Too many required pre-existing fractures (over fifteen) by the HTPF method restrict the wide application under various stress measurement conditions. Considering the shortcomings of HTPF method, a rock mechanics equation to describe the shearing stresses intrinsically present on geological discontinuity planes is established, and at the same time, the least square fitting method and the trial searching algorithm code to determine the frictional coefficient of preexisting fractures are utilized to determine complete stress tensors by inversions. Theoretically, each hydraulic test on every preexisting fracture can help establish two mechanical equations, and then three tests on preexisting fractures can determine a complete stress tensor. However, practically, in order to guarantee the inversion code to be convergent, at least four to five pre-existing fractures are needed. Here, this method is named modified hydraulic testing on pre-existing fractures method, abbreviated as M-HTPF method. The M-HTPF method was applied in a stress measurement campaign in Weifang area, Shandong Province. During this stress measurement campaign, the shut-in pressures determined by the hydraulic fracturing test on pre-existing fractures, and azimuth and dip angle data defined by televiewer logging of five geological fractures were utilized to determine the complete stress tensor. The stress tensor is characterized by: σ1=8.85MPa, N58.12°W ∠ 14.18σ; σ2=6.61MPa, N26.2°E ∠ -21.54°; σ3=5.01, N62.86°E ∠ 63.86°. The M-HTPF method offers a new access to determine a complete stress tensor using a single borehole.
In 1984, Cornet and Valette proposed that the hydraulic test on pre-existing fractures could be utilized to measure the normal stress on the fracture plane so as to determine in-situ stress tensors. On the first international rock stress symposium, Cornet (1986) named this method to determine in-situ stresses the hydraulic test on pre-existing fractures (abbreviated as HTPF). After that, many scientists applied the HTPF method in different field measurements and compared the application effects between the HTPF method and the Hydraulic fracturing (HF) method, and generally, they got fairly good results. Cornet and Burlet (1992) summarized 8 HTPF application cases in France, and then gave some key points and suggestions for carrying out the campaign of HTPF, which offered lots of reference experiences and cases for the popularization of HTPF. In order to determine in-situ stress in a mountainous region, Cornet et al. (1997) conducted HTPF measurements in an inclined borehole, and they combined a genetic algorithm and a Monte Carlo technique to optimize the inversion procedure, finally they got satisfied stress measurement results. In 2003, Cornet et al. developed new HTPF test equipment, the key downhole component of which is a probe combining electrical imaging with an inflatable straddle packer, and consequently, they can determine the normal stress applied on the tested fracture planes by analyzing both hydraulic and electrical signals observed during shut-in phases simultaneously. At the same time, Haimson and Cornet (2003) jointly published a paper to recommend HF and HTPF methods to determine in-situ stresses, sponsored by the International Society for Rock Mechanics (ISRM). In China mainland, Chinese scientists also used this methods in different engineering cases.