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We developed a hydraulic fracturing simulator by coupling a flow simulator to a geomechanics code, namely T+M simulator. Modeling of the vertical fracture development involves continuous updating of the boundary conditions and of the data connectivity, based on the finite element method for geomechanics. The T+M simulator can model the initial fracture development during the hydraulic fracturing operations, after which the domain description changes from single continuum to double or multiple continua in order to rigorously model both flow and geomechanics for fracture-rock matrix systems. The T+H simulator provides two-way coupling between fluid-heat flow and geomechanics, accounting for thermoporomechanics, treats nonlinear permeability and geomechanical moduli explicitly, and dynamically tracks changes in the fracture(s) and in the pore volume. We also fully accounts for leak-off in all directions during hydraulic fracturing. We first validate the T+M simulator, matching numerical solutions with the analytical solutions for poromechanical effects, static fractures, and fracture propagations. Then, from numerical simulation of various cases of the planar fracture propagation, shear failure can limit the vertical fracture propagation of tensile failure, because of leak-off into the reservoirs. Slow injection causes more leak-off, compared with fast injection, when the same amount of fluid is injected. Changes in initial total stress and contributions of shear effective stress to tensile failure can also affect formation of the fractured areas, and the geomechanical responses are still well-posed.
We developed a hydraulic fracturing simulator by coupling a flow simulator to a geomechanics code, namely T+M simulator. Modeling of the vertical fracture development involves continuous updating of the boundary conditions and of the data connectivity, based on the finite element method for geomechanics. The T+M simulator can model the initial fracture development during the hydraulic fracturing operations, after which the domain description changes from single continuum to double or multiple continua in order to rigorously model both flow and geomechanics for fracture-rock matrix systems. The T+H simulator provides two-way coupling between fluid-heat flow and geomechanics, accounting for thermoporomechanics, treats nonlinear permeability and geomechanical moduli explicitly, and dynamically tracks changes in the fracture(s) and in the pore volume. We also fully accounts for leak-off in all directions during hydraulic fracturing. We first validate the T+M simulator, matching numerical solutions with the analytical solutions for poromechanical effects, static fractures, and fracture propagations. Then, from numerical simulation of various cases of the planar fracture propagation, shear failure can limit the vertical fracture propagation of tensile failure, because of leak-off into the reservoirs. Slow injection causes more leak-off, compared with fast injection, when the same amount of fluid is injected. Changes in initial total stress and contributions of shear effective stress to tensile failure can also affect formation of the fractured areas, and the geomechanical responses are still well-posed.
Tight gas and shale oil play an important role in energy security and in meeting an increasing energy demand. Hydraulic fracturing is a widely used technology for recovering these resources. The design and evaluation of hydraulic fracture operation is critical for efficient production from tight gas and shale plays. The efficiency of fracturing jobs depends on the interaction between hydraulic (induced) and naturally occurring discrete fractures. In this work, a coupled reservoir-fracture flow model is described which accounts for varying reservoir geometries and complexities including non-planar fractures. Different flow models such as Darcy flow and Reynold's lubrication equation for fractures and reservoir, respectively are utilized to capture flow physics accurately. Furthermore, the geomechanics effects have been included by considering a multiphase Biot's model. An accurate modeling of solid deformations necessitates a better estimation of fluid pressure inside the fracture. The fractures and reservoir are modeled explicitly allowing accurate representation of contrasting physical descriptions associated with each of the two. The approach presented here is in contrast with existing averaging approaches such as dual and discrete-dual porosity models where the effects of fractures are averaged out. A fracture connected to an injection well shows significant width variations as compared to natural fractures where these changes are negligible. The capillary pressure contrast between the fracture and the reservoir is accounted for by utilizing different capillary pressure curves for the two features. Additionally, a quantitative assessment of hydraulic fracturing jobs relies upon accurate predictions of fracture growth during slick water injection for single and multistage fracturing scenarios. It is also important to consistently model the underlying physical processes from hydraulic fracturing to long-term production. A recently introduced thermodynamically consistent phase-field approach for pressurized fractures in porous medium is utilized which captures several characteristic features of crack propagation such as joining, branching and non-planar propagation in heterogeneous porous media. The phase-field approach captures both the fracture-width evolution and the fracture-length propagation. In this work, the phase-field fracture propagation model is briefly discussed followed by a technique for coupling this to a fractured poroelastic reservoir simulator. We also present a general compositional formulation using multipoint flux mixed finite element (MFMFE) method on general hexahedral grids with a future prospect of treating energized fractures. The mixed finite element framework allows for local mass conservation, accurate flux approximation and a more general treatment of boundary conditions. The multipoint flux inherent in MFMFE scheme allows the usage of a full permeability tensor. An accurate treatment of diffusive/dispersive fluxes owing to additional velocity degrees of freedom is also presented. The applications areas of interest include gas flooding, CO2 sequestration, contaminant removal and groundwater remediation.
Discrete Fracture Network Modeling of Hydraulic Stimulation describes the development and testing of a model that couples fluid-flow, deformation, friction weakening, and permeability evolution in large, complex two-dimensional discrete fracture networks. The model can be used to explore the behavior of hydraulic stimulation in settings where matrix permeability is low and preexisting fractures play an important role, such as Enhanced Geothermal Systems and gas shale. Used also to describe pure shear stimulation, mixed-mechanism stimulation, or pure opening-mode stimulation. A variety of novel techniques to ensure efficiency and realistic model behavior are implemented, and tested. The simulation methodology can also be used as an efficient method for directly solving quasistatic fracture contact problems. Results show how stresses induced by fracture deformation during stimulation directly impact the mechanism of propagation and the resulting fracture network.
This dissertation intends to advance fundamental understanding of two areas of interest in the petroleum industry: complex stimulated fracture network during hydraulic fracturing treatments and induced seismicity during wastewater disposal operations. Successful completion of hydraulic fractures in unconventional formations has been the primary source of increased oil and gas production in the US. However, field observations suggest that the hydraulic fracture networks are much more complex and different from the classical description of bi-wing planar fractures. Thus, the attempts to optimize this stimulation technique are hindered by the uncertainties in predicting the complex fracture network. A by-product of massive improvement in oil and gas production is a significant amount of water being co-produced from these formations. The common practice in the industry is to recycle wastewater for hydraulic fracturing purposes or reinject it into the reservoir through disposal wells. In certain regions of the US, this wastewater injection has led to historically high seismicity rates and earthquakes of Magnitude 5 and above which caused the public to be concerned. To maintain the social license to continue such operations, these concerns need to be addressed, and the physics behind such induced events need to be understood. Two novel hydraulic fracturing and induced seismicity simulators are developed that implicitly couple fluid flow with the stresses induced by fracture deformation in large, complex, three-dimensional discrete fracture networks. The simulators can describe the propagation of hydraulic fractures and opening and shear stimulation of natural fractures. Fracture elements can open or slide, depending on their stress state, fluid pressure, and mechanical properties. Fracture sliding occurs in the direction of maximum resolved shear stress. Nonlinear empirical relations are used to relate normal stress, fracture opening, and fracture sliding to fracture aperture and transmissivity. Field-scale hydraulic fracturing simulations were performed in a dense naturally fractured formation. Height containment of propagating hydraulic fractures between bedding layers is modeled with a vertically heterogeneous stress field or by explicitly imposing hydraulic fracture height containment as a model assumption. The propagating hydraulic fractures can cross natural fractures or terminate against them depending on the natural fracture orientation and stress anisotropy. The simulations demonstrate how interaction with natural fractures in the formation can help explain the high net pressures, relatively short hydraulic fracture lengths, and broad regions of microseismicity that are often observed in the field during stimulation in low permeability formations, some of which were not predicted by classical hydraulic fracturing models. Depending on input parameters, our simulations predicted a variety of stimulation behaviors, from long hydraulic fractures with minimal leakoff into surrounding fractures to broad regions of dense fracturing with a branching network of many natural and newly formed fractures. Induced seismicity simulator was developed to investigate the effects of multiple operational, hydraulic, and geophysical parameters on the magnitude of induced earthquakes. The rate-and-state framework is implemented to include the effect of fault nonlinear friction evolution and to model unstable earthquake rupture. The Embedded Discrete Fracture Model (EDFM) technique is used to model the fluid flow between the matrix and fractures efficiently. The results show that high-rate injections are more likely to induce a more significant earthquake, confirming the statistical correlation attributing induced events to high-rate injection wells. To understand the seismic occurrence outside of the injection zone, the effect of fault permeability structure on seismicity is studied by assigning non-uniform permeabilities as an input parameter. The model shows that the fault rupture is dominantly controlled by initial pressure and stress heterogeneity which ultimately affect the magnitude of an induced earthquake event
Fluid injection and production in highly fractured unconventional reservoirs could induce complex stress reorientation and redistribution. The strong stress sensitivity of fractured formations may also lead to non-negligible fracture opening or closure under the reservoir loading or unloading process. Hence, a coupled flow and geomechanics model is in high demand to assist with stress prediction and production forecast in unconventional reservoirs. In this dissertation, an enhanced geomechanics model is developed for fractured reservoirs and integrated with the in-house compositional reservoir simulator – UTCOMP for coupled flow and geomechanics modeling. The multiphase flow model is solved using the finite volume method (FVM) with an embedded discrete fracture model (EDFM) to represent flow through complex fractures. Based on static fracture assumption, the finite element method (FEM) is applied to solve the geomechanics model by incorporating fracture effects on rock deformation through pore pressure changes. An iterative coupling procedure is implemented between fluid flow and geomechanics, and the 3D coupled model is applied to predict spatiotemporal stress evolution in single-layer and multilayer unconventional reservoirs. To consider dynamic fracture properties, the geomechanics model is further enhanced by the extended finite element method (XFEM) with a modified linear elastic proppant model. The fracture surface is under the coeffects of pore pressure and proppant particles, and various enrichment functions are introduced to reproduce the discontinuous fields over fracture paths. The enhanced geomechanics model is validated against classical Sneddon and Elliot’s problem and presents a first-order spatial convergence rate. Numerical studies indicate that modeling fracture closure is necessary for poorly propped, highly stressed, or fast depleted reservoirs, and fracture opening can be significant under high permeability and low stiffness conditions. The coupled flow and geomechanics model is finally combined with a displacement discontinuity method (DDM) hydraulic fracture model to establish an integrated reservoir-geomechanics-fracture model for the end-to-end optimization of secondary stimulations. It is applied to Permian Basin and Sichuan Basin tight formations to optimize parent-child well spacing at different infill times. The integrated model provides hands-on guidelines for refracturing and infill drilling in multilayer unconventional reservoirs and can be easily adapted to other basins under their unique data
Numerical modeling plays a key role in assessing, developing, and managing energy resources (such as oil, gas and heat) from subsurface formations. Fluids are injected into wellbores during hydraulic fracturing, water flooding, parent well pre-loading, and improved oil recovery. Oil, gas and water are produced back to the surface during flowback, primary/secondary/tertiary production, and geothermal operations. Results from modeling these subsurface energy resources assist engineers and geologists in the decision-making process. Geomechanics, fluid/solid flow, and heat transport are coupled in the reservoir, fracture, and wellbore domains. The purpose of this dissertation is to develop integrated hydraulic fracturing and reservoir simulator that can accurately model multi-component, multi-phase fluid flow, geomechanics, fracture propagation and thermal processes in the reservoir, fracture and wellbore domains. In this dissertation, fully coupled reservoir, fracture, and wellbore domains are modeled. Geomechanics, fluid flow, and heat transport are modeled in an integrated manner in each domain and between each domain. Thermo-poro-elasticity, fracture opening/closing, and fracture propagation are modeled based on the stresses and strains computed in the domain. Four flow types including single-phase flow, multi-phase black-oil flow, multi-phase compositional flow, and water-steam two-phase flow are developed for different applications. Temperature and enthalpy formulations are developed to model the energy balance within the fully coupled system. A novel proppant transport model formulation which couples fracture opening/closing has also been developed. The governing equations are discretized in space using the finite volume/area methods. Multiple fully implicit Newton solvers have been developed to solve different sets of nonlinear systems of equations. A fully distributed memory parallelization workflow is constructed. The simulator is also coupled with simpler (analytical and DDM) fracturing models to achieve shorter run times. The modeling capability of the simulator has been demonstrated in the dissertation through many example applications. Typical applications of the simulator include multi-stage, multi-cluster, hydraulic fracture propagation, proppant settling and fracture closure analysis, mini-frac analysis, parent-child well interference, fracture monitoring, reservoir cooling and induced fracture propagation from water injectors, production analysis, gas huff-n-puff injection, improved oil recovery, geothermal reservoir production, and enhanced geothermal system analysis. These applications demonstrate the wide variety of problems that our simulator can be used to model
Hydraulic fracturing in horizontal wells has enabled economic production from ultra-low permeability reservoirs. The productivity of these hydraulically fractured wells depends on the fracture dimensions, conductivity, connectivity to the wellbore, and applied drawdown pressure. Traditional numerical simulation models used to analyze the productivity of hydraulically fractured wells assume a planar bi-wing fracture that is open and connected to the wellbore. However, several core-through field studies and fracture propagation models have demonstrated that a hydraulic fracturing process can create non-planar complex fracture networks. The conductivity and connectivity of these complex fractures are highly dependent on the in-situ stress changes due to production. Hence it is critical to consider complex fractures and the impact of geomechanics in the simulation models for analyzing fractured well productivity. A finite-volume method based geomechanics coupled reservoir model was developed to simulate production from complex fracture networks. An automated meshing method was developed to create the reservoir, and fracture mesh for any given arbitrarily shaped fracture network. The reservoir-fracture network model accounts for fracture closure effects during production. The model developed in this dissertation was used to investigate the impact of drawdown strategy (choke management) on the productivity of wells producing from complex fracture networks. The competing phenomenon of higher initial production rate and faster fracture closure depending on the applied drawdown strategy was observed. Based on NPV maximization, an optimum drawdown strategy can be calculated. The model was also applied to estimate the effective permeability of the SRV (stimulated reservoir volume) to account for complex fractures in upscaled traditional reservoir simulation models. Tracer transport was implemented in the geomechanical reservoir simulation model to analyze the impact of (a) fracture geometry, (b) fracture propagation and closure effects, and (c) fracture complexity on the tracer response curves. An effective model was created to simulate tracer tests in complex fracture networks. Closure of activated natural fractures can explain the multiple peaks in the tracer response curves observed in the field tests. A neural network-based inverse modeling was performed to estimate effective connected fracture length using peak tracer concentration values, peak times, and tracer recovery from chemical tracer flowback data. Observations from the chemical tracer analysis were combined with radioactive proppant tracer and pressure interference tests to diagnose well interference for the Hydraulic Fracturing Test Site #1
Discrete Fracture Network Modeling of Hydraulic Stimulation describes the development and testing of a model that couples fluid-flow, deformation, friction weakening, and permeability evolution in large, complex two-dimensional discrete fracture networks. The model can be used to explore the behavior of hydraulic stimulation in settings where matrix permeability is low and preexisting fractures play an important role, such as Enhanced Geothermal Systems and gas shale. Used also to describe pure shear stimulation, mixed-mechanism stimulation, or pure opening-mode stimulation. A variety of novel techniques to ensure efficiency and realistic model behavior are implemented, and tested. The simulation methodology can also be used as an efficient method for directly solving quasistatic fracture contact problems. Results show how stresses induced by fracture deformation during stimulation directly impact the mechanism of propagation and the resulting fracture network.
The expansion of unconventional petroleum resources in the recent decade and the rapid development of computational technology have provided the opportunity to develop and apply 3D numerical modeling technology to simulate the hydraulic fracturing of shale and tight sand formations. This book presents 3D numerical modeling technologies for hydraulic fracturing developed in recent years, and introduces solutions to various 3D geomechanical problems related to hydraulic fracturing. In the solution processes of the case studies included in the book, fully coupled multi-physics modeling has been adopted, along with innovative computational techniques, such as submodeling. In practice, hydraulic fracturing is an essential project component in shale gas/oil development and tight sand oil, and provides an essential measure in the process of drilling cuttings reinjection (CRI). It is also an essential measure for widened mud weight window (MWW) when drilling through naturally fractured formations; the process of hydraulic plugging is a typical application of hydraulic fracturing. 3D modeling and numerical analysis of hydraulic fracturing is essential for the successful development of tight oil/gas formations: it provides accurate solutions for optimized stage intervals in a multistage fracking job. It also provides optimized well-spacing for the design of zipper-frac wells. Numerical estimation of casing integrity under stimulation injection in the hydraulic fracturing process is one of major concerns in the successful development of unconventional resources. This topic is also investigated numerically in this book. Numerical solutions to several other typical geomechanics problems related to hydraulic fracturing, such as fluid migration caused by fault reactivation and seismic activities, are also presented. This book can be used as a reference textbook to petroleum, geotechnical and geothermal engineers, to senior undergraduate, graduate and postgraduate students, and to geologists, hydrogeologists, geophysicists and applied mathematicians working in this field. This book is also a synthetic compendium of both the fundamentals and some of the most advanced aspects of hydraulic fracturing technology.