Fractures are ubiquitous in the subsurface, and they provide dominant pathways for fluid flow in low permeability formations. Therefore, fractures usually play an essential role in many engineering fields, such as hydrology, waste disposal, geothermal reservoir and petroleum reservoir exploitation. Since fractures are invisible and have variable sizes from micrometers to kilometers, there is limited knowledge of their structure. We aim to deepen the understanding of fracture networks in the subsurface from their topological structures, hydraulic connectivity and characteristics at different scales. We adopt the discrete fracture network method and develop an efficient C++ code, HatchFrac, to make in-depth investigations possible. We start from generating stochastic fracture networks by constraining fracture geometries with different stochastic distributions. We apply percolation theory to investigate the global connectivity of fracture networks. We find that commonly adopted percolation parameters are unsuitable for the characterization of the percolation state of complex fracture networks. We implement the concept of global efficiency to quantify the impact of fracture geometries on the connectivity of fracture networks. Furthermore, we constrain the fracture networks with geological data and geomechanics principles. We investigate the correlation of fracture intensities with different dimensionality and find that it is not feasible to obtain correct 3D intensity parameters from 1D or 2D samples. We utilize a deep-learning technique and propose a pixel-based detection algorithm to automatically interpret fractures from raw outcrop images. Interpreted fracture maps provide abundant resources to investigate fracture intensities, lengths,orientations, and generations. For large scale faults, we develop a method to generate fault segments from a rough fault trace on a seismic map. Accurate fault geometries have significant impacts on damage zones and fault-related flow problems. For small scale fractures, we consider the impact of fracture sealing on the percolation state of orthogonal fracture networks. We emphasize the importance of non-critically stressed and partially sealed fractures, which are usually neglected because usually they are nonconductive. However, with significant stress perturbations, those noncritically stressed and partially sealed fractures can also contribute to the production by enlarging the stimulated reservoir volume.

Stress and permeability variations around a wellbore and in the reservoir are of much interest in petroleum and geothermal reservoir development. Water injection causes significant changes in pore pressure, temperature, and stress in hot reservoirs, changing rock permeability. In this work, two- and three-dimensional finite element methods were developed to simulate coupled reservoirs with damage mechanics and stress-dependent permeability. The model considers the influence of fluid flow, temperature, and solute transport in rock deformation and models nonlinear behavior with continuum damage mechanics and stress-dependent permeability. Numerical modeling was applied to analyze wellbore stability in swelling shale with two- and three-dimensional damage/fracture propagation around a wellbore and injection-induced microseismic events. The finite element method (FEM) was used to solve the displacement, pore pressure, temperature, and solute concentration problems. Solute mass transport between drilling fluid and shale formation was considered to study salinity effects. Results show that shear and tensile failure can occur around a wellbore in certain drilling conditions where the mud pressure lies between the reservoir pore pressure and fracture gradient. The fully coupled thermo-poro-mechanical FEM simulation was used to model damage/fracture propagation and microseismic events caused by fluid injection. These studies considered wellbore geometry in small-scale modeling and point-source injection, assuming singularity fluid flux for large-scale simulation. Damage mechanics was applied to capture the effects of crack initiation, microvoid growth, and fracture propagation. The induced microseismic events were modeled in heterogeneous geological media, assuming the Weibull distribution functions for modulus and permeability. The results of this study indicate that fluid injection causes the effective stress to relax in the damage phase and to concentrate at the interface between the damage phase and the intact rock. Furthermore, induced-stress and far-field stress influence damage propagation. Cold water injection causes the tensile stress and affects the initial fracture and fracture propagation, but fracture initiation pressure and far-field stress are critical to create a damage/fracture plane, which is normal to the minimum far-field stress direction following well stimulation. Microseismic events propagate at both well scale and reservoir-scale simulation; the cloud shape of a microseismic event is affected by permeability anisotropy and far-field stress, and deviatoric horizontal far-field stress especially contributes to the localization of the microseismic cloud.

Uniaxial consolidation tests of resedimented mudrocks from the offshore Gulf of Mexico reveal compression and permeability behavior that is in many ways similar to those of intact core specimens and field measurements. Porosity (n) of the resedimented mudrock also falls between field porosity estimates obtained from sonic and bulk density well logs at comparable effective stresses. Laboratory-prepared mudrocks are used as testing analogs because accurate in-situ measurements and intact cores are difficult to obtain. However, few direct comparisons between laboratory-prepared mudrocks, field behavior, and intact core behavior have been made. In this thesis, I compare permeability and compressibility of laboratory-prepared specimens from Gulf of Mexico material to intact core and field analysis of this material. I resediment high plasticity silty claystone obtained from Plio-Pleistocene-aged mudrocks in the Eugene Island Block 330 oilfield, offshore Louisiana, and characterize its compression and permeability behavior through constant rate of strain consolidation tests. The resedimented mudrocks decrease in void ratio (e) from 1.4 (61% porosity) at 100 kPa of effective stress to 0.34 (26% porosity) at 20.4 MPa. I model the compression behavior using a power function between specific volume (v=1+e) and effective stress ([sigma]'v): v=1.85[sigma]'v-⁰̇¹⁰⁸. Vertical permeability (k) decreases from 2.5·10-¹⁶ m² to 4.5·10-²⁰ m² over this range, and I model the permeability as a log-linear function of porosity (n): log₁₀ k=10.83n - 23.21. Field porosity estimates are calculated from well logs using two approaches; an empirical correlation based on sonic velocities, and a calculation using the bulk density. Porosity of the resedimented mudrock falls above the sonic-derived porosity and below the density porosity at all effective stresses. Measurements on intact core specimens display similar compression and permeability behavior to the resedimented specimens. Similar compression behavior is also observed in Ursa Basin mudrocks. Based on these similarities, resedimented Gulf of Mexico mudrock is a reasonable analog for field behavior. / text

Petroleum and Geosystems Engineering / Production from unconventional reservoirs became popular in the last decade in the U.S. Promising production results and predictions, as well as improvements in hydraulic fracturing and horizontal drilling technology made unconventional reservoirs economically feasible. Therefore, an effective and efficient reservoir model for unconventional resources became a must. In order to model production from such resources, analytical, semi-analytical, and numerical models have been developed, but analytical models are frequently used due to their practicality, relative simplicity, and also due to limited availability of field data. This research project has been accomplished in two main parts. In the first part, two analytical models for unconventional reservoirs, one with infinite hydraulic fracture conductivity assumption proposed by Patzek et al. (2013), while the other one with finite hydraulic fracture conductivity assumption developed by Ozkan et al. (2011) are compared. Additionally, a commercial reservoir simulator (CMG, IMEX, 2012) is employed to compare the results with the analytical models. Sensitivity study is then performed to identify the critical parameters controlling the production performance of unconventional reservoirs. In the second part, naturally and hydraulically fractured unconventional reservoir is considered. In addition, geomechanical effects on natural and hydraulic fractures are examined. A simple analytical dual porosity model, which represents the natural fractures in unconventional reservoirs, is improved to handle the constant bottom-hole pressure production scenario to identify the production performance differences between the cases with and without geomechanical effects. Finally, geomechanical effects are considered for combined natural and hydraulic fractures, and an evaluation of the circumstances in which the geomechanical effects cause a significant production loss is carried out. / text

Uncertainty in rockmass and in situ stress parameters poses a critical design challenge in geotechnical engineering. This uncertainty stems from natural variability (aleatory) due to the complex history of formation and continual reworking of geological materials as well as knowledge-based uncertainty (epistemic) due to a lack of site specific information and the introduction of errors during the testing and design phases. While such uncertainty can be dealt with subjectively through the use of conservative design parameters, this leads to a lack of understanding of the variable ground response and the selection of an over-conservative design that can have a negative impact on both the project cost and schedule. Reliability methods offer an alternative approach that focuses on quantifying the uncertainty in ground conditions and utilizing it directly in the design process. By doing so, a probability of failure can be calculated with respect to a prescribed limit state, providing a measure of design performance. When multiple design options are considered, reliability methods can be paired with a quantitative risk analysis to determine the optimum design on the basis of safety and minimum cost rather than subjective conservatism. Despite the inherent benefits of such an approach, the adoption of reliability methods has been slow in geotechnical engineering due to a number of technical and conceptual challenges. The research conducted pertaining to this thesis aims to address these issues and remove the perceived “cloak of mystery” that surrounds the use of reliability methods. The scientific and engineering research in this thesis was divided into four sections: (1) the assessment of uncertainty in geotechnical input parameters, (2) a review of reliability methods in the context of geotechnical problems, (3) the development of a reliability-based, quantitative risk approach for underground support design and (4) the application of such a method to existing case studies. The completion of these areas is critical to the design of underground structures and may bring about a shift in design philosophy in the geotechnical industry. / Thesis (Ph.D, Geological Sciences & Geological Engineering) -- Queen's University, 2013-09-18 10:35:26.265

This PhD thesis contains the results of comprehensive research into the Pravčická brána Arch and surrounding sandstone massifs with focus on gaining more knowledge about natural dynamics and evolution of this rock formation, its current level of stability and the weathering processes it displays. Non-destructive methods were used for this comprehensive study; these ranged from detailed field documentation to monitoring temperature regime of the rock and included application of a geophysical survey and control monitoring of the course of arch body deformation. Laboratory testing was carried out for strength parameters and salt efflorescences together with weathered sandstones were analysed for chemical compounds. Main operating factors were monitored simultaneously, which particularly involved changes in external temperature, degree of sunlight and chemical composition of rainfall. Conventional as well as entirely new assessment procedures were used in synthesis and interpretation of the data collected, including knowledge of nonlinear dynamics of complex systems. The survey was designed to fully respect the protective conditions of the site, to make follow-up activities possible in future and to monitor any possible negative changes in the rock massif. The main results incorporate description of...

To safely store carbon dioxide in enhanced oil recovery/ CO2 sequestration projects it is important to ensure the integrity of the caprock during and after production and injection. A change in fluid pressure and temperature within a porous reservoir will generally induce stress changes within the reservoir and the rocks that surround it. Amongst the potential hazards resulting from these induced stress changes is the reactivation of existing faults or fractures and inducing new fractures, which may breach the hydraulic integrity of the caprock that bounds the reservoir.<p> The theories of inclusions and inhomogeneities have been used in this research to derive semi-analytical and closed-form solutions for induced stress change during pore pressure change within a reservoir and in the surrounding rock, under plane strain and axisymmetric conditions. Methods have been developed to assess fault reactivation and induced fracturing during injection or production within a reservoir. The failure stress change concept for a Coulomb failure criterion has been used to study the likelihood of fault reactivation and induced fracturing within the reservoir. Formulations have been adopted to calculate the critical pressure change for fault reactivation and induced fracturing within the reservoir and in the surrounding rock during injection and production. Sensitivity analysis has been performed to study the effects of different parameters such as initial in-situ stress, reservoir geometry, reservoir depth, reservoir tilt or dip , material property contrast between the reservoir and surrounding rock, fault geometry, fault strength, and intact rock strength. General patterns of induced stress change, in-situ stress evolution, fault reactivation, and induced fracturing have been identified.<p> The developed methodologies have been applied to six different case studies: fault reactivation analysis in the entire field for a synthetic case study; induced fracturing analysis in the entire field in a synthetic case study; fault reactivation and induced stress change analysis within the Ekofisk oil reservoir in North Sea; fault reactivation analysis in the Lacq gas reservoir in France; the Weyburn-Midale EOR/CO2 Storage project in southeast Saskatchewan; and acid gas injection in Zama oil field, Alberta. The results of these case studies show good consistency with field observation, and physical and numerical models.<p> The generality, simplicity, and straightforwardness of the developed methodologies, along with their flexibility to model different plausible scenarios and their ease of implementation for systematic sensitivity analyses makes them suitable for decision-making and uncertainty management, specifically in early stages of reservoir development or site assessment for geological sequestration of carbon dioxide.

To safely store carbon dioxide in enhanced oil recovery/ CO2 sequestration projects it is important to ensure the integrity of the caprock during and after production and injection. A change in fluid pressure and temperature within a porous reservoir will generally induce stress changes within the reservoir and the rocks that surround it. Amongst the potential hazards resulting from these induced stress changes is the reactivation of existing faults or fractures and inducing new fractures, which may breach the hydraulic integrity of the caprock that bounds the reservoir.<p> The theories of inclusions and inhomogeneities have been used in this research to derive semi-analytical and closed-form solutions for induced stress change during pore pressure change within a reservoir and in the surrounding rock, under plane strain and axisymmetric conditions. Methods have been developed to assess fault reactivation and induced fracturing during injection or production within a reservoir. The failure stress change concept for a Coulomb failure criterion has been used to study the likelihood of fault reactivation and induced fracturing within the reservoir. Formulations have been adopted to calculate the critical pressure change for fault reactivation and induced fracturing within the reservoir and in the surrounding rock during injection and production. Sensitivity analysis has been performed to study the effects of different parameters such as initial in-situ stress, reservoir geometry, reservoir depth, reservoir tilt or dip , material property contrast between the reservoir and surrounding rock, fault geometry, fault strength, and intact rock strength. General patterns of induced stress change, in-situ stress evolution, fault reactivation, and induced fracturing have been identified.<p> The developed methodologies have been applied to six different case studies: fault reactivation analysis in the entire field for a synthetic case study; induced fracturing analysis in the entire field in a synthetic case study; fault reactivation and induced stress change analysis within the Ekofisk oil reservoir in North Sea; fault reactivation analysis in the Lacq gas reservoir in France; the Weyburn-Midale EOR/CO2 Storage project in southeast Saskatchewan; and acid gas injection in Zama oil field, Alberta. The results of these case studies show good consistency with field observation, and physical and numerical models.<p> The generality, simplicity, and straightforwardness of the developed methodologies, along with their flexibility to model different plausible scenarios and their ease of implementation for systematic sensitivity analyses makes them suitable for decision-making and uncertainty management, specifically in early stages of reservoir development or site assessment for geological sequestration of carbon dioxide.

Natural hydraulic fractures (NHFs) are fractures whose growths are driven by fluid loading. The fluid flow properties of the host rock have a primary, but hitherto little appreciated control on the NHF propagation rates. This study focuses on investigating the impacts of host rock fluid flow on the propagation and pattern development of multiple NHF in a poroelastic media. A realistic geomechanical model is developed to combine both the fluid flow and mechanical interactions between multiple fractures. The natural hydraulic fracture propagation is observed to consist of a series of crack-seal processes indicating incremental stop-start growth. Growth timing is on the scale of millions of years based on recent natural fracture growth reconstructions. These time scales are compatible with some model scenarios. My newly developed numerical model captures the crack-seal process for multiple NHF propagation. A sensitivity study conducted to investigate the impacts of different fluid flow properties on NHF propagation shows that permeability is a predominate influence on the timescale of NHF development. In low-permeability rocks, fractures have more stable initiation and much longer propagation timing compared to those in high-permeability rocks. Another aspect of great interest is the influence of fluid flow on fracture spacing and pattern development for multiple NHFs propagation in a poroelastic environment. My new poroelastic geomechanial model combines the natural hydraulic fracturing mechanism with the mechanical interactions between fractures. The numerical results show that as host rock permeability decreases, more fractures can propagate and a much smaller spacing is reached for a given fracture set. The low permeability slows down the propagation of long fractures and prevents them from dominating the fracture pattern. As a result, more fractures are able to grow at a similar speed and a more closely spaced fracture pattern is achieved for either regularly spaced or randomly distributed multiple fractures in low-permeability rocks. Investigation is also conducted in analyzing the distributions of fracture attributes (length, aperture and spacing) in low- and high-permeability rocks. For shales with high subcritical index, low permeability helps the fractures propagate more closely spaced instead of clustering. Meanwhile, in low-permeability rocks, factures have relatively smaller apertures, which lead to a slower fracture opening rate. The competition between the slow fracture opening rate and quartz precipitation rate will affect the effective permeability and porosity of the naturally fractured reservoir. However, the competition is trivial in high-permeability rocks. Other factors, such as reservoir boundary condition, layer thickness, subcritical index and pattern development stage, all have considerable impact on fracture pattern development and attribute distribution in a poroelastic media. / text

At present, most of the energy power consumed in the world is produced by fossil fuel combustion, which has raised increasing interest in renewable energy technologies, non-conventional oil and gas reservoirs, and nuclear power. Innovative nuclear fuels and reactors depend on the economical and environmental impacts of waste management. Disposals in mined geological formations are viewed as potential consolidated storage facilities before final disposition. Different stress paths during construction result in different kinds of failure mechanisms, which alter rock strength and induce anisotropy of rock elastic properties. Crack propagation in rock can be originated by these engineering activities (excavation, drilling, mining, building overburden), or by changes of the natural environment (tectonic processes, erosion or weathering). Damage is a mathematical variable that can represent a variety of microstructure changes, such as crack density, length, aspect ratio and orientation. The framework of Continuum Damage Mechanics allows modeling the resulting reduction in strength and stiffness, as well as the associated stress-induced anisotropy and irreversible deformation. This work presents a modeling framework for anisotropic crack propagation in rock, in conditions of stress typical of geological storage and oil and gas extraction. Emphasis is put on the prediction of the damage zone around cavities and ahead of pressurized fracture tips. An original model of anisotropic damage, the Differential Stress Induced Damage (DSID) model, is explained. The Drucker-Prager yield function is adapted to make the damage threshold depend on damage energy release rate and to distinguish between tension and compression strength. Flow rules are derived with the energy release rate conjugate to damage, which is thermodynamically consistent. The positivity of dissipation is ensured by using a non-associate flow rule for damage, while nonelastic deformation due to damage is computed by an associate flow rule. Stress paths simulated at the material point illustrate damaged stiffness and deformation variations in classical rock mechanics tests. The maximum likelihood method was employed to calibrate and verify the DSID model against stress-strain curves obtained during triaxial compression tests and uniaxial compression tests performed on clay rock and shale. Logarithmic transformation, normalization and forward deletion allowed optimizing the formulation of the DSID model, and reduce the number of damage constitutive parameters from seven to two for clay rock. The DSID model was implemented in ABAQUS Finite Element (FE) software. The iterative scheme was adapted in order to account for the non-linearities induce both by damage and damage-induced deformation. FE simulations of laboratory tests capture size an intrinsic anisotropy effects on the propagation of damage in rock. Smeared DSID zones representing shale delamination planes avoid some convergence problems encountered when modeling discontinuities with debonded contact surface elements. FE simulations of tunnel excavation, fracture propagation and borehole pressurization were performed to illustrate the evolution of the damage zone and the impact on energy dissipation, anisotropy of deformation, and loss of stiffness. Future work will focus on coupling the propagation of fractures with the evolution of the damage process zone, and on the transition from continuum damage to discrete fracture upon crack coalescence.