hydraulic and thermal fractures. One of the purposes of this work was to validate the concept of creating thermal fractures that have faces perpendicular to the maximum horizontal earth stress. The bench top experimental analog that has been created to accomplish this was uniaxially loaded and therefore only has one principal stress. Regardless of this, thermal fractures were created for the first time, in 3 dimensional specimens, that have faces perpendicular to the maximum principal stress.

Finally a finite difference thermoelastic code with a linear elastic fracture mechanics application was created. It has the capability of applying various types of heat transfer and evaluating the thermal stresses and fracture nucleation potential. It was concluded that the circumferential fractures that were created experimentally in acrylic occurred from flaws that are at least four times larger in that orientation from drilling.