Development and use of a high pressure packer for measuring horizontal stress using the hydraulic fracturing method

摘要

Research conducted by the National Institute for Occupational Safety and Health (NIOSH) has shown that knowledge of the direction and magnitude of the in situ horizontal stress can be critical to the success of many underground mining operations. One technique employed by NIOSH for determining in situ stress has been hydraulic fracturing from boreholes drilled in mine. However, in some cases the combined rock strength and stress magnitude have been too high to allow successful stress determination using available hydraulic fracturing equipment. NIOSH developed high pressure straddle and impression packers capable of operating at pressures up to 103 MPa in a 51 mm diameter borehole. The high pressure hydraulic fracturing system was used to measure horizontal and vertical stresses in a western Pennsylvania limestone mine with a high incidence of stress related roof failures. Breakdown pressures ranged from 43.9 to 95.7 MPa and in two cases the rock could not be broken down even at the maximum system pressure, 103 MPa. The estimated ranges for the maximum horizontal principal stresses were 27.2 to 65.9 MPa. The measured vertical stress was 3.8 MPa. The calculated K ratio was between 7 and 17. The stress directions, determined from the crack directions in roof and floor holes, were N76°E and N66°E, respectively. Because of the high breakdown pressures required to initiate the fractures, neither of the horizontal stress measurements could have been made with any other existing hydraulic fracturing equipment. The author believes the packers represent a significant improvement in the state of the art of in situ stress measurement under near mine conditions.