A novel semi-analytical tool for stress interpretation using borehole breakouts

Research Project, 2018 – 2019

Boreholes and other cavities of a circular cross-section are relevant to a wide variety of applications, such as tunneling, geothermal energy production, oil and gas production, nuclear waste disposal and stress measurement, either with the method of hydrofracturing or through the interpretation of borehole breakouts. Breakouts are of importance in all above applications, both as a threat to borehole or tunnel stability and as a tool for the primary stress state interpretation. The necessary simulations are challenging from a numerical point of view, as the problem is linked to loss of numerical stability for ductile rock and loss of continuity for brittle rock. The first case can be tackled using advanced continuum formulations, such as micropolar of nonlocal models, to model the material behavior. The second is more challenging and no continuum based solutions are currently available. The present project suggests a semi-analytical method for both the prediction of breakouts in brittle rock and their use to assess the primary in situ stress. The project is subdivided into three steps. In the first, a tool will be developed for the prediction of the form and size of the breakouts for given primary stresses and material parameters. The correctness will be verified by means of comparison to analytical solutions. In the second step, using also the method developed in the first step, a tool will be developed for the estimation of the primary in situ stress for given geometries of the borehole breakouts and given material properties. Finally, the applicability and efficiency of both resulting tools will be validated against data from the literature and field experiments. The aim is to provide a simple, fast and easy to use tool for the assessment of borehole and tunnel stability and for the estimation of the in situ stress in brittle rock.Boreholes and other cavities of a circular cross-section are relevant to a wide variety of applications, such as tunneling, geothermal energy production, oil and gas production, nuclear waste disposal and stress measurement, either with the method of hydrofracturing or through the interpretation of borehole breakouts. Breakouts are of importance in all above applications, both as a threat to borehole or tunnel stability and as a tool for the primary stress state interpretation. The necessary simulations are challenging from a numerical point of view, as the problem is linked to loss of numerical stability for ductile rock and loss of continuity for brittle rock. The first case can be tackled using advanced continuum formulations, such as micropolar of nonlocal models, to model the material behavior. The second is more challenging and no continuum based solutions are currently available. The present project suggests a semi-analytical method for both the prediction of breakouts in brittle rock and their use to assess the primary in situ stress. The project is subdivided into three steps. In the first, a tool will be developed for the prediction of the form and size of the breakouts for given primary stresses and material parameters. The correctness will be verified by means of comparison to analytical solutions. In the second step, using also the method developed in the first step, a tool will be developed for the estimation of the primary in situ stress for given geometries of the borehole breakouts and given material properties. Finally, the applicability and efficiency of both resulting tools will be validated against data from the literature and field experiments. The aim is to provide a simple, fast and easy to use tool for the assessment of borehole and tunnel stability and for the estimation of the in situ stress in brittle rock.