In-situ stresses are one of the most important elements for the design and stability assessment of rock engineering structures and earthquake science. Many in-situ stress inference and measurements techniques are devised and they are broadly classified into direct or indirect techniques. Drilling and blasting technique is widely used as an excavation technique in rock engineering practice. The experiments on specimens clearly indicated that the fracture zones around the blasthole was larger in the direction of the maximum load. Some parts of the blastholes remain following blasting. The author proposes a method how to infer the in-situ stresses from the damage zones around the blastholes in this article and it is named as the blasthole-damage method. The fundamentals of this method are described and it is applied to several sites where in-situ stress states are obtained by using direct or other indirect techniques. The inferences are compared with measurements and the validity of the method are discussed in view of the measurements or inferences from other methods.
The rates and magnitudes of fracture diagenesis on commercially available proppants were determined from flow through experiments at difference stresses (depths) and temperatures and for hydraulically-open and –closed systems. A lumped-parameter conceptual model addressing diagenetic compaction is extended to accommodate the specific nature of the compaction of the designated proppant. Mechanisms include mineral dissolution, transport, and re-precipitation of the resulting products in the particle interstices, resulting in a loss of porosity in proppant packs. The model used recovered thermodynamic and kinetic data for mineralogical composition of available proppants within rigid walled hydrothermal and thermomechanical reactors. Under reservoir temperatures of 135 to 191°C and stresses to 65.5 MPa, these ensemble data suggest that proppant packs may compact by up to 10%, resulting in porosity loss of up to 30%, and lose 75% of initial permeability. This may occur over a period of the order of 3 years. These predictions are consistent with observations from active wells where production may decrease to 10% over the period of a few years.
This study demonstrates the importance of considering anisotropy in borehole stability of transversely isotropic rock. Both analytical and numerical methods are conducted in order to analyze the horizontal borehole stability in transversely isotropic rock. The data for transversely isotropic rock are based on laboratory experiments on shale. The boundary condition is assumed as in situ stress based on the stress ratio of Pohang, Korea. The critical plane approach is applied to describe the anisotropic Mohr-Coulomb failure criterion. A various scenarios with depth in transversely isotropic rock are considered in order to examine the borehole breakout.
Since rock formations are always saturated by fluids, the coupled poroelastic models were developed for improved wellbore stability analysis. Poroelastic models allow to consider phenomena such as well communication with rock formation and pore pressure redistribution. This paper presents the analytical solution of one coupled poroelastic model and then departs to consider two common drilling scenarios, under-balanced and over-balanced drilling. Obtained results show that time-dependent delayed failure and failure initiation taking place inside the formation rather than at the wellbore wall can be predicted by virtue of the coupled model. Moreover, these two phenomena are different for the two drilling scenarios mentioned above. For the under-balanced drilling borehole stability will get enhanced with time due to poroelastic effect in the formation. On the contrary, the borehole stability status will get worse and time-delayed failure may happen for the over-balanced drilling.
We generalize our view of a bonded-particle model (BPM) to consist of a base material (of rigid grains joined by deformable and breakable cement at grain-grain contacts) to which larger-scale joints can be added and whose mechanical behavior is simulated by the distinct-element method using the two- and three-dimensional discontinuum programs PFC2D and PFC3D. The micromechanical processes that control brittle fracture and thus, should inform any micromechanical model, are summarized. The rich variety of microstructural models that can be produced by the bonded-particle modeling methodology are described and classified with respect to their microstructural and larger-scale features. These models provide a wide range of rock behaviors that encompass both compact and porous rock at both an intact and rock-mass scale, and examples are provided of how BPMs are being used to model rock at these scales. The examples include an intact anisotropic material that may swell and contract in response to changes in saturation, the behavior of two alternative BPMs that can match both the uniaxial and tensile strengths of compact rock and the embedding of an intact BPM within a larger continuum model to study fracturing around a gold-mine stope in quartzite.
Various factors such as physical and geometrical properties of pressure cell, properties of surrounding media and installation techniques seriously affect the accuracy of total pressure measurement within soil or concrete structure. In this study a numerical code was used to simulate some of factors affecting calibration coefficient and steps of installation in soil and tunnel concrete lining, for this purpose more than 1200 elastic 3D analysis for various installation situations performed. The results show for pressure cell in soils, geometric factors and compaction quality of the surrounding material greatly affect on the precision of measurement and for pressure cells that are installed in tunnel concrete lining, the most important factor in the installation method, is the place that pressure cell is installed in respect to the stress concentration.