Analysis Of Wellbore Image Data Applied To Environmental And Geotechnical Case Studies

Abstract

The application of borehole televiewer (BHTV) data analyses recorded in the shallow crust to environmental and hydrological investigations is fourfold: 1) fracture characterization, 2) in-situ stress measurement, 3) providing site geologic and lithologic constraints, and 4) geotechnical monitoring. One of the primary uses of the borehole televiewer data since the tool became operational in geophysical logging has been measurement of the orientation and distribution of planar features in a drillhole. The resolution of the borehole televiewer is capable of detecting a wide range of fracture sizes and these data can provide a wealth of information pertinent to understanding the mechanical behavior of the Earths crust at a drillsite. Fractures provide permeable pathways for fluids and affect rock properties by chemical alteration of the surrounding rock mass. Fracture geometry is therefore important to fluid flow and permeability in reservoirs and waste disposal sites and to the safety of groundwater aquifers. In situ stresses play a major role in the stability of natural and man made structures. The measurement of stress orientation and magnitude, possible through the analysis of BHTV image data, can provide information about the ambient tectonic stresses and information crucial to the design, construction and monitoring of dams, tunnels or aqueducts. Stress orientation and magnitude also has a major impact on the development of natural fractures and subsequent movement along preexisting fault surfaces. In shallow crustal wells the constraints placed on in-situ stress from the analysis of BHTV data are important to interpreting the interaction between pore pressure and hydrostatic pressure in a well and subsequent wellbore stability. Case studies from wells that exemplify the application of BHTV data analysis to geotechnical and environmental issues are presented. The first study involved in the siting and construction of a major aqueduct/tunnel in central Utah. The objectives of this study were to characterize the natural fractures within the shallow crust at the drillsite and to determine the orientation of in-situ stresses. A second study, conducted in two phases, concerned the analysis of fractures developed in a concrete dam abutment in Colorado. Detailed measurements of the aperture of fractures detected by BHTV imaging were made at a ten month interval to help establish the integrity of the dam. Finally, a study of wellbore image data recorded at county landfill in Oregon was performed to measure the orientation, aperture and depth distribution of fractures at the site. The image data was used in conjunction with geophysical logs to discriminate between bedding plane fractures and through-going fractures in siltstone units and fractures in basalt units.