Integrated 3-D stress determination by hydraulic fracturing in multiple inclined boreholes beneath an underground cavern

This paper presents a complete three-dimensional stress determination using hydraulic fracturing data from three inclined boreholes, drilled from the floor of an underground cavern at a depth of about 100 m. Both conventional hydraulic fracturing (HF) and hydraulic testing of pre-existing fractures (HTPF) were carried out at all test points to acquire reliable data and conduct the integrated stress analysis. We determined 3-D stress states using a numerical inversion code that integrates the entire data set from HF and HTPF methods, and employs a nonlinear least-squares optimization routine based on a modified Levenberg-Marquardt method and a finite-difference Jacobian algorithm. We present the trend of complete three-dimensional stress states, with depth expressed by correlation equations. We compare the 3-D stress inversion result for the integration of all data from the three boreholes with the results determined independently for each individual borehole. The results showed that the maximum principal stress was subhorizontal and oriented approximately NNE-SSW, and the ratio of maximum to minimum principal stress was 2.0 on average. The inverted scatter of misfit remained within 10% for both the integrated analysis of the entire data set and the measurement data for each individual borehole. From these findings, we conclude that the 3-D stress determination made by integration of HF and HTPF data from multiple inclined boreholes resulted in a reliable inversion of the 3-D stress field.