Abstract:Hydraulic fracturing has originated as a stimulation technique of oil reservoir,and has been developed as a rock stress measurement technique. Recently,it has also been developed to create artificial heat exchange surfaces for hot dry rock power generation system. To study the mechanism of crack extension by hyduaulic fracturing,the mass hyduaulic fracturing experiments are carried out in granodiorite at Fangshan District,Beijing,China. A FR borehole of 301 m deep in a fresh and intact rock is used for the experiments. During the fracturing process,electrical potential changes around the injection well are measured on the surface. Measurement points,total 49 points,distribute around the injection well on the grid with interval of 25 m in an area of 200 m×200 m. Hyduaulic fracturing experiments are carried out by selecting 3 spots without natural joints from the section of 110 m to 140 m in the depth of the FR borehole. These spontaneous potential(SP) anomalies show elliptical distribution in the map view. The values of the anomalies increase with the water flow. Most of the measurement results show that the positive elliptical anomalies have two long axes or one long axis with distortion to different directions. These directions coincide with the orientations of fracture and joint. In these cases,water is considered to flow into both the fracture and joint. The directions of crack extension obtained from the SP measurement agree with those evaluated one from the hydraulic fracturing stress measurement.