Abstract: Since many pre-existing fractures (i.e. more than 15) are required by the hydraulic testing of pre-existing fractures (HTPF) method, the application of HTPF method to various stress measurement conditions is restricted. To overcome the shortcomings, a mechanical equation is developed to describe the shearing stresses intrinsically occurring on geological discontinuity planes. The least square method and the trial searching algorithm code are used to calculate the frictional coefficient of pre-existing fractures and further to calculate in-situ stress tensors by the inversion technique. Theoretically, the hydraulic test on each pre-existing fracture is utilized to establish two mechanical equations, and three tests on pre-existing fractures is used to determine the in-situ stress tensor. Practically, to guarantee the convergence of the inversion code, at least five pre-existing fractures are required. Thus, the developed method is called the modified-HTPF method, which is further applied to measure in-situ stress in Weifang area, Shandong Province. During the process of stress measurement, the complete in-situ stress tensor is determined by the shut-in pressures from the hydraulic fracturing test on pre-existing fractures and the azimuth and dip angle data from Televiewer logging of five geological fractures. The in-situ stress tensors are characterized as 8.85 MPa, N58.12°W∠14.18°; 6.61 MPa, N26.2°E∠ 21.54°; and 5.01 MPa, N62.86°E∠63.86°. Compared with the data by the classic hydraulic fracturing (HF) method, the medium and minimum principal stresses determined by these two methods are similar, but there exists a large difference between the maximum principal stresses. The orientations of the maximum and minimum principal stresses determined by the modified-HTPF method are in good agreement with the classic HF method. The modified-HTPF method offers a new access to determine a complete stress tensor using a single borehole.

Key words: hydraulic fracturing, modified-HTPF, in-situ stress measurement, field operation, shut-in pressure