ABSTRACT: This paper focuses on the simulation of localized compaction in granular rocks. For this purpose, a continuum framework referred to as Breakage Mechanics is used to capture the role of microscopic crushing on the mechanical response of grain assemblies. In particular, grain size dependencies are introduced by connecting the physics of grain-scale fracture to the energetics of collective crushing. It is shown that this approach enables the simultaneous consideration of changes in grain sorting and average grain size, where the role of the latter is modeled via central splitting and contact fracture laws. Using this constitutive framework, the localization potential of Bentheim sandstone has been studied with the purpose to emphasize the role of grain scale characteristics in the inception of compaction banding. The analyses show that the model captures correctly the increase of the localization potential resulting from a coarser gradation or a narrow grain size distribution.
Understanding the processes that control compaction and faulting in reservoir rocks is crucial for many branches of geophysics and geo-engineering. In this context, a considerable challenge is due to compaction banding, a mechanism by which strains localize into narrow tabular zones (Aydin, 1978, Mollema and Antonellini, 1996, Fossen et al., 2007). These deformation bands are critical for the energy industry in that they lead to marked pore collapse and permeability loss, thus having considerable impact on reservoir integrity and injection/extraction efficiency (Wawersik et al., 2001).
A widely adopted tool to assess the localization potential of rocks is the bifurcation theory proposed by Rudnicki and Rice, 1975, often used in combination with elastoplastic constitutive laws. Among its many applications, this theory has been used to study compaction banding in porous rocks (Issen and Rudinicki, 2000) via cap-plasticity (Challa and Issen, 2004) or critical state models (Buscarnera and Laverack, 2013; Das and Buscarnera, 2014). These models are typically calibrated on the basis of the results of laboratory tests, which, in addition to measurements of stress-strain response, have often provided a link between shear-enhanced compaction and inelastic phenomena at the grain-scale (Wong et al., 1997, Klein et al., 2001, Wu et al., 2000, Besuelle et al., 2003, Baud et al., 2004). Recently, such multi-scale links have been exploited in the context of the theoretical and numerical modeling of compaction bands. An example is given by the work of Das et al., 2011, 2013, who studied compaction banding in sandstones through a microstructure-inspired constitutive framework referred to as Breakage Mechanics (Einav, 2007a,b). Starting from thermodynamic principles, this theory links the macroscopic yielding of a granular rock to the energy released by crushing events, thus enabling the simulation of the changes in grain size distribution due to high-pressure compaction. Despite its links with the micromechanics of grain crushing, recent findings by Zhang and Buscarnera, 2014 pointed out that the model lacks an inherent length scale related with the size of the largest grains, thus preventing the model from capturing major grain-size dependencies. Hence, to incorporate such length scale, Zhang et al., 2016 have recently augmented the framework by proposing a linear scaling law that connects the fracture properties of grains to the macroscopic critical breakage energy of the solid, thus bridging the physics of fracture at particle scale with the energetics of comminution at continuum scale.
ABSTRACT: Slope failure is often caused by heavy rain fall and it crashes into small pieces with water. However, this process could not be solved by conventional FEM. In this paper we introduce discontinuous DDA (Discontinuous Deformation analysis) and particle MPS (Moving Particle Simulation) analysis to solve a complex solid and fluid interaction problem, especially rock and water mixed debris flow. This study presents a new approach in which rock block movement is solved by DDA and water flow inside the rock mass is solved by MPS (Moving Particle Simulation) method separately. Then they are coupled in the calculation to account for the mutual interaction. To analyze the coupling between the water flow and the falling blocks of rock, this study introduces the equivalent external forces for the falling blocks of rock caused by the water flow. The velocities are calculated for the fluid flow based on MPS. MPS analyzes the part of fluid alone and DDA analyzes the part of solid alone to set the initial state of the analysis. After the results is superimposed, the coupling analysis is carried out.
Topographically the mountainous area in Japan occupy about 70% of the whole country and the earthquake and volcanic activities is very dynamic. The disaster due to earthquake and heavy rain has occurred frequently. In particular, the scale of slope disaster due to heavy rain tends to increase in recent years. Many researchers and the government have tried to predict the occurrence and damage of slope disasters. The analytic method also has been applied to the estimation of the damage, however it is not always successful. One of the reason is considered that continuum method such as FEM is applied to analyze the phenomenon with large movement and a breakdown occurring in slope disasters. On the other hand, discontinuous method such as Discontinuous Deformation Analysis (DDA) (Shi and Goodman, 1989) and Distinct Element Method (DEM) (Cundall, 1971) has also been used as analyses methods of the slope disasters. However, these methods cannot handle the mixed body of fluid and solid material appropriately. It is known that the water explicitly affect stability of rock slope and debris flow is a mixture of water and rock blocks.
Recently, a particle method, which is one of mesh free methods, has been used to analyze water movement. The particle method is simple but powerful method to solve the problem of fluid mechanics. In this paper we introduce the discontinuous DDA and particle MPS (Moving Particle Simulation) analysis to solve a complex solid and fluid interaction problem, especially rock and water mixed debris flow. In the analysis of the coupling between the water flow and falling blocks of rock, the equivalent external for the falling blocks of rock caused by the water flow is introduced.
ABSTRACT: Hydraulic fracture initiation and propagation in the presence of multiple fluid injection sources and under far-field stresses is investigated experimentally utilizing a novel fracturing cell. Porous test specimens with injection points in different configurations are placed between two transparent plates. Experiments are conducted to examine hydraulic fracture growth in proximity to a constant pressure injection source as well as propagation of multiple neighboring fractures under different injection conditions. The fracturing process is recorded using a high resolution digital camera. A variety of fracture behavior is observed in these experiments that demonstrate the interplay between high pore pressure regions, the applied contrast in far-field stresses, and fluid injection scheme and configuration of injectors.
Changes in pore pressure by fluid injection or production in oil and gas reservoirs can have a significant influence on the magnitude and orientation of in-situ stresses. Initiation and propagation of hydraulic fractures can, therefore, be affected by a perturbation of pore pressure field.
Bruno and Nakagawa (1991) performed laboratory experiments in which a hydraulic fracture was propagated from an injection port in thin sandstone slabs under the influence of a nearby fluid injection source maintained at constant pressure. The experiments reported by the authors were performed without applying far-field stresses, i.e. under isotropic far-field stress state. Bouteca et al. (1983) reported an experimental result showing hydraulic linkage between two injection ports that were offset by 10 degrees from an applied maximum far-field stress without reporting the specifics of the experiment. Analytical and numerical evaluation of similar problems were reported by Berchenko and Detournay (1997) and Wang et al. (2013).
In this work, we performed an experimental study that examines the effect of pore pressure on hydraulic fracture growth using novel experimental capabilities. A series of tests were performed in a systematic manner under various fluid injection schemes and configurations and varying contrast in the applied far-field stresses.
ABSTRACT: Steam assisted gravity drainage (SAGD) adopts a stimulation process through water injection to a pair of horizontal wells. Two deformation mechanisms named shear dilation and tensile parting occur in this process. Triaxial shear and hydrostatic pressure tests were conducted on specimens sampled from field-collected oil sand cores, so as to investigate the compressibility of shear dilation and tensile parting dilation of oil sand under the real water injection pressure as well as in-situ stress and temperature states in the field. It is revealed that the compressibility of the specimens under shear increases with increasing mean effective stress until strain-softening takes place, but displays an opposite trend thereafter. Moreover, such a compressibility increases with decreasing effective confining stress and temperature, or an increase in uniaxial strain. The tensile dilation tests, on the other hand, disclosed that the associated compressibility changes inversely proportional to the mean effective stress or temperature. Interestingly, an increased temperature of the injected fluid suppresses rather than promoting the compressibility of an oil sand reservoir. A wellpair in the Fengcheng oilfield of Karamay was examined to predict its ultimate injected water volume, producing a result very close to that recorded in the field.
Micro-fracturing technology was originally applied in the development of marine oil sands in Alberta, Canada. Here micro-fracturing or water injection instead of hydraulic fracturing is a more appropriate term due to the absence of stress shadow effects in weak rocks (Lin et al. 2015&2016). Resulting from the historical glacial action, the solid particles of marine oil sands in Canada are characterized by an interlocking fracture similar to the over-consolidated soil (Oldakowski 1994, Yuan et al. 2011). On the other hand, the oil sands in Karamay, northwest China, belong to land-facies deposit in absence of glacial activities and their compaction, resulting in a loose fabric with scattered mud stringers (Yuan et al. 2013, Lin et al. 2015). Micro-fracturing by water injection has been widely implemented in the steam assisted gravity drainage (SAGD) projects on oil sand reservoirs in Karamay, aiming to expand the pore space of the domain encompassing the dual wells, such that the preheating period to shorten the preheating stage and improve the propagation of steam chamber (Lin et al. 2016&2017). The micro-fracturing physical models of oil sand reservoirs in Karamay are given in Fig. 1.
ABSTRACT: Rockbursts under high stress environment is of prime concerns during underground excavations. Infrared camera thermography could be of some value for assessing hot spots at the excavation face, which may result in pop-ups, spalling or rockbursting. The authors undertook an experimental and monitoring program on the possibility of the utilization of the Infrared Camera Thermography (IRCT) for assessing the real-time stability of underground excavations. First, a series of laboratory tests on rock specimens and large rock blocks having a circular hole are tested under compression environment in relation to the deformation and rupture processes. Later, some observations were carried out at the excavation face of Taru-Toge tunnel before, during and after blasting operation in relation to assess the real-time stability of the excavation face. The authors present the outcomes of this unique experimental and monitoring program for real-time stability assessments of underground excavations and discuss their implications in actual applications.
Stability problems such as rockburst under high stress environment are of prime concerns during underground excavations. Aydan and his group performed a series of tests on minerals and rocks ranging from soft to hard, which showed distinct heat releases before the rupture (Aydan et al. 2014, 2015). This fact implied that infrared camera thermography should be of some value for assessing hot spots at the excavation face, which may result in pop-ups, spalling or rockbursting.
The authors undertook an experimental and monitoring program on the possibility of the utilization of the Infrared Camera Thermography (IRCT) for assessing the real-time stability of underground excavations. First, a series of laboratory tests on rock specimens and large rock blocks having a circular hole are tested under compression environment in relation to the deformation and rupture processes. Later, some observations were carried out at the excavation face of Taru-Toge tunnel before, during and after blasting operation in relation to assess the real-time stability of the excavation face. The authors present the outcomes of this unique experimental and monitoring program for real-time stability assessments of underground excavations and discuss their implications in actual applications.
Zertsalov, M. G. (Moscow State University of Civil Engineering (National Research University)) | Khokhlov, I. N. (Moscow State University of Civil Engineering (National Research University)) | Nikishkin, M. V. (Moscow State University of Civil Engineering (National Research University))
ABSTRACT: In the article the interaction of drilled shafts with rock mass, folded by rocks of low to medium strength, is analyzed. The analysis is based on the results of numerical three-dimensional simulation of the behavior of drilled shafts under the action of vertical compressive load. The results of the research allow to obtain the factor dependencies (regression equation) for calculating of bearing capacity and settlements of shafts. As independent variable factors in the equations were adopted: the ratio of the modulus of elasticity of the shaft to the modulus of deformation of intact rock, the ratio of the length of the drilled shaft to its diameter and RQD of the rock mass. The effect of the shaft - rock mass interface characteristics on the settlements of shafts were studied separately. Results of numerical simulation studies were compared with the results of in-situ tests of shafts.
Socketed shafts are usually designed and constructed as foundations of high-rise buildings and bridge structures, when layers of loose soil overlie bedrock. The behavior of socketed shaft both in rock and in soil has a lot in common. However, the structure of a rock mass and their highly variable mechanical characteristics considerably complicate the calculation of bearing capacity and settlement of socketed shafts.
In soils, the load corresponding to the bearing capacity of shaft (Qp), is calculated by simple summation of the shear resistance force along its side surface and the resistance force of the base of the shaft (Qs and Qb). In the case of rocks, different researches show that this approach is possible only, if the failure of the sidewall interface (shaft – rock) is plastic and a load (Qb), transmitted to the base of the shaft, can be determined (NCHRP Synthesis 360).
Especially it should be emphasized the influence of the sidewall interface on the interaction of the shaft with surface of the rock. Variability in the properties of the interface and the character of its failure can have a considerable impact on the bearing capacity and the settlement of the shaft.
ABSTRACT: The aim of this paper is to present workflow of implementing 1-D geomechanical analysis for optimization hydraulic fracturing design. Well stress modelling is very important in case of optimal stimulation of unconventional shale formations. It can be achieved by generating complex fracture system around the horizontal wellbore. Properly designed hydraulic fracturing is currently the best way to improve contact between well and unconventional reservoir. Data gathered during hydraulic fracturing treatments resulted in calibration existing geological models and optimization of future well completions. Geomechanical modelling and results obtained from Lublin Basin Silurian shale formation hydraulic stimulation are good example of cooperation between geology and engineering teams. Similar analysis can be transferred to other unconventional basins, regarding to acquire enough data. The examples showed in this paper illustrate iterative workflow of the ORLEN Upstream Ltd. in improving hydraulic fracturing design. Furthermore, it also demonstrates production potential of shale formations of the Lublin Basin and determines way for exploration unconventional rocks in Poland.
The unconventional hydrocarbon resources may affect more and more on global energy balance. A number of international oil and gas companies, including Polish ones, already done several projects to acquire commercial hydrocarbon production from shale formations in Poland. ORLEN Upstream Ltd. has encountered experience on exploring unconventional Paleozoic shale formations of the Lublin Basin. Described in the article operations had been conducted in the years 2011-2015 and leaded by a highly qualified team, based in Warsaw, representing upstream segment in the PKN ORLEN S.A., the biggest company in central and eastern Europe, originally downstream oriented and developing its oil & gas activity since 2006.
Exploration and production of unconventional plays requires integration and cooperation across geological and engineering teams. As mentioned previously by Wikel, 2011, geomechanics is recently becoming more appreciated bridge between geological sciences and engineering practice. It is also one of the most impacting parameters of shale reservoirs. Geomechanical parameters determines how borehole instability features appears, how hydraulic fractures grows and how much proppant can be pumped into formation.
ABSTRACT: We propose an interfacial contact/damage model for simulating dynamic fracture in rocks. An interfacial damage parameter, D, models the evolution of damage on fracture interfaces, while relative contact and contact-stick fractions model contact-separation and stick-slip transitions. The damage rate is determined by an effective stress, written as a scalar function of the normal and tangential components of the Riemann traction solution for assumed bonded conditions. We propose alternative definitions of the effective stress that generate failure criteria that resemble the Tresca and Mohr-Coulomb criteria for compressive stress states, and we compare their compressive strengths and fracture angles under a compressive loading. We adopt a stochastic Weibull model for crack-nucleation in which cracks nucleate at points where the effective stress exceeds the probabilistic fracture strength. We implement the nucleation model with an h-adaptive asynchronous spacetime discontinuous Galerkin (aSDG) method that captures accurately the complex fracture patterns that arise under dynamic loading conditions. Numerical examples illustrate the effects on fracture response of varying the stochastic nucleation parameters and the alternative definitions of the effective stress.
Understanding the stress states that cause rock failure is critical to the reliable analysis and safe design of structures in rocks. In situ rock is typically subjected to compressive stress fields, and experimental observations indicate that the compressive strength of rock increases with increasing confining pressure. Failure occurs by shearing along planes oriented at a rock-type-specific angle, θ, defined relative to the direction of maximum compressive stress . Failure criteria describe the variation of compressive strength with confining pressure and, in general, the stress states at which rock fails.
A number of failure criteria have been proposed in rock mechanics. The Tresca criterion assumes that a material fails on planes with maximum shear stress. While it is sometimes used for failure analysis of rock , the Tresca criterion is more appropriate for ductile materials as its corresponding shear strength is independent of the confinement pressure. The Mohr-Coulomb (MC) failure criterion depends linearly on the normal and shear stress components. This implies a linear relation between confinement pressure and compressive strength. However, this straight-line relation does not always fit experimental data , and the extension of the linear relation into the tensile loading regime generally over-predicts the tensile strength of rock. Experiments show that the rates of increase of the shear and compressive strengths decrease as confinement pressure increases. In fact, beyond a certain confinement pressure, rock reaches a critical state at which the shear strength no longer increases, similar to the constant shear strength of the Tresca model . Beyond the limitations associated with linearity of the Mohr-Coulomb model, various studies demonstrate that fully three-dimensional failure criteria are required to capture the influence of the intermediate principal stress [5-8]. However, due to its simplicity and the challenges involved in calibrating the more advanced models, the Mohr-Coulomb model is still the most popular and widely used-failure criterion for rock.
ABSTRACT: The permeability of shale gas reservoir is very low. Hydraulic fracturing is needed to form an effective production capacity. However, creep properties of shale gas reservoir rocks have effects on the development of shale gas, for instance, artificial fracture flow conductivity is very sensitive to the creep deformation of the crack. In this work, we make numerical model of the creep behavior of shale gas reservoir and the constitutive relation is based on creep experiments in triaxial deformation apparatus under room temperature and room humidity conditions. Model result shows that after one year the creep deformation is around 9% of fracture width and it will enhance proppant embedment and fracture closure during production process. The capacity of fracture flow conductivity can strongly decrease based on the creep deformation of shale rock after a few decades. In conclusion, shale gas reservoir rock has creep deformation controlled by power-law function of the time. Creep behavior leads to the fracture closure and decrease of fracture flow conductivity in the reservoir.
Creep is a phenomenon in which material deformation increases with time. It has an important effect on the long-term deformation, strength and seepage characteristics of materials. It widely plays important roles in mine, tunnel, nuclear waste storage and large-scale gas storage construction. The various types of rocks have different degrees of creep characteristics. Normally creep deformation studies are mostly for high creep performance of soft rock such as mudstone and salt rock. Zhou et al. (1985) and Deng et al. (1993) established rock triaxial creep experimental devices in the laboratory and the creep experiment was carried out on mudstone and salt rock. The creep empirical mode and constitutive equation were established to analyze the stress on casing and wellbore stability.
At present, shale gas has become one of China's important oil and gas replacement resources. Shale gas reservoirs are generally more brittle rock and the permeability is very low. The key to the success of fracturing is the formation of large-scale, mutual communication and stability of the fracture network. Sone and Zoback (2010) found that the shale gas reservoir has a certain creep. The existence of the artificial fractured network of shale gas reservoirs and its extremely sensitive to weak deformation make the study of shale gas reservoir creep become important. The shale gas reservoir after fracturing has the characteristics of long exploitation time, artificial network development and small crack opening, which make the ability of fracture conductivity to be very sensitive to weak creep deformation. Creep can cause cracks of shale gas reservoir to close and affect its flow conductivity. It has also an impact on gas well productivity and final recovery (Sone and Zoback, 2014; Li and Ghassemi, 2012; Rassouli and Zoback, 2015).
ABSTRACT: A finite element based approach to design of foundations on jointed rock is described. Focus is on computation of load-settlement (P-δ) plots that provide important design guidance in the form of foundation stiffness K where Ρ=Κδ. While bearing capacity is an ambiguous term that may imply load or settlement, foundation stiffness relates to both. Combinations of joint properties and geometry with properties of intact rock between joints are endless, so site-specific analysis is highly desirable. However, tabulation of jointed rock foundation stiffness for selected combinations of three-joint set geometries can provide useful guidance during preliminary design stages. Examples illustrate application of the approach using tabulated data.
Design of foundations on soil is evidently based on a legacy of limiting equilibrium analyses of strip footings (plane strain) from an era of pre-digital computer days. Collapse loads based slipline solutions to rigid-ideally plastic problems used with a judicious factor of safety or one specified by building codes are the norm. Such solutions are upper bound solutions. Lower bound estimates perhaps would be more useful and indeed much effort has been expended in developing such bounds (Chen and Han 1988, Chen 2008). However, estimates of collapse loads are silent on the critical question of settlement. To be sure, elasticity theory is also used in foundation analysis (Selvaduari and Davis 1996). While the literature concerning foundations on soils is enormous, the literature on rock foundations is sparse, indeed, and moreover tends to follow the soil mechanics tradition of focusing on collapse loads (Goodman 1980, Wyllie 1999, Kulhawy 2005).
A finite element based design approach uses current technology to obtain estimates of load and settlement quickly and accurately subject to the quality of available site data. A most important feature of such an approach is a technically sound accounting of joints at the site of interest. An equivalent properties computation that allows joint and intact rock between joints to yield as evolution of the stress field occurs under load meets this requirement (Pariseau 1999). Viability of the approach is demonstrated first in comparison with analytical solution to load-settlement of a strip footing without joints. The effect of joints on this solution is presented next. Example problems follow. A brief summary concludes this contribution.