Borehole breakouts are used to constrain the magnitude of maximum horizontal stress. However, when the borehole wall strength is higher than the in situ tangential stress, borehole wall failure does not develop. Additional compressive stress can be induced by heating borehole walls. To validate this concept experimentally, we conducted room-temperature and elevated temperature true-triaxial tests on Berea sandstone and Niagaran dolomite samples. We used acoustic emission sensors to capture the onset of breakout development, and we measured the temperature close to borehole wall to assess the magnitude of induced thermal hoop stress. The test results show that within a specific rock type, the breakouts develop in similar manner in room-temperature and elevated-temperature tests. Therefore, the maximum horizontal stress can be constrained from the following dataset: critical tangential stress at which breakout develops, minimum horizontal stress, elastic and thermal properties, and temperature change at the borehole wall.

1. INTRODUCTION

Knowledge of in situ stress is crucial for many geological engineering applications including borehole stability analysis or hydraulic fracturing design. The stress tensor is commonly analyzed in principal directions assuming the direction of gravity is one of them. In such conditions stress description consists of finding the magnitudes of the vertical stress σv, the minimum horizontal stress σh, the maximum horizontal stress σH and the azimuth of horizontal stresses. In the current study, we focus on maximum horizontal stress magnitude assessment and we investigate thermally induced borehole breakouts to constrain it.

Compressive failure of the wellbore wall, i.e. breakout, develops when the circumferential stress σθθ at the borehole wall is higher than the rock strength. If the rock strength and the minimum horizontal stress magnitude are known, widths of breakouts provide important constraints on the magnitude of maximum horizontal stress. However, no breakouts occur when the rock strength is too high, which limits the information available to constrain stress. In such case, additional compressive circumferential stress may be induced by increasing the borehole wall temperature, leading to formation of thermally-induced breakouts. To test the feasibility of the method, we performed a series of laboratory tests where thermal breakouts were induced experimentally in a true-triaxial apparatus with fully controlled stress boundary conditions.

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