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Xi Zhang - One of the best experts on this subject based on the ideXlab platform.
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Numerical methods for hydraulic fracture propagation: a review of recent trends
Journal of Natural Gas Science and Engineering, 2018Co-Authors: Brice Lecampion, Andrew P. Bunger, Xi ZhangAbstract:Development of numerical methods for hydraulic fracture simulation has accelerated in the past two decades. Recent advances in hydraulic fracture modeling and simulation are driven by increased industry and research activity in oil and gas, a drive toward consideration of more complex behaviors associated with layered and Naturally-Fractured Rock formations, and a deepening understanding of the underlying mathematical model and its intrinsic challenges. Here we review the basic approaches being employed. Some of these comprise enhancements of classical methods, while others are imported from other fields of mechanics but are completely new in their application to hydraulic fracturing. After a description of the intrinsic challenges associated with the mechanics of fluid-driven fractures, we discuss both continuum and meso-scales numerical methods as well as engineering models which typically make use of additional assumptions to reduce computational cost. We pay particular attention to the verification and validation of numerical models, which is increasingly enabled by an ever-expanding library of laboratory experiments and analytical solutions for simple geometries in a number of different propagation regimes. A number of challenges remain and are amplified with a drive toward fully-coupled, three-dimensional hydraulic fracture modeling that accounts for host-Rock heterogeneity. In the context of such a drive to complex models, we argue that the importance of best-practice development that includes careful verification and validation is vital to ensure progress is constrained by the appropriate underlying physics and mathematics with a constant attention to identifying conditions under which simpler models suffice for the intended modeling purposes.
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Hydraulic fracture growth in Naturally Fractured Rock
Porous Rock Fracture Mechanics, 2017Co-Authors: Robert G. Jeffrey, Xi Zhang, Zuorong ChenAbstract:Abstract Volcanic dikes and sills, sometimes exposed in outcrops, are examples of natural hydraulic fractures that interact with faults and natural fractures. The industrial use of hydraulic fracturing for stimulation of Naturally Fractured reservoirs and to modify Rock strength for mining has motivated study of hydraulic fracture growth in Naturally Fractured Rock in the petroleum, geothermal, and mining industries. Predicting the path and overall geometry of a hydraulic fracture growing through a Naturally Fractured Rock has proven to be difficult. A range of experimental, theoretical, and numerical studies are available in the literature that address the need for an accurate model to predict the outcome of hydraulic fracture interaction with natural fractures. However, consensus regarding hydraulic fracture growth in the presence of natural fractures has yet to be reached. This chapter provides a review of recent progress in this area and presents current thinking on this important topic.
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Reassessing Stimulation for EGS Reservoirs
2015Co-Authors: Robert G. Jeffrey, Xi Zhang, Reinhard JungAbstract:Although the original attempts to stimulate hot dry Rock or enhanced geothermal system (EGS) reservoirs employed conventional hydraulic fracturing methods, initial difficulty in obtaining sufficient connection between the stimulated zone and a targeted production well was experienced. Furthermore, the measurement of a cloud of microseismicity associated with the injections suggested shearing of natural fractures as an important process and led to development of stimulation methods that rely on shearinduced dilation to create reservoir permeability. Since this early change in approach, the process of stimulation applied to EGS reservoirs has continued to be founded on promoting shearing of natural fractures in the Rock mass by injection of large volumes of water into long open-hole sections of the well at rates that limit the pressure to be at or below the fracture opening pressure. Shearing of natural fractures with the formation of wing fractures that then link between otherwise unconnected natural fractures provides a mechanism for development of directional conductive pathways when stimulation is carried out at or below fracture opening pressure. In particular, placing multiple fractures along a wellbore completed using a cemented casing string is expected to produce a better stimulation result compared to injection into a long openhole section. In addition, analysis of opening mode hydraulic fractures placed into hard, Naturally Fractured Rock using pressures well above the minimum principal stress magnitude and then mapped in mineback experiments, show these fractures grew in the direction perpendicular to the minimum principal stress with offsets in their path developing at sites where the opening mode fracture interacts with and crosses natural fractures. This provides another mode of fracture growth that could be used to stimulate EGS reservoirs, provided cemented casing strings are used to complete the well.
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Role of overpressurized fluid and fluid-driven fractures in forming fracture networks
Journal of Geochemical Exploration, 2014Co-Authors: Xi Zhang, Robert G. JeffreyAbstract:Abstract A 2-D numerical study was carried out, using a fully coupled Rock deformation and fluid flow hydraulic fracturing model on fracture network formation in a low-permeability Naturally Fractured Rock, in which new fractures are allowed to nucleate and grow driven by overpressurized fluid. Fracture seeds either are defined as small flaws or are nucleated based on a stress and fracture energy criterion. Fracture intersection, fluid flow and frictional slip along fractures are also explicitly simulated in the model. In particular, we consider a few artificial fracture network geometries with sets of finite discrete or isolated fractures in order to study the roles of fluid viscosity, injection conditions, fracture intersections and offsetting at pre-existing or natural fractures in determining the paths which hydraulic fractures follow through the network. The newly fluid-created fracture segments, which are oriented locally normal to the least compressive stress, are the most conductive parts along the hydraulic fracture path. The resulting network will allow for long-term fluid flow to occur more easily provided that these segments retain their conductivity. However, the existence of intersections and offsets in the main fracture path act to impede fracture growth and fluid flow. A fracture nucleation event associated with higher fluid pressure may allow a fracture path to bypass these barriers, thus leading to a more planar fracture geometry. If fracture nucleation does not occur, fluid may enter other cross cutting natural fractures or enter along a barrier fracture to the tip of the barrier fracture, both processes that require a larger expenditure of energy reflected in an increasing fluid pressure. The results clearly demonstrate the importance of these stress and flow barriers in forming a preferential fracture and flow path. Fluid invasion into and fracture growth of natural fractures can still continue to occur at the high pressure region near the hydraulic fracture entry, even after a complete hydraulic fracture pathway has developed, showing strong similarity to the invasion percolation process. In contrast to fixed-aperture, connected fracture network models, the fracture growth-generated patterns also depend on the in situ stresses, which affect mechanical interaction between the main hydraulic fracture and natural fractures. The numerical results provide improved understanding of fluid-driven fracture growth into and through a network of pre-existing natural fractures.
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Measuring Hydraulic Fracture Growth in Naturally Fractured Rock
SPE Annual Technical Conference and Exhibition, 2009Co-Authors: Robert G. Jeffrey, Xi Zhang, Zuorong Chen, Andrew P. Bunger, Brice Lecampion, A. Van As, D. Allison, W. De Beer, J. Dudley, Eduard SiebritsAbstract:Note: SPE 124919 Reference EPFL-CONF-212832 Record created on 2015-10-08, modified on 2016-08-09
Robert G. Jeffrey - One of the best experts on this subject based on the ideXlab platform.
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Hydraulic fracture growth in Naturally Fractured Rock
Porous Rock Fracture Mechanics, 2017Co-Authors: Robert G. Jeffrey, Xi Zhang, Zuorong ChenAbstract:Abstract Volcanic dikes and sills, sometimes exposed in outcrops, are examples of natural hydraulic fractures that interact with faults and natural fractures. The industrial use of hydraulic fracturing for stimulation of Naturally Fractured reservoirs and to modify Rock strength for mining has motivated study of hydraulic fracture growth in Naturally Fractured Rock in the petroleum, geothermal, and mining industries. Predicting the path and overall geometry of a hydraulic fracture growing through a Naturally Fractured Rock has proven to be difficult. A range of experimental, theoretical, and numerical studies are available in the literature that address the need for an accurate model to predict the outcome of hydraulic fracture interaction with natural fractures. However, consensus regarding hydraulic fracture growth in the presence of natural fractures has yet to be reached. This chapter provides a review of recent progress in this area and presents current thinking on this important topic.
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Reassessing Stimulation for EGS Reservoirs
2015Co-Authors: Robert G. Jeffrey, Xi Zhang, Reinhard JungAbstract:Although the original attempts to stimulate hot dry Rock or enhanced geothermal system (EGS) reservoirs employed conventional hydraulic fracturing methods, initial difficulty in obtaining sufficient connection between the stimulated zone and a targeted production well was experienced. Furthermore, the measurement of a cloud of microseismicity associated with the injections suggested shearing of natural fractures as an important process and led to development of stimulation methods that rely on shearinduced dilation to create reservoir permeability. Since this early change in approach, the process of stimulation applied to EGS reservoirs has continued to be founded on promoting shearing of natural fractures in the Rock mass by injection of large volumes of water into long open-hole sections of the well at rates that limit the pressure to be at or below the fracture opening pressure. Shearing of natural fractures with the formation of wing fractures that then link between otherwise unconnected natural fractures provides a mechanism for development of directional conductive pathways when stimulation is carried out at or below fracture opening pressure. In particular, placing multiple fractures along a wellbore completed using a cemented casing string is expected to produce a better stimulation result compared to injection into a long openhole section. In addition, analysis of opening mode hydraulic fractures placed into hard, Naturally Fractured Rock using pressures well above the minimum principal stress magnitude and then mapped in mineback experiments, show these fractures grew in the direction perpendicular to the minimum principal stress with offsets in their path developing at sites where the opening mode fracture interacts with and crosses natural fractures. This provides another mode of fracture growth that could be used to stimulate EGS reservoirs, provided cemented casing strings are used to complete the well.
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Role of overpressurized fluid and fluid-driven fractures in forming fracture networks
Journal of Geochemical Exploration, 2014Co-Authors: Xi Zhang, Robert G. JeffreyAbstract:Abstract A 2-D numerical study was carried out, using a fully coupled Rock deformation and fluid flow hydraulic fracturing model on fracture network formation in a low-permeability Naturally Fractured Rock, in which new fractures are allowed to nucleate and grow driven by overpressurized fluid. Fracture seeds either are defined as small flaws or are nucleated based on a stress and fracture energy criterion. Fracture intersection, fluid flow and frictional slip along fractures are also explicitly simulated in the model. In particular, we consider a few artificial fracture network geometries with sets of finite discrete or isolated fractures in order to study the roles of fluid viscosity, injection conditions, fracture intersections and offsetting at pre-existing or natural fractures in determining the paths which hydraulic fractures follow through the network. The newly fluid-created fracture segments, which are oriented locally normal to the least compressive stress, are the most conductive parts along the hydraulic fracture path. The resulting network will allow for long-term fluid flow to occur more easily provided that these segments retain their conductivity. However, the existence of intersections and offsets in the main fracture path act to impede fracture growth and fluid flow. A fracture nucleation event associated with higher fluid pressure may allow a fracture path to bypass these barriers, thus leading to a more planar fracture geometry. If fracture nucleation does not occur, fluid may enter other cross cutting natural fractures or enter along a barrier fracture to the tip of the barrier fracture, both processes that require a larger expenditure of energy reflected in an increasing fluid pressure. The results clearly demonstrate the importance of these stress and flow barriers in forming a preferential fracture and flow path. Fluid invasion into and fracture growth of natural fractures can still continue to occur at the high pressure region near the hydraulic fracture entry, even after a complete hydraulic fracture pathway has developed, showing strong similarity to the invasion percolation process. In contrast to fixed-aperture, connected fracture network models, the fracture growth-generated patterns also depend on the in situ stresses, which affect mechanical interaction between the main hydraulic fracture and natural fractures. The numerical results provide improved understanding of fluid-driven fracture growth into and through a network of pre-existing natural fractures.
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Measuring Hydraulic Fracture Growth in Naturally Fractured Rock
SPE Annual Technical Conference and Exhibition, 2009Co-Authors: Robert G. Jeffrey, Xi Zhang, Zuorong Chen, Andrew P. Bunger, Brice Lecampion, A. Van As, D. Allison, W. De Beer, J. Dudley, Eduard SiebritsAbstract:Note: SPE 124919 Reference EPFL-CONF-212832 Record created on 2015-10-08, modified on 2016-08-09
Mohd Mustaqim Mohd-nordin - One of the best experts on this subject based on the ideXlab platform.
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Evolution of Joint Roughness Degradation from Cyclic Loading and Its Effect on the Elastic Wave Velocity
Rock Mechanics and Rock Engineering, 2015Co-Authors: Mohd Mustaqim Mohd-nordin, Ki-il Song, Dongwook Kim, Ilhan ChangAbstract:The application of stress to the Rock joint has a significant impact on morphological and mechanical properties of the joint. In particular, the evolution of the joint condition is dramatic on the surface of the weathered Rock whose material integrity is altered in a manner similar to the Rock joint behavior from weathering effects (Resende et al. 2010). As indicated in Kabeya and Legge (1997), a significant change in the weathered Rock joint surface can be induced by the modification of grain properties of the joint surface. The grain size distribution has been found to be strongly correlated with the shear behavior and the joint roughness coefficient (JRC). The open joint or gouge dominates in weathered joints of the Rock mass structure, thereby providing access for weathering agents to increase the failure likelihood from a slickenside (Wooet al. 2010).This granular disintegration leads to surface flakes to transform the joint aperture into a wider opening. In the Rock mass rating (RMR) system, the geomechanic classification method for Rock masses is associated with the roughness of the Rock joint surface as the discontinuity condition (Hoek 2007). Conventionally, the roughness of a Rock joint surface is rated and quantified in term of the JRC, which ranges from 0 to 20 (Barton and Choubey 1977). However, the validity of techniques and the accuracy of measurement methods for identifying and classifying Rock joint surfaces are generally questioned because of the subjectivity of JRC results. The undulation and unevenness of a Rock joint surface are recognized by its peak-to-valley height (Hotar and Novotny 2005) to classify the Rock joint surface profile. In particular, Mohd-Nordin et al. (2014) applied the JRC to Naturally Fractured Rock surfaces using the scan line technique through the implementation of the peak-to-valley height. In this regard, the maximum peak-to-valley height (Pmax) can be used as a meaningful quantitative indicator of the degree of roughness of a Rock joint surface. Previous studies have examined the propagation of elastic waves across multiple jointed Rock masses by considering joint surface conditions (Mohd-Nordin et al. 2014; Huang et al. 2014a; Cha et al. 2009). One of the main findings is that the roughness and unevenness of a replicated natural Rock joint surface have significant effects on the propagation of elastic waves. However, changes in & Ki-Il Song ksong@inha.ac.kr
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Long-Wavelength Elastic Wave Propagation Across Naturally Fractured Rock Masses
Rock Mechanics and Rock Engineering, 2014Co-Authors: Mohd Mustaqim Mohd-nordin, Ki-il Song, Gye-chun Cho, Zainab MohamedAbstract:Geophysical site investigation techniques based on elastic waves have been widely used to characterize Rock masses. However, characterizing jointed Rock masses by using such techniques remains challenging because of a lack of knowledge about elastic wave propagation in multi-jointed Rock masses. In this paper, the roughness of Naturally Fractured Rock joint surfaces is estimated by using a three-dimensional (3D) image-processing technique. The classification of the joint roughness coefficient (JRC) is enhanced by introducing the scan line technique. The peak-to-valley height is selected as a key indicator for JRC classification. Long-wavelength P-wave and torsional S-wave propagation across Rock masses containing Naturally Fractured joints are simulated through the quasi-static resonant column (QSRC) test. In general, as the JRC increases, the S-wave velocity increases within the range of stress levels considered in this paper, whereas the P-wave velocity and the damping ratio of the shear wave decrease. In particular, the two-dimensional joint specimen underestimates the S-wave velocity while overestimating the P-wave velocity. This suggests that 3D joint surfaces should be implicated to obtain the reliable elastic wave velocity in jointed Rock masses. The contact characteristic and degree of roughness and waviness of the joint surface are identified as a factor influencing P-wave and S-wave propagation in multi-jointed Rock masses. The results indicate a need for a better understanding of the sensitivity of contact area alterations to the elastic wave velocity induced by changes in normal stress. This paper’s framework can be a reference for future research on elastic wave propagation in Naturally multi-jointed Rock masses.
Zainab Mohamed - One of the best experts on this subject based on the ideXlab platform.
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Long-Wavelength Elastic Wave Propagation Across Naturally Fractured Rock Masses
Rock Mechanics and Rock Engineering, 2014Co-Authors: Mohd Mustaqim Mohd-nordin, Ki-il Song, Gye-chun Cho, Zainab MohamedAbstract:Geophysical site investigation techniques based on elastic waves have been widely used to characterize Rock masses. However, characterizing jointed Rock masses by using such techniques remains challenging because of a lack of knowledge about elastic wave propagation in multi-jointed Rock masses. In this paper, the roughness of Naturally Fractured Rock joint surfaces is estimated by using a three-dimensional (3D) image-processing technique. The classification of the joint roughness coefficient (JRC) is enhanced by introducing the scan line technique. The peak-to-valley height is selected as a key indicator for JRC classification. Long-wavelength P-wave and torsional S-wave propagation across Rock masses containing Naturally Fractured joints are simulated through the quasi-static resonant column (QSRC) test. In general, as the JRC increases, the S-wave velocity increases within the range of stress levels considered in this paper, whereas the P-wave velocity and the damping ratio of the shear wave decrease. In particular, the two-dimensional joint specimen underestimates the S-wave velocity while overestimating the P-wave velocity. This suggests that 3D joint surfaces should be implicated to obtain the reliable elastic wave velocity in jointed Rock masses. The contact characteristic and degree of roughness and waviness of the joint surface are identified as a factor influencing P-wave and S-wave propagation in multi-jointed Rock masses. The results indicate a need for a better understanding of the sensitivity of contact area alterations to the elastic wave velocity induced by changes in normal stress. This paper’s framework can be a reference for future research on elastic wave propagation in Naturally multi-jointed Rock masses.
D. Elmo - One of the best experts on this subject based on the ideXlab platform.
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Improving DFN-geomechanical model integration using a novel automated approach
Computers and Geotechnics, 2019Co-Authors: Ladan Karimi Sharif, D. Elmo, Doug SteadAbstract:Abstract In the past decade, numerical modelling has been increasingly used for simulating the mechanical behavior of Naturally Fractured Rock masses. The synthetic Rock mass approach often adopted has three main components: data collection and characterisation; discrete fracture network (DFN) modelling; and finally, geomechanical modelling to simulate Rock mass behaviour. The focus of this paper is the integration of the DFN with geomechanical models, with emphasis on the numerical aspects of embedding rather complex 2D fracture networks within a finite element mesh. A new method is introduced that simplifies fracture networks, while maintaining important fracture properties such as orientation, connectivity, and intensity. The resulting finite element mesh is of sufficient quality to both improve computational time and eliminate spurious results that may be associated with the presence of highly distorted elements. The proposed method uses a web-based code with two integral components (DFNAnalyzer and DFNCleaner) that analyse the properties of the original fracture network, and then perform simplifications to ensure an acceptable (user defined) mesh quality is achieved.
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The Development of a new Numerical Modelling Approach for Naturally Fractured Rock Masses
Rock Mechanics and Rock Engineering, 2006Co-Authors: R. J. Pine, J. S. Coggan, Z. N. Flynn, D. ElmoAbstract:An approach for modelling Fractured Rock masses has been developed which has two main objectives: to maximise the quality of representation of the geometry of existing Rock jointing and to use this within a loading model which takes full account of this style of jointing. Initially the work has been applied to the modelling of mine pillars and data from the Middleton Mine in the UK has been used as a case example. However, the general approach is applicable to all aspects of Rock mass behaviour including the stress conditions found in hangingwalls, tunnels, block caving, and slopes. The Rock mass fracture representation was based on a combination of explicit mapping of Rock faces and the synthesis of this data into a three-dimensional model, based on the use of the FracMan computer model suite. Two-dimensional cross sections from this model were imported into the finite element computer model, ELFEN, for loading simulation. The ELFEN constitutive model for fracture simulation includes the Rotating Crack, and Rankine material models, in which fracturing is controlled by tensile strength and fracture energy parameters. For tension/compression stress states, the model is complemented with a capped Mohr-Coulomb criterion in which the softening response is coupled to the tensile model. Fracturing due to dilation is accommodated by introducing an explicit coupling between the inelastic strain accrued by the Mohr-Coulomb yield surface and the anisotropic degradation of the mutually orthogonal tensile yield surfaces of the rotating crack model. Pillars have been simulated with widths of 2.8, 7 and 14 m and a height of 7 m (the Middleton Mine pillars are typically 14 m wide and 7 m high). The evolution of the pillar failure under progressive loading through fracture extension and creation of new fractures is presented, and pillar capacities and stiffnesses are compared with empirical models. The agreement between the models is promising and the new model provides useful insights into the influence of pre-existing fractures. Further work is needed to consider the effects of three-dimensional loading and other boundary condition problems.
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The Development of a new Numerical Modelling Approach for Naturally Fractured Rock Masses
Rock Mechanics and Rock Engineering, 2006Co-Authors: R. J. Pine, J. S. Coggan, Z. N. Flynn, D. ElmoAbstract:An approach for modelling Fractured Rock masses has been developed which has two main objectives: to maximise the quality of representation of the geometry of existing Rock jointing and to use this within a loading model which takes full account of this style of jointing. Initially the work has been applied to the modelling of mine pillars and data from the Middleton Mine in the UK has been used as a case example. However, the general approach is applicable to all aspects of Rock mass behaviour including the stress conditions found in hangingwalls, tunnels, block caving, and slopes. The Rock mass fracture representation was based on a combination of explicit mapping of Rock faces and the synthesis of this data into a three-dimensional model, based on the use of the FracMan computer model suite. Two-dimensional cross sections from this model were imported into the finite element computer model, ELFEN, for loading simulation.