The current state-of-the-art technology for in situ stress measurements involves an integrated approach that combines borehole breakout observations, drilling-induced tensile fractures, and hydraulic fracturing tests (i.e., mini-fracs); however, this methodology has several limitations that often prevent successful in situ stress measurements. One major limitation is that breakouts do not appear in every borehole and are generally a natural occurrence that cannot easily be controlled. Because borehole breakouts are used to measure the maximum horizontal in situ stress magnitude, the absence of borehole breakouts presents a major data gap for in situ stress measurements. As a response to this data gap a new thermal breakout technology is being developed that will provide a method for thermally inducing borehole breakouts and obtaining consistent measurements of the maximum horizontal stress magnitude. This thermal breakout technology involves heating the borehole and increasing the thermoelastic compressive stress in the rock until a breakout develops, which is directly correlated to the maximum horizontal stress magnitude. An initial component of the thermal breakout project involves developing a prototype downhole tool and performing field testing in the deep subsurface. The primary objective of the initial tool development and field testing is to provide a physical proof of concept for the thermal breakout technology. Three thermal breakout field tests were performed at approximately 4850 feet deep in the Sanford Underground Research Facility. Each test was performed in a borehole that had been previously used for overcoring stress measurements. The third and final test successfully created two diametrically opposed breakouts after deploying the thermal breakout tool for approximately 3 hours of continuous heating. Post-analysis of the thermally induced breakouts confirmed that the thermal breakout orientation directly corresponds to the known in situ stress orientation and magnitude.

1. INTRODUCTION AND BACKGROUND

The measurement of the state of rock stress has been an area of active research for more than 60 years (Zoback, 2007). Hydraulic fracturing measures the state of stress by creating a tensile fracture that opens normal to the minimum horizontal stress direction. The fundamental analysis assumes that the fluid pressure required to open or close the fracture is a measure of the stress normal to the fracture, which is the minimum horizontal stress (also known as the shut-in pressure) (Raaen and Brudy, 2001; Edwards et al., 2002). Using the minimum horizontal stress and fracture reopening pressure provides a basis for approximating the maximum horizontal stress, provided that the stress concentration around the borehole and fracture can be assumed from linear-elastic theory. Oriented impression or borehole image logs determine the orientation of the fracture, which is the maximum horizontal stress direction.

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