Vol. 8 No. 1 2016 JRMGE (OA journal)

Journal of Rock Mechanics and Geotechnical Engineering

Vol. 8 No. 1 2016

1. Ferri Hassani, Pejman M. Nekoovaght, Nima Gharib. The influence of microwave irradiation on rocks for microwave-assisted underground excavation. Journal of Rock Mechanics and Geotechnical Engineering. 2016, 8 (1): 1–15.

https://www.sciencedirect.com/science/article/pii/S1674775515001341             or

https://www.rockgeotech.org/qikan/manage/wenzhang/20160101.pdf

Abstract: Demand is growing for explosive-free rock breakage systems for civil and mining engineering, and space industry applications. This paper highlights the work being undertaken in the Geomechanics Laboratory of McGill University to make a real application of microwave-assisted mechanical rock breakage to full-face tunneling machines and drilling. Comprehensive laboratory tests investigated the effect of microwave radiation on temperature profiles and strength reduction in hard rocks (norite, granite, and basalt) for a range of exposure times and microwave power levels. The heating rate on the surface of the rock specimens linearly decreased with distance between the sample and the microwave antenna, regardless of microwave power level and exposure time. Tensile and uniaxial compressive strengths were reduced with increasing exposure time and power level. Scanning electron micrographs (SEMs) highlighted fracture development in treated basalt. It was concluded that the microwave power level has a strong positive influence on the amount of heat damage induced to the rock surface. Numerical simulations of electric field intensity and wave propagation conducted with COMSOL Multiphysics? software generated temperature profiles that were in close agreement with experimental results.

Keywords: Microwaves; Crack density; Microwave-assisted tunnel boring; Rock breakage

1. Chun-Liang Zhang. The stress–strain–permeability behaviour of clay rock during damage and recompaction. Journal of Rock Mechanics and Geotechnical Engineering. 2016,8 (1): 16–26.

https://www.sciencedirect.com/science/article/pii/S1674775515001225              or

https://www.rockgeotech.org/qikan/manage/wenzhang/20160102.pdf   

Abstract: Characterisation and understanding of the stress–strain–permeability behaviour of a clay host rock during damage and recompaction are essential for prediction of excavation damaged zone and for assessment of its impact on the repository safety. This important issue has been experimentally studied in triaxial compression tests on the Callovo-Oxfordian clay rock in this study. The samples were sequentially loaded by (1) hydrostatic precompaction to close up sampling-induced microcracks, (2) applying deviatoric stresses to determine damage and permeability changes, and (3) recompression along different loading paths to examine reversibility of the damage. The critical stress conditions at the onset of dilatancy, permeability percolation, failure strength, and residual strength are determined. An empirical model is established for fracturing-induced permeability by considering the effects of connectivity and conductivity of microcracks. The cubic law is validated for the variation of permeability of connected fractures with closure. The experiments and results are also presented and discussed.

Keywords: Clay rock; Deformation; Damage; Compaction; Permeability

1. Xinglin Lei, Takahiro Funatsu, Shengli Ma, Liqiang Liu. A laboratory acoustic emission experiment and numerical simulation of rock fracture driven by a high-pressure fluid source. Journal of Rock Mechanics and Geotechnical Engineering. 2016, 8 (1): 27-34.

https://www.sciencedirect.com/science/article/pii/S1674775515000621                  or

https://www.rockgeotech.org/qikan/manage/wenzhang/20160103.pdf  

Abstract: In order to improve our understanding of rock fracture and fault instability driven by high-pressure fluid sources, the authors carried out rock fracture tests using granite under a confining pressure of 80 MPa with fluid injection in the laboratory. Furthermore, we tested a number of numerical models using the FLAC3D modeling software to find the best model to represent the experimental results. The high-speed multichannel acoustic emission (AE) waveform recording system used in this study made it possible to examine the total fracture process through detailed monitoring of AE hypocenters and seismic velocity. The experimental results show that injecting high-pressure oil into the rock sample can induce AE activity at very low stress levels and can dramatically reduce the strength of the rock. The results of the numerical simulations show that major experimental results, including the strength, the temporal and spatial patterns of the AE events, and the role of the fluid can be represented fairly well by a model involving (1) randomly distributed defect elements to model pre-existing cracks, (2) random modification of rock properties to represent inhomogeneity introduced by different mineral grains, and (3) macroscopic inhomogeneity. Our study, which incorporates laboratory experiments and numerical simulations, indicates that such an approach is helpful in finding a better model not only for simulating experimental results but also for upscaling purposes.

Keywords: Laboratory experiment; Acoustic emission (AE); Fracture; Numerical simulation; Fluid injection

1. Chengbo Yu, Shaocheng Ji, Qi Li. Effects of porosity on seismic velocities, elastic moduli and Poisson's ratios of solid materials and rocks. Journal of Rock Mechanics and Geotechnical Engineering. 2016, 8 (1): 35-49.

https://www.sciencedirect.com/science/article/pii/S1674775515000967               or

https://www.rockgeotech.org/qikan/manage/wenzhang/20160104.pdf

Abstract: The generalized mixture rule (GMR) is used to provide a unified framework for describing Young's (E), shear (G) and bulk (K) moduli, Lame parameter (λ), and P- and S-wave velocities (Vp and Vs) as a function of porosity in various isotropic materials such as metals, ceramics and rocks. The characteristic J values of the GMR for E, G, K and λ of each material are systematically different and display consistent correlations with the Poisson's ratio of the nonporous material (ν0). For the materials dominated by corner-shaped pores, the fixed point at which the effective Poisson's ratio (ν) remains constant is at ν0 = 0.2, and J(G) > J(E) > J(K) > J(λ) and J(G) < J(E) < J(K) < J(λ) for materials with ν0 > 0.2 and ν0 < 0.2, respectively. J(Vs) > J(Vp) and J(Vs) < J(Vp) for the materials with ν0 > 0.2 and ν0 < 0.2, respectively. The effective ν increases, decreases and remains unchanged with increasing porosity for the materials with ν0 < 0.2, ν0 > 0.2 and ν0 = 0.2, respectively. For natural rocks containing thin-disk-shaped pores parallel to mineral cleavages, grain boundaries and foliation, however, the ν fixed point decreases nonlinearly with decreasing pore aspect ratio (α: width/length). With increasing depth or pressure, cracks with smaller α values are progressively closed, making the ν fixed point rise and finally reach to the point at ν0 = 0.2.

Keywords: Porous rocks; Seismic velocities; Elastic moduli; Poisson's ratio; Porosity

1. Chaoshui Xu, Peter Dowd, Qi Li. Carbon sequestration potential of the Habanero reservoir when carbon dioxide is used as the heat exchange fluid. Journal of Rock Mechanics and Geotechnical Engineering. 2016, 8 (1): 50-59.

https://www.sciencedirect.com/science/article/pii/S1674775515000694                or

https://www.rockgeotech.org/qikan/manage/wenzhang/20160105.pdf  

Abstract: The use of sequestered carbon dioxide (CO2) as the heat exchange fluid in enhanced geothermal system (EGS) has significant potential to increase their productivity, contribute further to reducing carbon emissions and increase the economic viability of geothermal power generation. Coupled CO2 sequestration and geothermal energy production from hot dry rock (HDR) EGS were first proposed 15 years ago but have yet to be practically implemented. This paper reviews some of the issues in assessing these systems with particular focus on the power generation and CO2 sequestration capacity. The Habanero geothermal field in the Cooper Basin of South Australia is assessed for its potential CO2 storage capacity if supercritical CO2 is used as the working fluid for heat extraction. The analysis suggests that the major CO2 sequestration mechanisms are the storage in the fracture-stimulation damaged zone followed by diffusion into the pores within the rock matrix. The assessment indicates that 5% of working fluid loss commonly suggested as the storage capacity might be an over-estimate of the long-term CO2 sequestration capacity of EGS in which supercritical CO2 is used as the circulation fluid.

Keywords: Carbon sequestration; Carbon dioxide (CO2) geological storage capacity; Enhanced geothermal system (EGS); CO2-EGS; Habanero project

1. Matthew A. Perras, Mark S. Diederichs. Predicting excavation damage zone depths in brittle rocks. Journal of Rock Mechanics and Geotechnical Engineering. 2016, 8 (1): 60-74.

https://www.sciencedirect.com/science/article/pii/S1674775515001407   or

https://www.rockgeotech.org/qikan/manage/wenzhang/20160106.pdf

Abstract: During the construction of an underground excavation, damage occurs in the surrounding rock mass due in large part to stress changes. While the predicted damage extent impacts profile selection and support design, the depth of damage is a critical aspect for the design of permeability sensitive excavations, such as a deep geological repository (DGR) for nuclear waste. Review of literature regarding the depth of excavation damage zones (EDZs) indicates three zones are common and typically related to stress induced damage. Based on past developments related to brittle damage prediction using continuum modelling, the depth of the EDZs has been examined numerically. One method to capture stress induced damage in conventional engineering software is the damage initiation and spalling limit (DISL) approach. The variability of depths predicted using the DISL approach has been evaluated and guidelines are suggested for determining the depth of the EDZs around circular excavations in brittle rock masses. Of the inputs evaluated, it was found that the tensile strength produces the greatest variation in the depth of the EDZs. The results were evaluated statistically to determine the best fit relation between the model inputs and the depth of the EDZs. The best correlation and least variation were found for the outer EDZ and the highly damaged zone (HDZ) showed the greatest variation. Predictive equations for different EDZs have been suggested and the maximum numerical EDZ depths, represented by the 68% prediction interval, agreed well with the empirical evidence. This suggests that the numerical limits can be used for preliminary depth prediction of the EDZs in brittle rock for circular excavations.

Keywords: Excavation damage zones (EDZs); Deep geological repository (DGR); Empirical depth prediction; Numerical depth prediction; Damage depth sensitivity; Damage initiation and spalling limit (DISL)

1. J.G. Wang, Yang Ju, Feng Gao, Jia Liu. A simple approach for the estimation of CO2 penetration depth into a caprock layer. Journal of Rock Mechanics and Geotechnical Engineering. 2016, 8 (1): 75-86.

https://www.sciencedirect.com/science/article/pii/S1674775515001328             or

https://www.rockgeotech.org/qikan/manage/wenzhang/20160107.pdf  

Abstract: Caprock is a water-saturated formation with a sufficient entry capillary pressure to prevent the upward migration of a buoyant fluid. When the entry capillary pressure of caprock is smaller than the pressure exerted by the buoyant CO2 plume, CO2 gradually penetrates into the caprock. The CO2 penetration depth into a caprock layer can be used to measure the caprock sealing efficiency and becomes the key issue to the assessment of caprock sealing efficiency. On the other hand, our numerical simulations on a caprock layer have revealed that a square root law for time and pore pressure exists for the CO2 penetration into the caprock layer. Based on this finding, this study proposes a simple approach to estimate the CO2 penetration depth into a caprock layer. This simple approach is initially developed to consider the speed of CO2 invading front. It explicitly expresses the penetration depth with pressuring time, pressure difference and pressure magnitude. This simple approach is then used to fit three sets of experimental data and good fittings are observed regardless of pressures, strengths of porous media, and pore fluids (water, hydrochloric acid, and carbonic acid). Finally, theoretical analyses are conducted to explore those factors affecting CO2 penetration depth. The effects of capillary pressure, gas sorption induced swelling, and fluid property are then included in this simple approach. These results show that this simple approach can predict the penetration depth into a caprock layer with sufficient accuracy, even if complicated interactions in penetration process are not explicitly expressed in this simple formula.

Keywords: Fracture-matrix system; Fully coupled model; Two-phase flow model; Square root law; Simple approach; CO2 penetration depth; Caprock sealing efficiency

1. Qianlin Zhu, Qianlong Zhou, Xiaochun Li. Numerical simulation of displacement characteristics of CO2 injected in pore-scale porous media. Journal of Rock Mechanics and Geotechnical Engineering. 2016, 8 (1): 87-92.

https://www.sciencedirect.com/science/article/pii/S1674775515001079              or

https://www.rockgeotech.org/qikan/manage/wenzhang/20160108.pdf  

Abstract: Pore structure of porous media, including pore size and topology, is rather complex. In immiscible two-phase displacement process, the capillary force affected by pore size dominates the two-phase flow in the porous media, affecting displacement results. Direct observation of the flow patterns in the porous media is difficult, and therefore knowledge about the two-phase displacement flow is insufficient. In this paper, a two-dimensional (2D) pore structure was extracted from a sandstone sample, and the flow process that CO2 displaces resident brine in the extracted pore structure was simulated using the Navier–Stokes equation combined with the conservative level set method. The simulation results reveal that the pore throat is a crucial factor for determining CO2 displacement process in the porous media. The two-phase meniscuses in each pore throat were in a self-adjusting process. In the displacement process, CO2 preferentially broke through the maximum pore throat. Before breaking through the maximum pore throat, the pressure of CO2 continually increased, and the curvature and position of two-phase interfaces in the other pore throats adjusted accordingly. Once the maximum pore throat was broken through by the CO2, the capillary force in the other pore throats released accordingly; subsequently, the interfaces withdrew under the effect of capillary fore, preparing for breaking through the next pore throat. Therefore, the two-phase displacement in CO2 injection is accompanied by the breaking through and adjusting of the two-phase interfaces.

Keywords: Level set method; Displacement; Pore-scale porous media; Numerical simulation

1. Mohsen S. Masoudian. Multiphysics of carbon dioxide sequestration in coalbeds: A review with a focus on geomechanical characteristics of coal. Journal of Rock Mechanics and Geotechnical Engineering. 2016, 8 (1): 93-112.

https://www.sciencedirect.com/science/article/pii/S1674775515000992              or

https://www.rockgeotech.org/qikan/manage/wenzhang/20160109.pdf 

Abstract: To reduce the emissions of carbon dioxide (CO2) into the atmosphere, it is proposed to inject anthropogenic CO2 into deep geological formations. Deep un-mineable coalbeds are considered to be possible CO2 repositories because coal is able to adsorb a large amount of CO2 inside its microporous structure. However, the response of coalbeds is complex because of coupled flow and mechanical processes. Injection of CO2 causes coal to swell, which leads to reductions in permeability and hence makes injection more difficult, and at the same time leads to changes in the mechanical properties which can affect the stress state in the coal and overlying strata. The mechanical properties of coal under storage conditions are of importance when assessing the integrity and safety of the storage scheme. On the other hand, the geomechanical response of coalbed will also influence the reservoir performance of coalbed. This paper provides an overview of processes associated with coalbed geosequestration of CO2 while the importance of geomechanical characteristics of coalbeds is highlighted. The most recent findings about the interactions between gas transport and geomechanical characteristics of coal will be discussed and the essence will be delivered. The author suggests areas for future research efforts to further improve the understanding of enhanced coalbed methane (ECBM) and coalbed geosequestration of CO2.

Keywords: Coalbed geosequestration; Carbon dioxide (CO2); Reservoir geomechanics; Elasticity; Rock failure

1. Yankun Sun, Qi Li, Duoxing Yang, Xuehao Liu. Laboratory core flooding experimental systems for CO2 geosequestration: An updated review over the past decade. Journal of Rock Mechanics and Geotechnical Engineering. 2016, 8 (1): 113-126.

https://www.sciencedirect.com/science/article/pii/S1674775515001419            or

https://www.rockgeotech.org/qikan/manage/wenzhang/20150610.pdf 

Abstract: Carbon dioxide (CO2) geosequestration in deep saline aquifers has been currently deemed as a preferable and practicable mitigation means for reducing anthropogenic greenhouse gases (GHGs) emissions to the atmosphere, as deep saline aquifers can offer the greatest potential from a capacity point of view. Hence, research on core-scale CO2/brine multiphase migration processes is of great significance for precisely estimating storage efficiency, ensuring storage security, and predicting the long-term effects of the sequestered CO2 in subsurface saline aquifers. This review article initially presents a brief description of the essential aspects of CO2 subsurface transport and geological trapping mechanisms, and then outlines the state-of-the-art laboratory core flooding experimental apparatus that has been adopted for simulating CO2 injection and migration processes in the literature over the past decade. Finally, a summary of the characteristics, components and applications of publicly reported core flooding equipment as well as major research gaps and areas in need of further study are given in relevance to laboratory-scale core flooding experiments in CO2 geosequestration under reservoir conditions.

Keywords: Carbon capture and storage (CCS); Geosequestration; Trapping mechanism; Core flooding; Saline aquifer; Experimental apparatus