Stress Measurement Campaign in Scientific Deep Boreholes: Focus on Tools and Methods

As part of the Sectoral Plan for Deep Geological Repositories, three potential siting regions are currently being investigated by means of a focused geological exploration program in northern Switzerland. The program involved drilling eight vertical boreholes and one deviated deep borehole with at least two vertical boreholes per siting region. Stress testing was undertaken with a wireline formation testing tool in each borehole (around 20 stress tests per borehole). Improvements in the tool string were introduced step by step to sharpen the range of the stress estimates and enable 100% coverage of the desired lithological column. This is the first time that a single tool string with three packers has been run to perform the complete combination of sleeve fracturing (SF), hydraulic fracturing, and sleeve reopening (SR) tests. A dedicated stress testing protocol was developed to ensure the most robust estimate of the stress in a large variety of formations. A detailed planning process was developed to maximize the success rate and coverage of stress test stations, integrating all available information as it becomes available. A review of the techniques enabled by the new tool string for estimating the closure stress from a stress test, especially in low-permeability formations, is presented, and detailed stress testing examples are provided. A preliminary comparison between the stress estimates for the first two boreholes in the campaign is shown.