In this report, we develop a computational procedure to investigate propagation of 3D cracks in isotropic linearly elastic media in a 2D framework. We reduce the 3D weakly singular, weak-form traction integral equation to its 2D analog by introducing a constraint on crack profile in out-of-plane direction. Symmetric Galerkin boundary element method based on the obtained 2D weak-form traction integral equation is adopted to model these fractures. In particular, we consider multiple interacting cracks in unbounded domain subject to internal pressure to model hydraulic fracture. / text

Most models developed for hydraulic fracturing in unconsolidated sands are based on Linear Elastic Fracture Mechanics (LEFM) and tensile fracture (Mode I fracture). However, in unconsolidated sand formations the field data shows that LEFM based models cannot properly predict the fracture behavior. Hydraulic fracture lab experiments in a true triaxial setup which was made as a part of this study are designed to investigate the failure mechanism around the crack tip in unconsolidated sands and effects of fluid rheology, leak off, and stress state are investigated. The results show that two mechanisms of tensile and shear failure are involved in fracture propagation in unconsolidated sands and depending on the fracturing fluid rheology and stress state of the formation one or both of them can happen at the crack tip. Several experiments with different fracturing fluids, rates, and different stress boundary conditions are categorized into two major categories based on whether we have a fracture or not. A subsequent categorization is used to categorize the fractured cases into Tensile Failure, Shear Failure and Mixed Failure categories. First the experimental observations are presented and subsequently observations are analyzed and compared in order to explain the observations and conclusions. ;Tensile failure category is happening in medium viscosity fracturing fluids in the order of 20,000 cP viscosity at unit 1/s shear rate. Shear failure category is mostly taking place in low viscosity fluids (200 cP viscosity at unit 1/s shear rate). Mixed mode fracturing is happening in high viscosity fluids (70,000 cP viscosity at unit 1/s shear rate) with high stress anisotropy. However, the same fluid will give a No Fracture result in the case of isotropic or near isotropic stress state. It is shown that higher stress anisotropy increases the tendency of shear failure and at the same time, the resulting fracture will propagate in a preferential direction. However, tilting and branching might happen due to high stress anisotropy which is more pronounced in case of thicker fluids. It was also observed that in case of vaseline injection, stress anisotropy decreases treatment breakdown pressure. / text

Multiphase and non-Darcy flow effects in hydraulically fractured gas wells reduce effective fracture conductivity. Typical proppant pack laboratory experiments are oriented in such a way such that phase segregation is not possible, which results in mixed flow. Tidwell and Parker (1996), however, showed that in proppant packs, gravity segregation occurs for simultaneous gas and liquid injection at laboratory scale (1500 cm2). Although the impact of gravity on flow in natural fractures has been described, previous work has not fully described the effect of gravity on multiphase non-Darcy flow in hydraulic fractures. In this work, reservoir simulation modeling was used to determine the extent and impact of gravity segregation in a hydraulic fracture at field scale. I found that by ignoring segregation, effective fracture conductivity can be underestimated by up to a factor of two. An analytical solution was developed for uniform flux of water and gas into the fracture. The solution for pressures and saturations in the fracture agrees well with reservoir simulation. Gravity segregation occurs in moderate-to-high conductivity fractures. Gravity segregation impacts effective fracture conductivity when gas and liquid are being produced at all water-gas ratios modeled above 2 Bbls per MMscf. More realistic, non-uniform-flux models were also run with the hydraulic fracture connected to a gas reservoir producing water. For constant-gas-rate production, differences in pressure drop between segregated cases and mixed flow cases range up to a factor of two. As the pressure gradient in the fracture increases above 1 to 2 psi/ft, the amount of segregation decreases. Segregation is also less for fracture half-length-to-height ratios less than or close to two. When there is less segregation, the difference in effective conductivity between the segregated and mixed flow cases is reduced. I also modeled the water injection and cleanup phases for a typical slickwater fracture treatment both with and without gravity effects and found that for cases with segregation, effective fracture conductivity is significantly higher than the conductivity when mixed flow occurs. Gravity segregation is commonly ignored in design and analysis of hydraulically fractured gas wells. This work shows that segregation is an important physical process and it affects effective fracture conductivity significantly. Hydraulic fracture treatments can be designed more effectively if effective fracture conductivity is known more accurately.

One of the most critical parameters in the success of a hydraulic fracturing treatment is to have sufficiently high fracture conductivity. Unbroken polymers can cause permeability impairment in the proppant pack and/or in the matrix along the fracture face. The objectives of this research project were to design and set up an experimental apparatus for dynamic fracture conductivity testing and to create a fracture conductivity test workflow standard. This entirely new dynamic fracture conductivity measurement will be used to perform extensive experiments to study fracturing fluid cleanup characteristics and investigate damage resulting from unbroken polymer gel in the proppant pack. The dynamic fracture conductivity experiment comprises two parts: pumping fracturing fluid into the cell and measuring proppant pack conductivity. I carefully designed the hydraulic fracturing laboratory to provide appropriate scaling of the field conditions experimentally. The specifications for each apparatus were carefully considered with flexibility for further studies and the capability of each apparatus was defined. I generated comprehensive experimental procedures for each experiment stage. By following the procedure, the experiment can run smoothly. Most of dry runs and experiments performed with sandstone were successful.

The term biosolids refers to the nutrient-rich organic materials resulting from the treatment of domestic sewage at a wastewater treatment facility. It is a widely acceptable term for sewage sludge that has been treated at a wastewater treatment plant and is beneficially recycled. Biosolids inherently come from sewage sludge, so they have the same origin and biological nature, but a different applicability. The quantity of municipal biosolids produced increases annually in the United States. The production of biosolids has increased because of both the advance of sanitation and wastewater treatment and the growth of population. Sludge or biosolids are contaminated by varying amounts of heavy metals or hazardous organic compounds from industrial and commercial wastewater. Therefore, society has to face the potential for increased negative impacts on the environment from the increasing volume of biosolids being produced. Public concerns about applied biosolids treatment or reuse methods are potential health, environmental, or aesthetic impacts (such things as disease, odors), because of the pollutants in the biosolids. The most commonly used methods for biosolids treatment and recycling are briefly reviewed in the first two chapters of this thesis. However, the current biosolids treatment or recycling options have their own defects. A new and innovative technology, deep biosolids injection, is proposed for the treatment of biosolids and is to be implemented by Los Angeles where the City has been granted underground solids injection control permits under Class V wells by the US Environmental Protection Agency. Deep biosolids injection is a process referred to as one type of several deep underground injection techniques. It shares many similarities with slurried solids injection above the fracture pressure, which has been successfully used for the treatment of slurried non-hazardous solid materials produced in the oil industry such as drill cuttings, viscous emulsions with clay, oily sand, NORMs (naturally occurring radioactive materials), pipe scale, tank bottoms, soil from spill clean-up, and so on. The distinctive biosolids properties result in injection mechanisms different from other slurry injection processes. Filtration and consolidation processes occur simultaneously along with injection of biosolids, and these must be understood in order to properly design and manage a biosolids injection operation. Hydraulic fracture mechanisms, filtration theory and consolidation principles provide the basis for the interpretation of biosolids injection process. A semi-analytical hydraulic fracture model for injection of a compressible substance (biosolids) is developed as a modification of the Perkins-Kern-Nordgren (PKN) hydraulic fracture model. The PKN model is modified with a pseudo-dynamic leak-off function that describes the deposition of biosolids (filtration) and plugging effect of biosolids on the fracture wall in a porous medium. The pseudo-dynamic leak-off function is given in terms of the net pressure and the resistance of the filter cake to flow. The hydraulic fracture model is employed to compute the volume of biosolids slurry remaining in an open induced fracture. The consolidation process in the closure phase of deep biosolids injection is described using the biosolids properties under different stress conditions. A Terzaghi-type relationship is used to compute the volume change in the closure phase using compressibility data available from published literature. In contrast to the conventional PKN leak-off model, simulation results using the new model show that the induced fracture volume is much larger because of the impaired leak-off and because of the volumetric effects and consolidation of the biosolids in the fracture. Solids contents and biosolids compaction behavior have significant impacts on the geometry of fracture (width, length, volume) over time. The model was developed to help guide large-scale injection of municipal and animal biosolids as an environmentally more secure method of treatment than surface approaches.

The term biosolids refers to the nutrient-rich organic materials resulting from the treatment of domestic sewage at a wastewater treatment facility. It is a widely acceptable term for sewage sludge that has been treated at a wastewater treatment plant and is beneficially recycled. Biosolids inherently come from sewage sludge, so they have the same origin and biological nature, but a different applicability. The quantity of municipal biosolids produced increases annually in the United States. The production of biosolids has increased because of both the advance of sanitation and wastewater treatment and the growth of population. Sludge or biosolids are contaminated by varying amounts of heavy metals or hazardous organic compounds from industrial and commercial wastewater. Therefore, society has to face the potential for increased negative impacts on the environment from the increasing volume of biosolids being produced. Public concerns about applied biosolids treatment or reuse methods are potential health, environmental, or aesthetic impacts (such things as disease, odors), because of the pollutants in the biosolids. The most commonly used methods for biosolids treatment and recycling are briefly reviewed in the first two chapters of this thesis. However, the current biosolids treatment or recycling options have their own defects. A new and innovative technology, deep biosolids injection, is proposed for the treatment of biosolids and is to be implemented by Los Angeles where the City has been granted underground solids injection control permits under Class V wells by the US Environmental Protection Agency. Deep biosolids injection is a process referred to as one type of several deep underground injection techniques. It shares many similarities with slurried solids injection above the fracture pressure, which has been successfully used for the treatment of slurried non-hazardous solid materials produced in the oil industry such as drill cuttings, viscous emulsions with clay, oily sand, NORMs (naturally occurring radioactive materials), pipe scale, tank bottoms, soil from spill clean-up, and so on. The distinctive biosolids properties result in injection mechanisms different from other slurry injection processes. Filtration and consolidation processes occur simultaneously along with injection of biosolids, and these must be understood in order to properly design and manage a biosolids injection operation. Hydraulic fracture mechanisms, filtration theory and consolidation principles provide the basis for the interpretation of biosolids injection process. A semi-analytical hydraulic fracture model for injection of a compressible substance (biosolids) is developed as a modification of the Perkins-Kern-Nordgren (PKN) hydraulic fracture model. The PKN model is modified with a pseudo-dynamic leak-off function that describes the deposition of biosolids (filtration) and plugging effect of biosolids on the fracture wall in a porous medium. The pseudo-dynamic leak-off function is given in terms of the net pressure and the resistance of the filter cake to flow. The hydraulic fracture model is employed to compute the volume of biosolids slurry remaining in an open induced fracture. The consolidation process in the closure phase of deep biosolids injection is described using the biosolids properties under different stress conditions. A Terzaghi-type relationship is used to compute the volume change in the closure phase using compressibility data available from published literature. In contrast to the conventional PKN leak-off model, simulation results using the new model show that the induced fracture volume is much larger because of the impaired leak-off and because of the volumetric effects and consolidation of the biosolids in the fracture. Solids contents and biosolids compaction behavior have significant impacts on the geometry of fracture (width, length, volume) over time. The model was developed to help guide large-scale injection of municipal and animal biosolids as an environmentally more secure method of treatment than surface approaches.

One of the most critical parameters in the success of a hydraulic fracturing treatment is to have sufficiently high fracture conductivity. Unbroken polymers can cause permeability impairment in the proppant pack and/or in the matrix along the fracture face. The objectives of this research project were to design and set up an experimental apparatus for dynamic fracture conductivity testing and to create a fracture conductivity test workflow standard. This entirely new dynamic fracture conductivity measurement will be used to perform extensive experiments to study fracturing fluid cleanup characteristics and investigate damage resulting from unbroken polymer gel in the proppant pack. The dynamic fracture conductivity experiment comprises two parts: pumping fracturing fluid into the cell and measuring proppant pack conductivity. I carefully designed the hydraulic fracturing laboratory to provide appropriate scaling of the field conditions experimentally. The specifications for each apparatus were carefully considered with flexibility for further studies and the capability of each apparatus was defined. I generated comprehensive experimental procedures for each experiment stage. By following the procedure, the experiment can run smoothly. Most of dry runs and experiments performed with sandstone were successful.

Numerical simulation method is used in this work to solve the problem of transient and pseudosteady-state flow of fluid in a rectangular reservoir with impermeable boundaries. Development and validation of the numerical solution for various well-fracture configurations are the main objectives of this research. The specific case of horizontal well intersected by multiple transverse fractures is the focus of the investigation. The solutions for different operating conditions, constant rate and constant pressure, are represented in the form of transient – peudosteady-state productivity indices. The numerical simulator is validated by comparing results to known analytical solution for radial flow, existing models of productivity for vertical well intersected by vertical fracture, and also with published tables of shape factors. Numerical simulation is a powerful tool to predict well performance. The complexities of well-fracture configurations can be modeled in a truly 3-dimensional system and the pressure and productivity responses for all of the flow regimes can be computed efficiently, enabling optimization of the well-fracture system.

This investigation carried out the analysis of fracture conductivity in a tight reservoir using laboratory experiments, by applying the procedure known as the dynamic fracture conductivity test. Considering the large number of experiments necessary to evaluate the effect of each parameter and the possible interaction of their combinations, the schedules of experiments were planned using a fractional factorial design. This design is used during the initial stage of studies to identify and discharge those factors that have little or no effect. Finally, the most important factors can then be studied in more detail during subsequent experiments. The objectives of this investigation were focused on identifying the effect of formation parameters such as closure stress, and temperature and fracture fluid parameters such as proppant loading over the final conductivity of a hydraulic fracture treatment. With the purpose of estimating the relation between fracture conductivity and the design parameters, two series of experiments were performed. The first set of experiments estimated the effects of the aliases parameters. The isolated effect of each independent parameter was obtained after the culmination of the second set of experiments. The preliminary test results indicated that the parameters with major negative effect over the final conductivity were closure stress and temperature. Some additional results show that proppant distribution had a considerable role over the final fracture conductivity when a low proppant concentration was used. Channels and void spaces in the proppant pack were detected on these cases improving the conductivity of the fracture, by creating paths of high permeability. It was observed that with experiments at temperatures around 250 degrees F, the unbroken gel dried up creating permeable scales that resulted in a significant loss in conductivity. The results of this investigation demonstrated that dynamic fracture conductivity test procedure is an excellent tool to more accurately represent the effects of design parameters over the fracture conductivity. These results are also the first step in the development of a statistical model that can be used to predict dynamic fracture conductivity.

Hydraulic fracturing is an indispensable technique for developing unconventional resources such as shale gas and tight oil. When hydraulic fractures interact with pre-existing natural fractures, it can result in a complex fracture network. The interaction depends on in-situ stresses, rock and natural fracture mechanical properties, approach angle and hydraulic fracture treatment parameters. Most simulation studies treat natural fractures as frictional interfaces with cohesive properties. However, from core observation, partially cemented and fully cemented natural fractures are widely present and it is not clear whether they would fit the common description. In this study, semi-circular bending test is utilized to examine the propagation paths and strength of samples with pre-existing cemented fractures. Synthetic hydrostone samples are used to represent the rock and different inclusion slices with different mechanical properties are used to mimic cemented natural fractures. In a series of experiments, we assess the influence of the fracture approach angle, inclusion strength, and inclusion thickness on fracture propagation. Current results show that fractures tend to cross the inclusion when the approach angle is high and divert into the inclusion when the approach angle is low. The crossing surface is not a clean cut, but often has a jog distance. The thickness of the inclusion does not change the crossing/diverting behavior for orthogonal approaching samples, however it does change the jog distance along the interface. Preliminary simulation results using finite element software, ABAQUS, are presented better to analyze the experimental observations. The assessments of fracture interaction in this study are in good agreement with previous work and theories. / text