ABSTRACT:

Identification of in situ stress state is crucial in developing an enhanced geothermal system. Currently, there are no reliable methods to measure in situ stress in geothermal boreholes because conventional borehole technologies (i.e., over-coring, hydraulic fracturing, image log analysis of wellbore failures) cannot withstand the harsh environment of a geothermal reservoir. Sponsored by the United States Department of Energy, a new approach is being developed for measuring the in-situ stress state for deep geothermal applications. This approach involves directional cooling induced fracturing (DCIF) on the borehole surface by controlling the rock's thermoelastic contraction.

The primary objective of this research paper is to presents select results on the applicability of the DCIF technology and sensitivity of horizontal principal stress (SHmax and Shmin) magnitudes and orientation to material parameters and delta temperature (i.e., amount of cooling required to initiate a tensile fracture). Probability and sensitivity studies were conducted to analyze the impact of inherent variability in geologic materials on the predicted magnitudes of SHmax, Shmin, and Θ (i.e., azimuth angle from SHmax direction). Probability and sensitivity analyses results show that the variability in geologic materials impacts the magnitudes of SHmax, Shmin, and Θ. The results also confirm that a reasonably accurate calculation of SHmax, Shmin, and Θ is possible through the DCIF technology.

1. Introduction and Background

Current borehole-based in situ stress measurements are either unreliable or incomplete, especially for geothermal applications. A robust methodology to obtain the complete stress tensor in a deep geothermal well is still absent and is the focus of this paper.

Hydraulic fracturing (Hubbert and Willis, 1957) is only able to determine the minimum principal stress (Shmin) magnitude and its direction. This method also suffers in geothermal settings because current packer materials used to seal off a segment of the borehole are only rated up to 260°C (Halliburton, 2019), whereas geothermal reservoirs reach over 300°C.

This content is only available via PDF.
You can access this article if you purchase or spend a download.