Numerical analyses of the long term behavior of enhanced geothermal system (EGS)

Enhanced Geothermal System (EGS) technology is a challenging extrapolation of conventional hydrothermal geothermal systems but expected to significantly contribute to U.S. energy in near future. This technology is still going through a steep learning phase and current research studies are mostly focused on reservoir performance and initial stages of the EGS. The main focus of this research is to gain a better understanding of thermal stresses generated during the heat recovery process as a result of induced cooling of rocks and interaction of thermal stresses with existing lithologic stresses. The long term behavior of the EGS is assessed in terms of stress redistribution and reservoir performance based on selected operation, design, and geologic variables. Existing concepts of stress generation and redistribution are revisited, improved and supported with numerical models. Eighteen different models are developed using COMSOL, a finite element numerical modeling environment, based on the three study variables, i.e., production rate, number and pattern of boreholes, and thermal properties of rock, to provide scenario comparisons, effect quantification, analytical reasoning, and factual explanations. The three study variables are found to be functioning interdependently to define the performance and the final stress state of the EGS. Production rates are shown to delineate rates of heat removal, number and pattern of injection and production borehole(s) are shown to define flow paths, and thermal properties of rock are shown to have control over heat flows. Magnitudes of thermal stresses generated are found to be 35 to 45 MPa; stress redistribution is shown to relieve applied in-situ stresses by magnitudes equal to the tensile thermal stresses. The findings of this study provide original quantification of stress redistribution in the EGS with respects to study variables. Outcomes of this research will serve as a foundation for assessing the long term effects of the EGS and the extent of the effects on the surrounding rock mass. Also, considering the acceptable magnitudes of stresses produced by full scale numerical models of multiple EGS scenarios, the EGS is suggested to successfully and safely fulfill a portion of our future energy needs.

Name thesis:degree_name

Doctor of Philosophy (Ph.D.)

Level thesis:degree_level

Doctoral

Discipline thesis:degree_discipline

Mining Engineering

Grantor dc:publisher

Colorado School of Mines. Arthur Lakes Library

Year dc:date.issued

2016

Author dc:creator

• Arshad, Hafiz Syed Mahmood

Advisor dc:contributor.advisor

• Nakagawa, Masami

Committee members dc:contributor.committeemember

• Griffiths, D. V.
• Holley, Elizabeth A.
• Kaunda, Rennie

• enhanced geothermal system
• stress redistribution
• thermal stresses
• geothermal
• EGS
• thermal flow

Statement dc:rights

• Copyright of the original work is retained by the author.

Language dc:language.iso

eng, English

Identifier

T 8174

OAI identifier oai:identifier

oai:repository.mines.edu:11124/170609