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

As part of the Sectoral Plan for Deep Geological Repositories, three candidate sites are currently examined by a focused geological exploration program in Northeastern Switzerland. The program involves 3D seismic surveys and drilling of at least two deep boreholes at each site. Stress testing is being undertaken with a wireline formation testing tool in each borehole (around 20 stress tests per borehole). Improvements in the toolstring 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 toolstring with three packers has been run to perform the complete combination of sleeve fracturing, hydraulic fracturing and sleeve reopening 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 has been 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 toolstring for estimating the closure stress from a stress test, especially in low-permeability formations, is presented, and detailed stress testing examples are provided. Preliminary comparison between the stress estimates for the first two boreholes in the campaign are shown.