Interaction between in situ stress states and tectonic faults:A comment

Understanding the in situ stress state is crucial in many engineering problems and earth science research.The present article presents new insights into the interaction mechanism between the stress state and faults.In situ stresses can be influenced by various factors,one of the most important being the existence of faults.A fault could significantly affect the value and direction of the stress components.Reorientation and magnitude changes in stresses exist adjacent to faults and stress jumps/discontinuities across the fault.By contrast,the change in the stress state may lead to the transformation of faulting type and potential fault reactivation.Qualitative fault reactivation assessment using characteristic parameters under the current stress environment provides a method to assess the slip tendency of faults.The correlation between in situ stresses and fault properties enhances the ability to predict the fault slip tendency via stress measurements,which can be used to further refine the assessment of the fault reactivation risk.In the future,stress measurements at greater depths and long-term continuous real-time stress monitoring near/on key parts of faults will be essential.In addition,much attention needs to be paid to distinguishing the genetic mechanisms of abnormal stress states and the type and scale of stress variations and exploring the mechanisms of pre-faulting anomaly and fault reactivation.

In situ stress state and seismic hazard in the Dayi seismic gap of the Longmenshan thrust belt

In the Longmenshan thrust belt,the Dayi seismic gap,an area with few earthquakes,is located between the ruptures of the 2008 Wenchuan Earthquake and the 2013 Lushan Earthquake,with a length of approximately 40–60 km.To date,however,the extent of the seismic hazard of the Dayi seismic gap and whether this gap is under high stress are still hotly debated.To further evaluate the seismic hazard of the Dayi seismic gap with regard to stress,two boreholes(1,000 and 500 m deep)were arranged to carry out hydraulic fracturing in situ stress measurement on either side of the Shuangshi-Dachuan fault zone.This zone has a high seismic hazard and the capacity to undergo surface rupture.Through the analogy of this new data with stability analysis using Byerlee’s Law and existing stress measurement data collected before strong earthquakes,the results show that the area surrounding the Shuangshi-Dachuan fault zone in the Dayi seismic gap(Dachuan Town)is in a state of high in situ stress,and has the conditions necessary for friction slip,with the potential hazard of moderate to strong earthquakes.Our results are the first to reveal the in situ stress profile at a depth of 1,000 m in the Dayi seismic gap,and provide new data for comprehensive evaluation of the seismic hazard in this seismic gap,which is of great significance to explore the mechanism of earthquake occurrence and to help mitigate future disaster.

Mutual impact of true triaxial stress, borehole orientation and bedding inclination on laboratory hydraulic fracturing of Lushan shale

Unconventional resources like shale gas has been the focus of intense research and development for two decades. Apart from intrinsic geologic factors that control the gas shale productivity (e.g. organic matter content, bedding planes, natural fractures, porosity and stress regime among others), external factors like wellbore orientation and stimulation design play a role. In this study, we present a series of true triaxial hydraulic fracturing experiments conducted on Lushan shale to investigate the interplay of internal factors (bedding, natural fractures and in situ stress) and external factors (wellbore orientation) on the growth process of fracture networks in cubic specimens of 200 mm in length. We observe relatively low breakdown pressure and fracture propagation pressure as the wellbore orientation and/or the maximum in situ stress is subparallel to the shale bedding plane. The wellbore orientation has a more prominent effect on the breakdown pressure, but its effect is tapered with increasing angle of bedding inclination. The shale breakdown is followed by an abrupt response in sample displacement, which reflects the stimulated fracture volume. Based on fluid tracer analysis, the morphology of hydraulic fractures (HF) is divided into four categories. Among the categories, activation of bedding planes (bedding failure, BF) and natural fractures (NF) significantly increase bifurcation and fractured areas. Under the same stress regime, a horizontal wellbore is more favorable to enhance the complexity of hydraulic fracture networks. This is attributed to the relatively large surface area in contact with the bedding plane for the horizontal borehole compared to the case with a vertical wellbore. These findings provide important references for hydraulic fracturing design in shale reservoirs.