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.

Cretaceous-Cenozoic regional stress field evolution from borehole imaging in the southern Jinzhou area, western Liaoning, North China Craton

The Mesozoic Yanshanian Movement affected the tectonic evolution of the North China Craton(NCC).It is proposed that Mesozoic cratonic destruction peaked~125 Ma,possibly influenced by subduction of the western Pacific Plate beneath the Euro-Asian Plate in the Early Cretaceous.The southern Jinzhou area in the eastern block of the NCC preserves clues about the tectonic events and related geological resources.Studies of the regional stress field evolution from the Cretaceous to the Cenozoic can enhance our understanding of the tectonics and dynamics of the NCC.Borehole image logging technology was used to identify and collect attitudes of tensile fractures from 11 boreholes;these were subdivided into four groups according to dip direction,i.e.,NNW-SSE,NWW-SEE,W-E and NE-SW.The development of these fractures was controlled primarily by the regional tectonic stress field;temperature,lithology,and depth contributed to some extent.In 136-125 Ma in the Early Cretaceous,the area was characterized by extension that was oriented NNW-SSE and NWW-SEE;from 125-101 Ma the extension was oriented W-E;after 101 Ma it was NE-SW.This counterclockwise trend has persisted to the present,probably related to oblique subduction of the Pacific Plate,and is characterized by ongoing extension that is nearly N-S-oriented and NEE-SWW-oriented compression.

DYNAMIC FRACTURE ANALYSIS OF AGED GLASSY POLYCARBONATE BY THE METHOD OF CAUSTICS

The purpose of this work is to investigate the dynamic fracture properties of glassy polycarbonate （PC） with different aging times. The optical method of caustics is adopted in which the shadow spot patterns are recorded by a high speed camera during the dynamic fracture process. Then, the dynamic crack propagation, the stress intensity factor （SIF） and the dynamic fracture toughness of aged PC are obtained through an analysis of the characteristic size of caustic pattern. Moreover, by combining with the investigation of the fracture surface and the energy release rate analysis, the influence of aging time on the dynamic fracture behavior is discussed. Results show that the dynamic fracture toughness and critical energy release rate of PC decreases with aging time for short aging times, whereas they have little change or even increase for longer aging times. Therefore, aging modifies the mechanical properties especially the dynamic fracture properties of PC nonlinearly, not linearly as generally thought of.