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Reliable Estimation of Horizontal Stress Magnitudes from Borehole Breakout Data

Underground » Strata Control and Windblasts

Published: September 19Project Number: C26063

Get ReportAuthor: Joung Oh, Ismet Canbulat, Bruce Hebblewhite, Hossein Masoumi & Huasheng Lin | The University of New South Wales, Monash University

The main objective of the project is to develop a reliable and simple and cost-effective technique that can be used to estimate the magnitude of horizontal stresses based on borehole breakout data. It is well known that ground stresses have a major impact upon roof and rib behaviour and stability. In underground coal mines, stress magnitudes can vary significantly according to direction. In situ stress orientation and magnitude are also altered when adjacent to major geological structures. These changes in stress can have a substantial adverse impact on mining conditions such as increasing the risk of violent failures, via coal burst or other major roof failures. Knowledge of changes in stress orientation and magnitude will indicate the high risk zones within a mine site which can enable mine operators to implement appropriate controls.

Four areas of studies were carried out and discussed in this report, including literature review, laboratory testing, numerical simulation and back analysis of field data. Based on the review of current underground stress measurement techniques, the advantages and disadvantages of each are summarised.

As conventional techniques are generally expensive, time consuming and difficult to be applied to weak strata, there is a need to develop a stress estimation technique that can supplement the limitations. Borehole breakout is a phenomenon due to the excessive stresses around the borehole, especially in the weak strata. It has been argued that the breakout depth and angular span are influenced by horizontal stresses. To date, it has been used as a reliable indicator of horizontal stress orientation and there are no universally accepted methods for estimation of stress magnitudes currently available. In Australia, every borehole is geologically and geophysically logged,  so borehole breakout analysis has a great potential for a tool of horizontal stress estimation.

To investigate the relationship between breakout geometries and horizontal stress magnitudes, a series of experimental work was carried out at School of Minerals and Energy Resources Engineering, UNSW. Experimental results revealed clearly stress dependency of breakout geometries, suggesting that horizontal stress magnitudes could be derived from breakout data. It was also found that the borehole size plays a crucial role on breakout formation. The smaller borehole size amplifies the stress required for the breakout initiation. The additional experiments on plaster also showed the influence of rock strength on the breakout geometries.

Numerical simulations were conducted via PFC (Particle Flow Code) and RS2 (RocScience 2D) for further studies on breakout behaviour under various conditions it was found that a set of numerical simulation results presents a different trend than the experimental observation, where breakout angular span becomes narrower with increasing horizontal principal stress ratio. This contradicts with the popular argument that two geometries are dependent on each other and horizontal stresses cannot be derived from the two redundant factors. After the further analytical investigation, this 'unconventional' breakout trend was able to be explained. In addition, the analysis of previous experimental studies also suggests that the two breakout geometries are not solely dependent on each other. Their relationship is heavily influenced by the change in horizontal stress magnitudes. The finding here indicates that two geometries are not redundant factors during the calculation, thus the estimation of two horizontal stresses from breakout geometries is possible.

To assess the reliability of the horizontal stress estimation technique via borehole breakout, field data was collected from literature as well as the collaborated mine site with stress measurement in the nearby depth. Sonic log data was also used for the purpose of rock strength estimation, where the coring data information was not available. A procedure for data extraction and processing is described in the Appendix of the report.

Aa guideline for horizontal stress estimation using breakout geometries was proposed and presented in this report. This guideline consists of two independent models that can estimate the horizontal stress ratio and maximum horizontal stress magnitude, respectively. Based on the back analysis of field data, the models yielded the average error rates at 21.53% and 27.55%, respectively. The guideline enables the quantitative analysis on horizontal stress magnitudes from downhole logging data.

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