Determination of Horizontal In-Situ Stress Profiles and Rock Deformation Moduli in Karamay Basin Using a Multiobjective Optimization Technique

Summary We introduce a quick and cost-effective method of estimating horizontal in-situ stress profiles and rock elastic moduli vs. depth from geophysical logs taken in vertical well sections. A multiobjective optimization approach finds the optimum solution for the inversion of in-situ stresses and the rock mechanical parameters from elastic borehole deformations measured by the commonly available four-arm caliper tools. The four-arm caliper log responses also permit quality control (QC) of input and identification and classification of borehole sections that display breakouts and sloughing. The method is applied in the estimation of horizontal in-situ stress profiles and rock deformation moduli vs. depth in Karamay Basin, Northwestern China. The results have demonstrated good agreement with available field in-situ stress measurements, indicating promising broader applications of the method.

Brittle deformation

By nature, brittle deformation is discontinuous. It is often studied through mechanical tests, both in laboratories and outdoors, in mines and quarries. Brittle deformation also concerns civil engineering (road maintenance, strength of retaining structures such as bridges, dams, galleries etc.) and is well integrated with investigations in rock mechanics. Hydraulic fracturing is extensively used in the geothermal sector, for oil or gas production enhancement, or recovery of shale gas. Along with in-situ stress measurements, it has expanded the interest of geologists within the domain of rock mechanics. A solid knowledge of the mechanisms governing rock failure is necessary to understand the processes operating at the origin of earthquakes and volcanic eruptions, as well as the genesis of ore vein deposits. Beyond the elastic threshold of mechanical tests, rock failure takes place after development of a certain amount of non-elastic deformation. The fact that a progressive transition exists between ductile and brittle deformation suggests that these two behaviours are not mutually exclusive. Indeed, the study of the brittle-ductile transition paves the way to new concepts that enrich our understanding of the mechanisms of failure, in turn allowing practical applications. In this chapter, a presentation of the relationships between fracture orientation and principal stress directions will be followed by an examination of the macroscopic and microscopic aspects of brittle deformation.

Hydrofracturing in Situ Stress Measurements in Itabirite Brazilian Ferriferous Quadrilater, and their Limitations

This article presents a first attempt to carry out measurements (magnitudes and orientations) of the in situ stress in itabirite rocks in the region of the Brazilian Ferriferous Quadrilater obtained by hydraulic fracture tests at a depth of 399 m. Previous studies available in this rock mass consider estimated values of k index (Sh / Sv), and it is not a practice adopted to carry out in situ stress tests in this region and rockmass to support geotechnical analysis. The area of study is located at a depth of 500 m in a pit; therefore, the determination of the in situ stress distribution is very important to assess the stability of the mining open pit. The activities, from the planning to the execution of the tests, and the results are presented. The rock mass under study shows the presence of different geological structures, such as banding and foliation, which resulted in difficulties with performing the tests, and only 12.5% of the tests were successful. The results contribute to understanding the strains and stresses induced by mining activities in slopes in the Brazilian Ferriferous Quadrilater and their impacts on surrounding structures. For a better determination of the regional in situ stresses in the rock mass of the Brazilian Ferriferous Quadrilater, it is recommended to perform hydraulic tests on pre-existing fractures.

Linking bulk modulus to an unilateral damage yield criterion: A thermodynamic modeling approach

This work presents a new damage criterion suitable for elastic, elastic-plastic/viscous or elastic-viscous-plastic materials involving rupture effects. Its derivation, made here within a thermodynamic framework, follows previous scalar-valued damage mechanics approaches. Such approaches are appropriate to many geophysical problems involving quasi-brittle materials for which there is no clear physical justification for the level of complexity of a tensorial damage variable. Distinction between the mechanical response to compressive and tensile stresses is therefore not introduced by the damage itself but via a special definition of the Helmholtz free energy. This scheme differs from previous ones in that it combines with an evolution of Poisson’s ratio with the level of damage, which allows expressing the damage criterion in the principal stresses space. Moreover, there is no need to compute the stress eigensystem, which makes it simpler to implement than the Mohr–Coulomb damage criterion. Here we derive this damage criterion and compare it to observations of the variations of the bulk modulus in damaged geomaterials. We also compare it to in-situ stress measurements and find a good agreement in terms of the shape of the criterion in the stress space. We tentatively interpret the results in the context of previous studies of rock and ice mechanics.