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4. NATURAL FRACTURE MAPPING Figure 4 displays the 3D model that resulted from mapping natural fractures in the mine. The model includes individual fractures traced out in the mine, and interpretations of the dominant fracture corridors. Two fracture systems are observed at the mine.' One runs approximately N70W and is similar to the dominant trend in the Piceance Basin. The other system is more dominant locally and runs N60-80E. This system is thought to result from uplift, erosion, and unloading. As will be seen later, Electrofracs EF 1 and EF3 both run in the direction of this second natural fracture system. Understanding these natural fracture systems and recognizing that hydraulic fractures might follow the same corridors influenced the design of Electrofrac field experiments. 5. HYDRAULIC FRACTURE PRETESTS The key results of hydraulic fracture pretests are illustrated in Figure 5. At each of three sites a 25- foot hole was drilled, cased, and cemented. These holes are shown in yellow. A pilot hole was then drilled out from each casing and a small hydraulic fracture job was pumped. The main phase of each fracture job was pumped with pink colored grout, composed of Portland cement, water and red pigment. Each fracture job included a fluid efficiency test to determine fracture leak-off characteristics, and the minimum in situ stress was determined from the instantaneous shut-in pressure. These important data were later used to design our larger fracture jobs. After each fracture was allowed to cure, three observation holes were cored through the expected locations of the fracture. These holes are shown in orange. Occurrences of colored grout in the core are indicated by the bluish disks, oriented as the fracture was oriented in the core. In all nine observation holes we successfully cored through the targeted grout-filled fractures. Key conclusions from these pretests were: • The natural fracture trend influences hydraulic fracture direction, • The minimum principle in situ stress in the mine is about 150 psi, and is parallel to the cliff face, and • Natural fractures exert stress on fracture fill. This last item is important because granular calcined coke is a poor conductor of electricity without stress being applied to enhance its grain- to-grain contact. 6. THERMAL CONDUCTIVITY PRETEST An in situ thermal conductivity pretest was conducted to understand the thermal properties of oil shale in the mine. This test also provided valuable experience running experiments in the mine and recording data automatically. The physical setup and test equipment are illustrated in Figure 6. The test utilized a pattern of 9 holes drilled 10 feet into the mine rib in the southwest corner of the mine. A 2-foot long, 250-watt cartridge heater was installed in the central hole. The surrounding holes had 3 thermocouples spanning the 2-foot section opposite the heater. 29`h Oil Shale Symposium Page 4 of 13 October 19-23, 2009 Figure 5. Key results of hydraulic fracture pretests. Figure 4.3-D model of natural fractures at Colony Mine.