**1. Introduction**

The potential of Enhanced Geothermal Systems (EGS) is well documented in the MIT led study titled "The Future of Geothermal Energy" [1]. With this technology, unconventional deep Hot Dry Rock (HDR) reservoirs are engineered with drilling and stimulation techniques to create a heat mining system for base load energy production. The methods needed for enabling EGS energy production also have the ability to improve production from traditional geothermal resources which are already being utilized today.

To provide the EGS reservoir stimulation, one of the most promising techniques is hydraulic fracturing. This method utilizes high pressure fluid injection into targeted reservoir intervals to enhance permeability and generate new flow paths through enhancing existing fractures and creating new fractures. With the installation of an injector-producer well scheme, the physical limitations of natural reservoir recharge and stored harvestable fluids may be overcome and a productive reservoir may be the end result. Hydraulic fracturing has been proven effective as a stimulation technique by the oil and gas industry since its first implementation in 1947 [2].

**2.1. Heated true triaxial cell**

**Figure 1.** Layout of the true-triaxial cell.

The layout of the heated true triaxial cell is shown in Figure 1. It consists of a cylindrical loading rig made of high strength steel. Flatjacks apply pressures on all six faces of a 30x30x30 cm3

Scale Model Simulation of Hydraulic Fracturing for EGS Reservoir Creation Using a Heated True-Triaxial Apparatus

rock sample. Freyssinet 350 mm flatjacks, which are pressurized with pumps, allow independ‐ ent control of the principal stresses of up to 12.5 MPa. The flatjack pressures can be controlled to achieve triaxial stress conditions with different magnitudes of overburden stress σ v, maxi‐ mum horizontal stress σ H, and minimum horizontal stress σ h. Externally mounted flexible silicone rubber heaters with proportional-integral-derivative (PID) control allow for dualzone heating with separate set points for the lateral and vertical heating elements. The heating system allows for the simulation of an EGS reservoir with a temperature of up to 180 ºC.

Figure2showspicturesofthecompletedtruetriaxialcellwithandwithoutthedrillingrigplaced on top of the cell. An orientated rotary-hammer drill press is used to drill boreholes into the sample at user selected positions and angles while the sample is under stresses and tempera‐ ture. This procedure allows for strategic borehole installations that are specific to the test and the particular stimulated fracturing plane. Borehole damage is replicated by using percussive drilling into the loaded sample instead of the more common cast-in-place pre-drilled borehole methods[4-7].Theboreholeistypicallydrilledwithoneuppercasedsegmenthavingamaximum outside diameter of 10 mm and a second uncased fracturing interval having a typical diameter of5.6mm.Thesedimensionswereselectedtobeassmallaspossibletoallowforthemosteffective

A programmable hydraulic injection system is used for both hydraulic fracture stimulation and post-fracture flow analysis. Precision high pressure flow is provided by a dual 65DM Teledyne Isco syringe pump system, a series of pneumatic-hydraulic automated valves, and a custom pump control program developed with LabVIEW. This system is capable of provid‐ ing pressures up to 70 MPa and precise controlled flow rates between 10 nL/min and 60

EGS reservoir simulation within the confines of the 30x30x30 cm3

**2.2. High pressure hydraulic injection system**

block

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cubical sample blocks.

Currently, only a small number of EGS field trials have been performed due to the high economic risk of the procedure and the significant probability of failure. Thus, performing controlled EGS experiments in the laboratory setting may be able to provide some of the crucial data and experience needed for advanced fracture model calibration and full scale testing in the field. This is especially true considering that most hydraulic fracturing design techniques, as developed by the petroleum industry, are more dependent upon historical data than on theoretical analysis [3]. In the case of EGS development, this historical data does not yet exist in sufficient quantities.

To fill the knowledge gap, laboratory scale EGS reservoir testing is being performed at the Colorado School of Mines using a heated true-triaxial apparatus. Some completed test results and observations are presented along with technical information on the equipment and procedures used. Focus is given to series of tests performed on a hydraulically fractured granite sample with a binary injector-producer borehole scheme installed.
