The evolution of enhanced geothermal systems (EGS) entails spatially and temporally evolving permeability fields. During non-isothermal fluid injection, thermo-elastic stress and fluid pressure changes act upon partially open or hydrothermally altered fracture sets to enhance formation permeability. The physical couplings that drive this behavior are non-linearly dependent upon one another to varying degrees. To explore these interactions we are developing a simulator capable of coupling the dominant physics of shear stimulation using a variety of methods, allowing flexibility in the use of monolithic or staggered numerical schemes. The new simulator uses standard Galerkin and control-volume finite elements to balance fluid mass, mechanical deformation, and thermal energy with consideration of local thermal non-equilibrium and/or dual-porosity heat exchange between fluids and solids or fractures and intact rock. Similarly, changes in mechanical stress and fluid pressure can be rigorously coupled in single or multiple continua. Permeability is allowed to evolve under several constitutive models tailored to both porous media and fractures, considering the influence of thermo-hydromechanical stress, creep, and elasto-plastic shear and dilation in a ubiquitously fractured medium. From this basis we explore the coupled physical processes that control the evolution of permeability during shear stimulation and long-term evolution of a geothermal reservoir.
Microseismic monitoring of hydraulic fracturing has provided great value for understanding hydraulic fracturing in unconventional reservoirs, including measurement of fracture geometry and optimization of stimulations, completions, and field development. Nevertheless, microseismic monitoring is a complex endeavor and many issues of fielding, analysis, uncertainty, and geophysics should be carefully assessed. The geomechanics of the generation of microseismicity are still being investigated, as well as the source mechanisms and how it all relates to the fracturing process. Besides the value for field development and resource recovery, microseismic monitoring has also proved useful for evaluating environmental and safety issues. Data from thousands of fractures show that the levels of induced seismicity in typical relaxed sedimentary basins are well below any levels that would be of concern for safety or damage. Similarly, data from thousands of fractures show that hydraulic fractures in shale reservoirs do not propagate into aquifers.
Monitoring of the Boguchany concrete gravity dam rock foundation condition includes collection and analysis of the data on external impacts and measurement of the diagnostic parameters of the dam rock foundation. The field observations were initiated at the start of the dam construction in 1983. During the period from 1998 to 2006 actually there was no construction work at the dam, however, instrumental observations continued. Construction of 96 m high Boguchany dam on the Angara River in Siberia was finished in 2012. On the 16-th of April 2012, impounding of the storage reservoir for the Boguchany hydropower project was started ,which is the 4-th hydraulic unit in the chain of hydro developments on the Angara river,. This paper sets out the basic results of observations covering this period and analyzes the findings .in the context of the general picture of the concrete dam foundation condition during the initial period of reservoir filling.'ÉP.
In-situ stress magnitude and orientation have long been analyzed separately using classical statistics, despite the fact that stress is a tensor, and it should be analyzed using tensor-related methods. In this paper we investigate the applicability of a two dimensional aleatory model that can handle the stress tensor as a single entity. Firstly, a multivariate normal distribution of tensor components is assumed and the marginal probability functions of the eigen-parameters (principal stresses and rotational angle) derived. Using published in-situ stress data, random stress tensors are generated to compare the distributions of eigen-parameters obtained using classical and tensor statistics. We conclude that for stress magnitude, these two methods give the identical results, whereas for orientation only tensor statistics gives the correct result. Additionally, it is only tensor statistics that is invariant with regard to orientation of the coordinate system. Therefore, we conclude that in practical cases that deal only with stress magnitude, either method can give reliable result, but any analysis involving principal stress orientation requires tensor statistics.
The present work initially presents an overview and the theoretical background of the Material Point Method (MPM) and details of its numerical implementation for coupled fluid-mechanical problems. This method is particularly useful when analyzing large strain problems in solid/fluid media including coupled problems, in particular, for geomechanical and geological media. The method possesses both Eulerian and Lagrangian characteristics which makes it suitable for the solution of a number of problems especially when compared to the usual techniques such as the Finite Element Method (FEM). Using the FEM, sometimes remeshing can make the analysis of certain problems particularly cumbersome. In particular, in the present work the MPM is used firstly for the determination of the complete failure pattern of openings, from the initiation until its complete closure, in two different scales, laboratory and tunnel lengths. This problem may involve large strains and contact situations. The last example includes fluid-mechanical coupling under dynamic conditions. Here, the dynamic effects associated with the impact of a rock block in a saturated porous media in a slope is evaluated.
In the absence of natural fractures, hydraulic fractures can open against the minimum principal stress and propagate along continuously through the formation. However, field observations and microseismic (MS) activity suggest that, natural fractures can reactivate upon stimulation: forming complex/sheared discrete fracture networks (DFNs). Current industry practice primarily utilizes MS events alone to determine the extent of reactivation within a DFN and consequently the stimulated rock volume (SRV). However, MS activity does not necessarily distinguish between dry (i.e. reactivation with non-conductive fractures) vs. wet events (i.e. reactivation with conductive fractures). Indeed, wet events are most likely to contribute towards a Productive-SRV as these conductive fractures have an increased chance to remain open during production. This paper presents a series of geomechanical sensitivity analysis that quantify the difference between fracture events within the framework of a complex discrete fracture network simulator coupled with fluid flow and fracture reactivation. Our results show that the extent of Productive-SRV is controlled primarily by a combination of reservoir, geomechanical and stimulation parameters. These parameters include: DFN geometry (e.g. patterns, spacing, density, and orientation), stress anisotropy/isotropy, frictional strength, the duration of stimulation, and the number of stages. Results are further coupled with an unconventional reservoir simulator to quantify ultimate recovery/production for different scenarios. This integrated study/workflow provides guidelines in the field to optimize stimulation operations and to better interpret microseismic field data.'ép.
Understanding the geometry of a hydraulic fracture is key to predicting its behavior and performance. Physical measurement of field hydraulic fracture geometries beyond the borehole is difficult and typically cost prohibitive with the only published examples being mine-back studies and cores. Laboratory-scale hydraulic fracturing experiments can more accurately measure the fracture geometry due to smaller specimen size and improved monitoring capabilities. This paper presents laboratory work where hydraulic fracture treatments were performed using epoxy injection such that a propagating fracture could be stabilized and preserved at near-critical state. Constant backpressure was applied after hydraulic breakdown but before cessation of fracture extension to maintain near-critical state geometry. Preliminary results are presented giving measurement of fracture dimensions, including aperture, at the millimeter scale for a hydraulic fractured acrylic specimen. The pressure, flow rate, material strains, acoustic emissions, and video stills associated with this fracture are also presented and analyzed. A second experiment fracturing a 300×300×300 mm3 cubic foot granite block using epoxy is also discussed. Data regarding the interaction between shear and tensile dominated fractures is presented and discussed.
Hydraulic fracturing (HF) may lead to practical stress management possibilities by creating opportunities to control stress redistribution, or protecting locations where high stresses pose a threat to operations. These possibilities have application in petroleum engineering as well as mining. Understanding naturally fractured rock (NFR) behavior leads to better predictions of rock mass response to HF treatment and induced fracture initiation and propagation. Natural fractures exist in many different states, are reactivated by stress and pressure changes, and have alterable mechanical properties (e.g. stiffness, shear strength), leading to complex behavior in shear, opening, and closing reactions to stress changes. This article presents some attempts to understand and address simulation of HF interacting with a NFR using the Distinct Element Method (DEM) to represent the NFR. To this end, a coupled hydro-mechanical analysis is applied via the Universal Distinct Element Code (UDEC™) software to model both rock and fracture behavior in the HF/NFR system. In the current study, a Voronoi tessellated continuum has been generated to evaluate the effect of the stress ratio on flow into the joints by changing the differential principal compressive stresses. Given the difference in in situ stresses, pore pressure distribution is monitored and the distribution of slip and opening of fractures at different stress field anisotropy is investigated during pressurized hydraulic injection. Based on simulations, pore pressure decreases in a uniform pattern around the injection point in the isotropic stress state; however, the pressure distribution tends to become strongly anisotropic under a stronger differential stress. In addition, both normal and shear displacements show an increasing trend toward anisotropy under stronger stress differences. Applications may ensue with better understanding, such that HF strategies for strongly differential stress fields may evolve to be substantially different than for near-isotropic stress fields, and similar conclusions may ensue for random NFR fabrics, compared to cases with strongly oriented natural fracture fabric.
This paper is devoted to micromechanical modeling of the overall elastoplastic behavior and damage evolution in clayey rocks. The studied material is composed of a porous matrix which is embedded by linear elastic mineral inclusions. The solid phase of porous matrix is described by a pressure sensitive plastic model with a non associated flow rule. With a two-level homogenization procedure, the macroscopic plastic criterion of the heterogeneous material is deduced and takes into account effects of pores and mineral inclusions. Then it is assumed that the material damage is related to progressive debonding of mineral inclusions. The Weibull's statistical function is used to describe the varying probability of inclusion debonding. Finally, the proposed micro-macro model is applied to describe the macroscopic behavior of a typical clayey rock. Comparisons between the numerical results and experimental data show that the proposed model is able to capture the main features of the mechanical behavior of the studied material.
Cohesion debonding and friction mobilization along discontinuities are studied in the laboratory by monitoring seismic wave propagation through a discontinuity and measuring slip displacements using Digital Image Correlation (DIC). Slip was imposed on two individual blocks of gypsum through a contact surface that had both cohesion and friction. Investigation of slip was carried out by acquiring transmitted and reflected compressional, P-, and shear, S-, waves prior to and during failure of the specimens. DIC was used to monitor the discontinuity by measuring the surface displacement field. Direct shear experiments performed on gypsum specimens with cohesion revealed that debonding (loss of cohesion) occurred before the peak shear strength was reached, and that friction mobilization and cohesion debonding do not occur simultaneously along the discontinuity. In fact, the amount of slip that is induced due to debonding is an order of magnitude smaller than the slip needed for mobilizing the full frictional strength of the discontinuity. The loss of cohesion has distinct seismic signatures and resulted in a significant decrease in transmitted amplitude, an increase in the reflected amplitude, a significant reduction in wave velocity, and a sudden reduction of the dominant frequency of the waveforms for both compressional and shear waves.