Stress tensor determination by modified hydraulic tests on pre-existing fractures: Method and stress constraints

The hydraulic testing of pre-existing fractures (HTPF) is one of the most promising in situ stress measurement methods, particularly for three-dimensional stress tensor determination. However, the stress tensor determination based on the HTPF method requires at least six tests or a minimum of 14–15 tests (under different conditions) for reliable results. In this study, we modified the HTPF method by considering the shear stress on each pre-existing fracture, which increased the number of equations for the stress tensor determination and decreased the number of tests required. Different shear stresses were attributed to different fractures by random sampling; therefore, the stress tensors were obtained by searching for the optimal solution using the least squares criterion based on the Monte Carlo method. Thereafter, we constrained the stress tensor based on the tensile strength criterion, compressive strength criterion, and vertical stress constraints. The inverted stress tensors were presented and analyzed based on the tensorial nature of the stress using the Euclidean mean stress tensor. Two stress-measurement campaigns in Weifang (Shandong Province, China) and Mercantour road tunnel (France) were implemented to highlight the validity and efficiency of the modified HTPF (M-HTPF) method. The results showed that the M-HTPF method can be applied for stress tensor inversion using only three to four tests on pre-existing fractures, neglecting the stress gradient. The inversion results were confined to relatively small distribution dispersions and were significantly reliable and stable due to the shear stresses on the fractures and the stress constraints employed. The M-HTPF method is highly feasible and efficient for complete stress tensor determination in a single borehole.