Browsing by Subject "Hydraulic fracturing"

Now showing 1 - 20 of 75

A data analytics framework for analyzing the effect of frac hits on parent well production
(2021-07-24) Guo, Yifei; Oort, Eric van; Ashok, Pradeepkumar, 1977-
Well interference, which is commonly referred to as frac hits, has become a significant factor affecting production in fractured horizontal shale wells with the increase in infill drilling in recent years. Today, there is still no clear understanding on how frac hits affect production. This work aims to develop a process to automatically identify the different types of frac hits and to determine the effects of stage-to-well distance and frac hit intensity on long-term parent well production. First, child well completions data and parent well pressure data are processed by a frac hit detection algorithm to automatically identify different frac hit intensities and duration within each stage. This algorithm classifies frac hits based on the magnitude of the differential pressure spikes. The frac stage to parent well distance is also calculated. Then, we compare the daily production trend before and after the frac hits to determine the severity of its influence on production. Finally, any evident correlations between the stage-to-well distance, frac hit intensity and production change are identified and investigated. In addition, this work also introduces a novel pressure integration approach to better understand the prolonged production effects of sustained frac hits. The methodology was to first characterize every induced pressure change in a parent well (i.e. to determine frac-hit intensity), and subsequently to estimate the average daily production change after each frac hit. This was accomplished by utilizing a frac hit auto-detection algorithm and a log-log analysis of daily production. To add the effect of interference time (i.e. the time of the pressure change caused by the frac hit), a “PCI” metric was calculated for each parent well. Finally, the effects of frac-hit intensity and interference time on parent well production were quantified through correlation analysis. This work utilizes 3 datasets covering 13 horizontal wells in the Bakken formation and 37 horizontal wells in the Eagle Ford Shale formation. These datasets included well trajectories, child well completions data, parent well pressure data and parent well production data. The data analysis results include factors influencing frac hit intensity and production responses to different types of frac hits. Also, the result shows that both frac hit intensity and interference time impacts parent well production. This work not only adds new understandings to frac hit study, but also challenges some of the previous work
A model for hydraulic fracturing and proppant placement in unconsolidated sands
(2017-12-07) Lee, Dongkeun; Sharma, Mukul M.; Olson, Jon E; Espinoza, D. Nicolas; McClure, Mark W; Kallivokas, Loukas F
Hydraulic fracturing in unconsolidated or poorly consolidated formations has been used as a technique for well stimulation and for sand control. Although a large number of hydraulic fracturing operations have been performed in soft formations, the exact mechanisms of failure and fracture propagation remain an unresolved issue. Conventional hydraulic fracturing models based on the theory of linear elastic fracture mechanics (LEFM) consistently predict lower net fracturing pressure, smaller fracture widths and longer fracture lengths in soft formations than observed in the field. Operators who want to design and analyze frac-pack treatments routinely use a hard rock model and need to calibrate and often manipulate input parameters beyond a physically reasonable range to match the net fracturing pressure and well performance data. In this dissertation, we have developed a fully-coupled, three-dimensional hydraulic fracture model for poro-elasto-plastic materials and fluid flow coupled with proppant transport. A computational framework for fluid-structure interaction (FSI) based on finite volume method was developed for modeling of hydraulic fracturing and proppant placement in soft formations. Two separate domains, a fracture and a reservoir domain, are discretized individually, separate equations are solved in the two domains, and their interactions are modeled. The model includes the fully coupled process of power-law fluid flow inside the fracture with proppant transport, fluid leak-off from the fracture into the porous reservoir, pore pressure diffusion into the reservoir, inelastic deformation of the poro-elasto-plastic reservoir, and fracture propagation using a cohesive zone model along with a dynamic meshing procedure. Fully-coupled processes between the two domains, and pressure, flow and displacement coupling within each domain are modeled by an iterative and segregated solution procedure, where each component of the field variable is solved separately, consecutively, and iteratively. We verified the essential components of the model by comparing our simulation results with several well-known analytical solutions (elastoplastic deformation and failure problem, KGD model in a 2-D elastic domain, and KGD model in storage-toughness dominated regime). We applied the model to design and analyze frac-pack operations conducted in a Gulf of Mexico oilfield. Our model is capable of capturing the high net fracturing pressure commonly observed during frac-packing operations without adjusting any input parameters. The model shows quantitatively that plasticity causes lower stress concentration around the fracture tip which shields the tip of the propagating fracture from the fracturing pressure, and retards fracture growth. Our model predicts shorter fracture lengths and wider widths compared to a hard rock model. Shear failure around the fracture and ahead of the tip are modeled. Low cohesion sands tend to fail in shear first then in tension if sufficient pore pressure builds up. We investigated the effect of fluid viscosity, injection rate, and proppant diameter on fracture growth and proppant placement using sensitivity studies. Higher apparent fluid viscosity and injection rate results in wider fractures with better proppant placement, when the fracture is expected to be contained within the payzone. Utilizing larger diameter of proppant leads to settling-dominant proppant placement resulting in the formation of a proppant bank at the bottom of the induced fracture. The new frac-pack model for the first time allows operators to design and analyze hydraulic fracturing stimulations in soft, elastoplastic formations when complex fracturing fluids are used. Our results also provide guidelines for the selection of fracturing fluid rheology, proppant size, and injection rates.
A techno-economic framework for mitigating environmental liabilities from unconventional oil and gas operations in the United States
(2018-08) Glazer, Yael Rebecca; Webber, Michael E., 1971-; Lawler, Desmond F; Werth, Charles J; Passalacqua, Paola; Kreitler, Charles
Unconventional oil and gas (O&G) activity is associated with many environmental liabilities including 1) high water use, 2) substantial volumes of generated wastewater, and 3) flaring of co-produced natural gas. The work in this dissertation aims to holistically examine and find strategies to mitigate these environmental challenges through three studies: 1. Designing a method to select the most appropriate wastewater treatment technology or product based on numerous metrics and across many potential options. 2. Conducting an inventory and engineering assessment of the flared gas and wastewater. 3. Building a decision tree model to investigate and compare the economic feasibility of several potential traditional and nontraditional produced water management pathways, including treatment, disposal, discharge, and crop production. Based on the results of these analyses, the following general conclusions are drawn: The first study shows, through the tremendous number of technologies and products that claim to handle wastewater associated with O&G activities, mechanical vapor recompression, and to a lesser extent, reverse osmosis are the top contenders when treating to freshwater standards is desired. In the process, a down-selection tool that can be tailored to an operator’s specific requirements and a database containing many of the available technologies and products were created. The second study shows that from the seven prominent shale regions included in this analysis, Marcellus/Utica (in the Northeast), Bakken (North Dakota), and Niobrara (Rocky Mountains) flared between 2 and 48 times the amount of natural gas needed to provide energy for treatment of their wastewater volumes. The Permian Basin, Eagle Ford, and Haynesville did not have sufficient flared gas to treat wastewater produced in each respective region. As such, these regions would require additional energy sources for wastewater treatment. The third study shows that several nontraditional produced water management pathways might be economically feasible depending on the realized 1) price for the commodities produced and 2) cost associated with implementing the strategy. In this case, the traditional pathway to minimally treat and discharge to a nearby stream had the highest expected value by a slim margin over growing switchgrass onsite. This result suggests that further investigation should be considered to determine, with greater certainty, the attainable price for switchgrass. These general conclusions, along with further details, provide insight into the challenges and mitigation strategies with some of the environmental liabilities associated with unconventional O&G activity. As onsite resources (e.g., available water) become more constrained and regulations become more stringent (e.g., curtailment of flaring), implementing these or similar approaches to the industry’s waste streams will become increasingly imperative.
Advanced hydraulic fracture modeling : peridynamics, inelasticity, and coupling to FEM
(2018-06-21) York, Jason Robert; Foster, John T., Ph. D.; Sharma, Mukul M.; Espinoza, David N; Gray, Kenneth E; Landis, Chad M
In the effort to create new technology to enhance our ability to retrieve hydrocarbons, the technique of hydraulic fracturing has shown to be extremely beneficial. This involves pumping fluids at high pressures and high rates to induce and propagate fractures near the wellbore to stimulate production in otherwise low permeability reservoirs. To better understand the physical processes involved, several models have been proposed for numeric simulation. This work expands on an existing hydraulic fracturing model based on the nonlocal theory of peridynamics, detailed in Ouchi et al. [3]. Peridynamics is a relatively new reformulation of continuum mechanics, applicable even when discontinuities such as fractures are introduced. To incorporate the influence of inelasticity in the established model, which may be significant for several geologic materials, a multi-surface yield model is proposed. This yield model builds on a Drucker-Prager related yield model formulated for peridynamics by Lammi et al. [46], adding a tension cut-off surface as well as a cap to include hardening effects associated with inelastic compaction. The formulation of these additional surfaces in the peridynamic framework will be detailed and numerically demonstrated in this dissertation. As the peridynamic based hydraulic fracture model continues to develop complex capabilities, such as inelasticity, computational expense continues to be an ever-growing concern. Although the peridynamic formulation has demonstrated the capability of modeling complex fracture behavior, the computational expense is noted to be quite expensive relative to classic local models. Recently, methods have been introduced for coupling nonlocal bond based peridynamic grids with local finite element meshes, detailed in Galvanetoo et al. [74]. These coupling methods have demonstrated applicability to static equilibrium mechanics problems, while introducing negligible errors. In this work, the coupling method is implemented with the nonlocal hydraulic fracturing model, using peridynamics near existent and propagating fractures, as well as a standard finite element formulation far from the influence of such features. To further increase computational efficiency, a dynamically adaptive mesh coarsened away from the peridynamic region is implemented with the capability of converting finite element nodes to peridynamic nodes. This novel method of coupling peridynamics with a highly efficient mesh in the hydraulic fracture model will be fully detailed in this dissertation. In addition, 2D and 3D results will be provided using this method, demonstrating the capability of the coupled model to simulate complex fracture behavior, as well as discuss its impact on simulation capabilities and performance.
Alternate-slug fracturing using foam
(2016-08) Shrivastava, Kaustubh; Sharma, Mukul M.; Mohanty, Kishore K.
The success of a hydraulic fracturing job depends primarily on the proper distribution of proppant inside the fracture. Fracture length and conductivity are the two prime characteristics that determine the productivity of fractured wells (Liu & Sharma, 2005). Slick-water fracturing involves the use of large volumes of water for fracturing shales and mudstones (Palisch, et al., 2010). The low viscosity of water increases the settling velocity of proppant, resulting in an ineffective lateral placement of the proppant. It also affects the vertical coverage of the proppant across the pay zone(s), rendering the fracturing process inefficient (Gadde, et al., 2004). To improve proppant placement, a new technique was proposed by Malhotra et al. (2014), that involves pumping slugs of high viscosity and low viscosity fluids alternately, with most of the proppant being carried by the low viscosity fluid. Alternate injection of high viscosity and low viscosity slugs creates a mobility contrast between the fluids and leads to the formation of viscous fingers. The viscous fingers provide a pathway for proppant transport. The higher velocity of the viscous fingers compared to the injection velocity of the fluid leads to deeper placement of proppant. In addition, viscous sweeps, due to the high viscosity slugs, push any proppant bank formed near the wellbore deeper into the fracture, thus creating longer fractures (Malhotra, et al., 2014). In this study, we conducted an experimental investigation to obtain a fundamental understanding of the viscous fingering phenomena when water and foam are used as the low and high viscosity fluids, over a wide range of viscosity ratios. We have derived a relationship between finger-tip velocity and viscosity ratio of the fluids. This relationship will help in designing Alternate-slug fracturing treatments for the foam-water system.
Analysis and interpretation of a hydraulic fracture treatment using offset vertical observation wells and a hydraulic fracture simulator
(2015-08) Griffith, Christopher Adam; McClure, Mark W. (Mark William); Espinoza, Nicolas
Analysis of hydraulic fracture treatments requires incorporating a wide range of data in order to make useful inferences about fracture properties. For example, microseismic monitoring and production decline analysis can be used to obtain the hydraulic fracture half-length, which is an important parameter for field development. The challenge in using these tools is that the methods used for analysis are open to interpretation and can make it difficult to rely on the results. This thesis integrates data from four horizontal wells that were hydraulically fractured in an unconventional shale play and results from a 2-dimensional hydraulic fracture simulator in order to make qualitative observations about fracture properties. The importance of the data set hinges on nine vertical observation wells that recorded pressure vs. time during the hydraulic fracture treatments. The observation wells were located at different distances and depths from the horizontal wells. This is important because it removes some of the ambiguity associated with making interpretations from microseismic data, production decline analysis, or other methods. Results from modeling and the data set indicated the following: (1) the networks of fractures created from these treatments were volumetric and complex, illustrated by the microseismic data and the pressure signals recorded at the observation wells, (2) microseismicity was generally successful in delineating where fluid progressed during pumping, (3) however, flow of fluid into fractures stimulated during previous stages was aseismic, a manifestation of the Kaiser effect, and (4) during long term production, fluid was not produced from the more distant parts of the reservoir that were pressurized and stimulated during the fracturing treatment. To explain these four observations, we hypothesize that proppant was not transported to the regions of the stimulated rock volume that were most distant from the stimulated wells. The stimulated, but unpropped, fractures in this region evidently lost much of their conductivity after closure that they did not contribute significantly to long term production.
Analysis of hydraulic fracture growth and segmentation : implications for the HFTS1 slant core, Wolfcamp Fm., Midland Basin, West Texas
(2021-07-22) Rysak, Bethany Grace; Gale, Julia F. W.; Laubach, Stephen E. (Stephen Ernest), 1955-
The 6TW slant core is part of the multidisciplinary Hydraulic Fracture Test Site (HFTS1) project in the Midland Basin. The slant core made a close pass by two horizontal wells on an 11-well pad and has yielded new insight into fracture networks created by the hydraulic fracturing process. Approximately ~600 ft of core was recovered through the Wolfcamp A and B, with fracture characterization identifying 375 hydraulic fractures (trending E-W), and 309 calcite-sealed natural fractures (Set 1 trending NE-SW; Set 2 trending WNW-ESE). Initial observations showed that the number of hydraulic fractures found in core was higher than the number estimated to have been created via the completion processes. This abundance may be closely tied to the examples of twist-hackle segmentation, diversion, and bifurcation seen in core. These features can be used to determine propagation direction and help build a clearer picture of fracture network growth and geometry. To further investigate the impact of these features on our current understanding of hydraulic fracture propagation, this research was divided into four parts, those being: 1) Analysis of hydraulic fractures in the slant core, 2) Observation of lab-generated hydraulic fracture morphology, 3) Observation of natural hydraulic fracture morphology in the field, and 4) Building of a 3D reservoir model for the HFTS1 pad to run fracture forward modeling. The key implications of this work provide a greater understanding of hydraulic fracture network propagation in the subsurface, and could have wider applications for evaluation, completion, production, and fracture modeling techniques in unconventional reservoirs
Analysis of hydraulic fracture propagation in fractured reservoirs : an improved model for the interaction between induced and natural fractures
(2009-05) Dahi Taleghani, Arash; Olson, Jon E.
Large volumes of natural gas exist in tight fissured reservoirs. Hydraulic fracturing is one of the main stimulating techniques to enhance recovery from these fractured reservoirs. Although hydraulic fracturing has been used for decades for the stimulation of tight gas reservoirs, a thorough understanding of the interaction between induced hydraulic fractures and natural fractures is still lacking. Recent examples of hydraulic fracture diagnostic data suggest complex, multi-stranded hydraulic fracture geometry is a common occurrence. The interaction between pre-existing natural fractures and the advancing hydraulic fracture is a key condition leading to complex fracture patterns. Large populations of natural fractures that exist in formations such as the Barnett shale are sealed by precipitated cements which could be quartz, calcite, etc. Even though there is no porosity in the sealed fractures, they may still serve as weak paths for fracture initiation and/or for diverting the path of the growing hydraulic fractures. Performing hydraulic fracture design calculations under these complex conditions requires modeling of fracture intersections and tracking fluid fronts in the network of reactivated fissures. In this dissertation, the effect of the cohesiveness of the sealed natural fractures and the intact rock toughness in hydraulic fracturing are studied. Accordingly, the role of the pre-existing fracture geometry is also investigated. The results provide some explanations for significant differences in hydraulic fracturing in naturally fractured reservoirs from non-fractured reservoirs. For the purpose of this research, an extended finite element method (XFEM) code is developed to simulate fracture propagation, initiation and intersection. The motivation behind applying XFEM are the desire to avoid remeshing in each step of the fracture propagation, being able to consider arbitrary varying geometry of natural fractures and the insensitivity of fracture propagation to mesh geometry. New modifications are introduced into XFEM to improve stress intensity factor calculations, including fracture intersection criteria into the model and improving accuracy of the solution in near crack tip regions. The presented coupled fluid flow-fracture mechanics simulations extend available modeling efforts and provide a unified framework for evaluating fracture design parameters and their consequences. Results demonstrate that fracture pattern complexity is strongly controlled by the magnitude of in situ stress anisotropy, the rock toughness, the natural fracture cement strength, and the approach angle of the hydraulic fracture to the natural fracture. Previous studies (mostly based on frictional fault stability analysis) have concentrated on predicting the onset of natural fracture failure. However, the use of fracture mechanics and XFEM makes it possible to evaluate the progression of fracture growth over time as fluid is diverted into the natural fractures. Analysis shows that the growing hydraulic fracture may exert enough tensile and/or shear stresses on cemented natural fractures that they may be opened or slip in advance of hydraulic fracture tip arrival, while under some conditions, natural fractures will be unaffected by the hydraulic fracture. A threshold is defined for the fracture energy of cements where, for cases below this threshold, hydraulic fractures divert into the natural fractures. The value of this threshold is calculated for different fracture set orientations. Finally, detailed pressure profile and aperture distributions at the intersection between fracture segments show the potential for difficulty in proppant transport under complex fracture propagation conditions. Whether a hydraulic fracture crosses or is arrested by a pre-existing natural fracture is controlled by shear strength and potential slippage at the fracture intersections, as well as potential debonding of sealed cracks in the near-tip region of a propagating hydraulic fracture. We introduce a new more general criterion for fracture propagation at the intersections. We present a complex hydraulic fracture pattern propagation model based on the Extended Finite Element Method as a design tool that can be used to optimize treatment parameters under complex propagation conditions.
Analyzing pressure interference between fractured wells in unconventional reservoirs
(2020-11-29) Seth, Puneet; Sharma, Mukul M.; Mohanty, Kishore; Foster, John T; DiCarlo, David A; Roussel, Nicolas P
In conventional reservoirs, pressure transient analysis has been well studied and is based on hydraulic diffusion in the reservoir. In such high permeability reservoirs, pressure interference tests have been widely used to gather information about inter-well communication and reservoir permeability in the vicinity of the tested wells. However, in unconventional ultra-low permeability reservoirs (100 nD - 1μD), hydraulic diffusion through the reservoir matrix is negligible, instead poroelastic deformation of the rock dominates the pressure transient response. This renders traditional pressure interference analyses and well testing techniques ineffective in unconventional reservoirs. Horizontal wells drilled in unconventional reservoirs are hydraulically fractured during multiple stages of injection to increase the surface area available for production of hydrocarbons from these reservoirs, and similar to conventional reservoirs, pressure interference is often observed between fractured wells drilled in close proximity in unconventional reservoirs. However, unlike conventional reservoirs, pressure interference between fractured wells in unconventional reservoirs is not well understood. In this research, pressure interference between fractured wells in unconventional reservoirs is analyzed by developing numerical simulation models and investigating field data to understand the mechanisms that result in pressure communication between fractured wells, both during stimulation and production. Additionally, pressure interference analysis has been applied as a fracture diagnostics technique to investigate real field scenarios at multiple locations (Hydraulic Fracturing Test Site #1, DJ Basin and Permian Basin). A 3-D, fully-coupled geomechanical model that can simulate fracture propagation from the treatment well while monitoring pressure changes inside a compliant fracture in a nearby offset well has been developed. Numerical simulations and field data analyses show that in unconventional reservoirs, pressure interference between fractured wells is caused either by reservoir stress alterations during hydraulic fracture propagation, or high-permeability fracture connections (hydraulic communication) between the wells. The application of pressure interference analysis to diagnose inter-well communication during production and as a fracture diagnostics tool during stimulation to estimate fracture geometry, SRV permeability, diagnose diversion effectiveness and stimulation efficiency is demonstrated. Compared to other techniques such as micro-seismic monitoring and fiber optics that are expensive and require additional equipment, pressure interference analysis is presented as a novel and inexpensive tool that enables fracture diagnostics during both production and stimulation in unconventional reservoirs
Application of displacement discontinuity method to hydraulic fracture propagation in heterogeneous rocks
(2020-03-27) Hirose, Sho; Sharma, Mukul M.; Foster, John T; Kinnas, Spyridon A; Prodanovic, Masa; Espinoza, David N
The development of multi-stage hydraulic fracturing technique in horizontal wells enables us to produce oil and gas at economic rate from shale formations, leading to the shale revolution in the United States. Field observations including production history, microseismic mapping, and coring in fractured zones have revealed that the heterogeneity of shale rocks such as natural fractures is likely to have a large impact on oil and gas production from shale reservoirs. In this dissertation, a new hydraulic fracturing model based on the displacement discontinuity method (DDM) was developed. The major achievements in this research include the extension of DDM to multilayered media, the modeling of the interaction with natural fractures in three dimensions, and the development of a DDM-based hydraulic fracturing simulator. The formulation of DDM was revisited, and the equivalence of DDM and BEM was mathematically demonstrated. DDM was extended to multilayered media by using the method of images. The new DDM was applied to a three-layered medium in plain strain containing vertical and horizontal cracks. A sensitivity study suggests that bi-material solutions are sufficient for three-layered media under plain strain conditions. A DDM-based hydraulic fracturing model was developed. The discretized DDM and flow equations were solved in a segregated or fully coupled manner. A new splitting scheme was proposed to improve the convergence speed of the segregated method. The interaction between hydraulic and natural fractures was modeled for both intersecting and remotely interacting cases in our simulator. Poroelastic effects were partially incorporated into DDM by assuming an undrained condition. It was found that poroelastic effects under the undrained condition were limited to the vicinity of hydraulic fractures. Hydraulic fracturing simulations were performed in the presence of synthetic natural fracture networks. Synthetic microseismic events were generated, and inversion analyses of the synthetic microseismic data were performed. It was suggested that the density of microseismic events was affected by both the areal density and length distribution of natural fractures
An approach for evaluating changes in land-use from energy sprawl and other anthropogenic activities with implications for biotic resource management
(Environmental Earth Sciences, 2018) Wolaver, Brad; Pierre, Jon Paul; Benjamin, Labay; Travis, LaDuc; Charles, Duran; Wade, Ryberg; Toby, Hibbitts
This study presents an improved approach for evaluating land-use changes caused by energy development and other anthropogenic activities. We illustrate this approach by assessing the landscape footprint of energy development in the Eagle Ford Shale Play and Permian Basin. These two hydrocarbon provinces in Texas saw rapid expansion in drilling during 2008–2012. We compare changes in land-use from oil and gas infrastructure construction during this time period with that of wind energy development in West Texas, urbanization in Central Texas, and extensive agricultural areas. This land-change mapping approach is novel because it evaluates a suite of anthropogenic activities in one study, whereas most prior research assessed land-use effects of energy activities separately without comparing them to agricultural and urbanization processes. We found that changes in land-use caused by anthropogenic factors affected 1.06% (3,456 km2) of the ~324,000 km2 study area. Oil and gas development (well pads and pipelines) was ~48% of total changes in land use, changes in agriculture caused ~26%, and urbanization was ~24%. Construction of wind turbine pads and high voltage power transmission lines was less important (~1%). This study is part of an ongoing, multi-year research program generating science to inform the federal Endangered Species Act listing decision for the Spot-tailed Earless Lizard (Holbrookia lacerata). We illustrate this approach for a single species (i.e., H. lacerata) in Texas. Additionally, this technique can facilitate effective management of a variety of biotic resources in other rapidly developing environments globally by identifying what anthropogenic activities are most important and where land-change is most intense so that on-the-ground conservation strategies can be implemented where they are needed most.
Atmospheric emissions and air quality impacts of natural gas production from shale formations
(2014-08) Zavala Araiza, Daniel; Allen, David T.; Webber, Michael; McDonald-Buller, Elena; Hildebrandt Ruiz, Lea; Edgar, Thomas
Natural gas is at the core of the energy supply and security debates; new extraction technologies, such as horizontal drilling and hydraulic fracturing, have expanded natural gas production. As with any energy system, however, natural gas has an environmental footprint and this thesis examines the air quality impacts of natural gas production. Greenhouse gas (GHG), criteria pollutant, and toxic emissions from natural gas production have been subject to a great amount of uncertainty, largely due to limited measurements of emission rates from key sources. This thesis reports direct and indirect measurements of emissions, assessing the spatial and temporal distributions of emissions, as well as the role of very high emitting wells and high emitting sources in determining national emissions. Direct measurements are used to identify, characterize and classify the most important sources of continuous and episodic emissions, and to analyze mitigation opportunities. Methods are proposed and demonstrated for reconciling these direct measurements of emissions from sources with measurements of ambient concentrations. Collectively, the direct source measurements, and analyses of ambient air pollutant measurements in natural gas production regions reported in this work improve the estimation, characterization, and methods for monitoring air quality implications of shale gas production.
Computational frameworks for hydraulically fractured well production analysis
(2021-03-29) Hu, Jing, Ph. D.; Mear, Mark E.; Rodin, Gregory J.; Landis, Chad M.; Huang, Rui; Olson, Jon E.
Modeling for predicting hydraulic fracture growth and forecasting unconventional resources extraction is of great interest to the petroleum industry. Numerous celebrated accomplishments have been achieved over the decades, however there is still a lack of rigorous numerical models to simulate the well production process in fractured subsurface reservoirs and relatively little work touched upon accurately simulating intersecting and merging cracks due to the complex geometrical considerations that are involved. To address these challenges, we propose three computational frameworks in this dissertation. In the first part of this dissertation, a computational framework for modeling steady state well production by coupling steady state Darcy flow with channel flow through the law of mass conservation, is presented. The governing equation of steady state Darcy flow is formulated as a weakly singular integral equation with symmetric Galerkin boundary element method (SGBEM) and that of channel flow is cast in a weak form by finite element method (FEM). An asymptotic analysis is conducted for the steady state flux field around the tip front in a porous matrix and a special crack tip element is developed correspondingly. A numerical integration scheme is elaborated for the singular integrals taking account of the special tip element shape functions. To regularize the coupled system in the bounded layer domain as the production zone, a datum condition is introduced. The numerical implementation is comprehensively validated through decoupled Darcy flow equation, decoupled channel flow equation and coupled equations, respectively. Secondly, a computational framework for modeling transient state well production by coupling transient state Darcy flow with channel flow is proposed. In this framework, the governing equation of transient state Darcy flow is formulated as a convolutional integral equation with SGBEM and the channel flow equation is cast into the same form as in the steady state analysis. The coupling of SGBEM-FEM renders a time marching scheme which involves a slowly converging series kernel at large but finite times for the bounded layer domain. A fast algorithm for evaluating the bounded layer kernel is proposed based on Ewald summation. An asymptotic analysis is conducted showing that the transient state flux field is of the same order as the steady state flux field and the same special tip element is employed in the transient analysis. The numerical implementation is validated with the solutions to both the decoupled transient Darcy flow equation and the coupled equations respectively. Lastly, we propose a computational framework to model the growth of intersecting and merging hydraulic fractures. We revisit the existed efficient SGBEM-FEM framework to simulate the growth of isolated height-contained hydraulic fractures. We formulate the efficient model in a variational form and augment it with Lagrange multipliers to prescribe physical constraints on intersections of hydraulic fractures. Pertinent numerical implementation techniques, including space and temporal discretizations, special tip element, crack propagation criteria and remeshing strategy, are presented. We validate the proposed framework with the analytical results of both star shaped cracks and symmetric branched cracks. The validation shows the necessity to properly enforce the physical constraints on the intersections of fractures.
Conductivity of proppant mixtures
(2014-05) Schulz, Eric Clinton; Mohanty, Kishore Kumar
Hydraulic fracturing is a physically complex phenomenon, and there are many variables, both environmental and operational, that affect the overall success of a fracture treatment. Amongst the operational variables, the process of proppant selection is key to ensuring that the induced fractures remain open and permeable. A variety of physical mechanisms act to degrade the permeability of a given proppant packing after deposition in a fracture, the most important of which is the magnitude of the confining stress. The goal of this work is to understand how mixtures of unlike proppants behave under various stress conditions. Specifically, the permeability and conductivity of various mixtures of unlike proppants are measured as a function of confining stress. A secondary investigation is also made into the dependence of permeability on the areal concentration of proppant. Choices of proppants are restricted to those which are currently most common in industry, in terms of both material and size. To that end, mixtures consisted of primarily ceramics and sands with appropriate grain size distributions. Additionally, a light-weight plastic proppant was included in the study. Simple laboratory methods are employed to measure the permeability of the various proppant packings. Values obtained from direct experimentation are compared with values obtained from an independent analytical model. Given the assumptions which are inherent in the analytical model, the experimental and analytical results are in satisfactory agreement. Also, a correlation is developed for single proppants and binary mixtures which predicts permeability as a function of stress, grain size, material, and weight fraction. One key conclusion is that for a binary mixture of proppants, the mixture permeability will not generally be a weighted linear combination of the pure proppant permeabilities. In other words, the permeability of a mixture comprised of 50% (by weight) of one component and 50% of the second component will generally not be halfway between the permeabilities of the single components. A hypothesis is presented which posits that there are threshold weight fractions for each proppant pair that control the permeability of the mixture.
Correlation of structural lineaments and fracture traces to water-well yields in the Edwards Aquifer, Central Texas
(1990) Alexander, Kenneth Bower, 1961-; Bennett, Philip C. (Philip Charles), 1959-
Lineaments are "straight lines visible from afar on the surface of the earth". In the Austin, Texas area, lineaments reflect the structural grain of the Balcones-Ouachita fault zone and may indicate subsurface geologic phenomena such as faults, fractures, and joints. These structural features often represent discrete zones of high permeability, and thus, areas of enhanced flow of groundwater capable of transmitting greater quantities of water than surrounding, non-fractured, rock. For this study more than 900 lineaments and fracture traces, identified in aerial photographs during a previous study, were detected in the Barton Springs section of the Edwards Aquifer. The endpoints of each linear feature were digitized and tagged with a unique identification label. Rose plots, Cartesian histograms, and a series of statistical operations were utilized to illustrate regional trends in the orientation of lineaments. As an indicator of well productivity, specific capacities of 27 wells in the area were obtained. Sixty-one water samples were collected and analyzed to test for possible chemical evidence of lineament-well interactions. The orientations of lineaments and fracture traces in the study area clearly display a bimodal distribution with a primary trend of N 40 E and a secondary peak of N 50 W. A general correlation exists between increased well productivity and decreased distances to the nearest lineament, particularly within 200 feet of lineaments. Also, 10 of the 13 largest specific-capacity values are from wells located southeast of southwest-northeast trending lineaments. Nonparametric statistical methods show that direction from lineaments is a significant factor in predicting water-well yields. Lineaments provide a tool for predicting possible sites of environmental sensitivity with respect to groundwater resources. Examples include the siting of groundwater monitoring wells for point sources of pollution, predicting the likely underground flow paths of a pollution plume or locating dam sites for recharge enhancement. Awareness of the location, orientation, and density of structural lineaments will allow the water-resource manager to identify discrete groundwater flow paths, and, thus, predict contaminant plume migration.
Coupled chemo-mechanical processes in reservoir geomechanics
(2018-06-15) Shovkun, Igor; Espinoza, David N.; Sharma, Mukul M; Foster, John T; Balhoff, Matthew T; Hesse, Marc E
Reservoir geomechanics investigates the implications of rock deformation, strain localization, and failure for completion and production of subsurface energy reservoirs. For example, effective hydraulic fracture placement and reservoir pressure management are among the most important applications for maximizing hydrocarbon production. The correct use of these applications requires understanding the interaction of fluid flow and rock deformations. In the past a considerable amount of effort has been made to describe the role of poroelastic and thermal effects in geomechanics. However, a number of chemical processes that commonly occur in reservoir engineering have been disregarded in reservoir geomechanics despite their significant effect on the mechanical behavior of rocks and, therefore, fluid flow. This dissertation focuses on the mechanical effects of two particular chemical processes: gas-desorption from organic-rich rocks and mineral dissolution in carbonate-rich formations. The methods employ a combination of laboratory studies, field data analysis, and numerical simulations at various length scales. The following conclusions are the results of this work: (1) the introduced numerical model for fluid flow with effects of gas sorption and shear-failure-impaired permeability captures the complex permeability evolution during gas production in coal reservoirs; the simulation results also indicate the presence non-negligible sorption stresses in shale reservoirs, (2) mineral dissolution of mineralized fractures, similar to pore pressure depletion or thermal cooling/heating can increase stress anisotropy, which can reactivate critically-oriented natural fractures; in-situ stress chemical manipulation can be used advantageously to enlarge the stimulated reservoir volume, (3) semicircular bending experiments on acidized rock samples show that non-planar fractures follow high porosity regions and large pores, and that fracture toughness correlates well with local porosity. Numerical modeling based on the Phase-Field approach shows that a direct relationship between fracture toughness and porosity permits replicating fracture stress intensity at initiation and non-planar fracture propagation patterns observed in experiments, and (4) numerical simulations based on a novel reactive fluid flow model coupled with geomechanics show that mineral dissolution (i) lower fracture breakdown pressure, (ii) can bridge a transition from a toughness-dominated regime to uncontrolled fracture propagation at constant injection pressures, and (iii) can increase fracture complexity by facilitating propagation of stalled fracture branches. The understanding of these chemo-mechanical coupled processes is critical for safe and effective injection of CO2 and reactive fluids in the subsurface, such as in hydraulic fracturing, deep geothermal energy, and carbon geological sequestration applications.
Coupled flow and geomechanics modeling for fractured poroelastic reservoirs
(2014-12) Singh, Gurpreet, 1984-; Wheeler, Mary F. (Mary Fanett)
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.
Coupled SGBEM-FEM for efficient simulation of height-contained hydraulic fractures
(2019-07-09) Mood, Charles Gordon; Mear, Mark E.; Demkowicz, Leszek F; Gonzalez, Oscar; Landis, Chad M; Rodin, Gregory J
An efficient computational model is developed to simulate the growth of vertically oriented, height-contained hydraulic fractures. A symmetric Galerkin boundary element method, used to model the behavior of the fracture, is specialized by exploiting knowledge of the fracture surface geometry and an assumption on the approximately elliptical, vertical cross section of the fracture. This geometric knowledge is used to reduce the governing weakly singular, weak-form traction integral equation from an integral over the fracture surface to an integral along the centerline of the fracture through an analytical integration with respect to the fracture height. The fluid flow within the fracture is treated using a Galerkin finite element method to model one-dimensional flow through an arbitrarily curved channel. Under the assumption that the fluid pressure is uniform over the fracture height (as in the case of a tunnel crack) and using the cross sectional form assumed by the fracture model, a specialized, weak-form, fluid flow equation is developed and integrated analytically with respect to the fracture height. The symmetric Galerkin boundary element method and Galerkin finite element method are coupled and the resulting system is solved using a Newton-Raphson method. The fracture propagation is governed by a mixed mode-I/II growth law based on linear elastic fracture mechanics, with stress intensity factors computed directly from the degrees of freedom associated with special crack tip elements designed to capture the square root behavior near the fracture tip. This new computational model is compared to an existing coupled SGBEM-FEM model designed for general, three-dimensional, non-planar fractures to illustrate the efficiency of the new model and the dramatic speedup it offers for modeling height-contained hydraulic fracturing scenarios. The model is extended to treat various conditions typical of hydraulic fracturing design including fracture growth near completed hydraulic fractures, staged fluid injection scenarios, fracture growth in multiple, vertically stacked pay zones, and the distribution of fluid injection to a set of fractures growing from a shared wellbore.
Coupling geomechanics with flow and tracer transport in complex fracture networks
(2020-12-01) Kumar, Ashish, Ph. D.; Sharma, Mukul M.; Mohanty, Kishore; Balhoff, Matthew; Okuno, Ryosuke; Bonnecaze, Roger
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
Developing coupled fluid flow and geomechanics simulators to model fracture deformation
(2019-10-10) Babazadeh, Mohsen; Olson, Jon E.; Schultz, Richard; Foster, John; Kallivokas, Loukas; Rathje, Ellen
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.