Thermally-induced breakouts: laboratory validation of a novel maximum horizontal stress measurement method

Abstract:

Knowledge of in situ stress is essential for several geological engineering applications as well as fault mechanics and seismic hazard assessment. It is commonly assumed that one of the principal stresses is vertical, and equals the weight of overburden rocks, and other two are horizontal. The least horizontal stress is constrained by hydraulic fracturing tests. The most common method to constrain maximum horizontal stress is by analysis of borehole wall breakouts. However, in many geological settings breakouts do not develop if the maximum horizontal stress is not high enough. We have developed a methodology to heat the borehole to induce breakouts with thermal stress which would otherwise not form.

To validate this concept experimentally, we conducted a series of room temperature and elevated-temperature true-triaxial tests on Berea sandstone and Niagaran dolomite samples with a predrilled borehole. We used acoustic emission sensors to capture the onset of breakout development, and thermocouples located close to the borehole wall to capture the temperature change. Additionally, we tested thermal properties, and assessed rock strength parameters in conventional triaxial compression tests.

Results show that in each rock type, breakouts develop at similar stress levels and acoustic emission characteristics in both room-temperature and elevated-temperature experiments. Mohr-Coulomb strength criteria does not predict the onset of breakouts sufficiently and the use of a triaxial strength criterion (e.g. Mogi, Nadai) is necessary. The thermal breakout method can therefore be used for maximum horizontal stress estimation in boreholes given information about the minimum horizontal stress, elastic and thermal properties, borehole strength, and temperature change at the borehole wall when breakout develops.