A study of fracturing and heat transfer in enhanced geothermal systems
Creators: name: Craig, Seth Lorenzo
Publisher: name: University of Utah
Year: 2016
Subjects: EGS
Experimentation
Heat Transfer
Hydraulic Fracturing
Raft River
Thermal Fracturing

description: Enhanced Geothermal Systems (EGS) have the potential to tap vast amounts of energy.  In order to improve EGS functionality, in depth experimental and computational studies of the heat transfer and fracture mechanics of bench top geothermal rock analogs were 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: 1.Heat transfer rates in a hot dry rock analog containing a circular hole were quantified experimentally and computationally for single-phase fluid flow, and for water vaporization resulting from pore pressure reduction. 2.An experimental examination of hydraulic and thermal fracturing in plane strain was conducted to validate theoretical results and study the fracture morphologies. 3.Thermal fracturing of cement paste, acrylic, and granite was examined experimentally and 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 flow, characteristic of conditions found in the reservoir away from the wellbore and 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 was created to study thermal fracturing by uniaxially loading the specimen, thus creating conditions with only one principal stress which is perpendicular to the axis of the hole.  Thermal fractures were created and observed with faces that are perpendicular to the maximum principal stress in 3-dimensional specimens for the first time since they were theorized in the 1970s.  Finally a finite difference thermoelastic code with a linear elastic fracture mechanics assessment was created in order to evaluate the effect of various types of heat transfer on 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.  In order to create thermal fractures in geologic reservoirs that are perpendicular to the maximum horizontal principal stress, half an order of magnitude larger flaws or preexisting fractures would have to exist in that orientation than features parallel to the maximum horizontal principal stress.