Fractures exert a strong influence on fluid flow in subsurface reservoirs, and hence an adequate understanding of fracture properties could provide useful information on how they may be managed optimally to produce oil and gas or to be used as repositories for carbon dioxide (CO2) to mitigate climate change. Since fractures are commonly aligned by the stress field, seismic anisotropy is a key tool in investigating their properties. Velocity anisotropy is now a well-established technique for determining properties such as fracture orientation and density, but in recent years, attention has focused on quantifying azimuthal variations in Pwave attenuation to provide additional information, especially on the fracture size. However, the practical application of this attribute in geophysical exploration is still limited due to the uncertainty associated with its measurement and the difficulty in its interpretation in terms of rock properties. There is still a lack of proper understanding of the physical processes involved in the mechanisms of attenuation anisotropy. In this thesis, I use the seismic modelling approach to study the effects of attenuation anisotropy in fractured porous media using P-waves with the main aim of improving the understanding of these effects and exploring the physical basis of using attenuation anisotropy as a potential tool for the characterization of fractured reservoirs. Fractures with length on the order of the seismic wavelength in reservoir rocks cause scattering of seismic waves which exhibits characteristic azimuthal variations. I study these scattering effects using complementary seismic physical (scale-model laboratory experiments) and numerical (finite difference) modelling approaches. The results of both approaches are consistent in delineating fracture properties from seismic data. The scattered energy is quantified through estimates of the attenuation factor (the inverse of the seismic quality factor Q) and shown to be anisotropic, with elliptical (cos2θ) variations with respect to the survey azimuth angle θ. The minor axis of the Q ellipse corresponds to the fracture normal. In this direction, i.e. across the material grain, the attenuation is a maximum. The major axis corresponds to the fracture strike direction (parallel to the material grain) where minimum attenuation occurs. Empirically, the magnitude of P-wave attenuation anisotropy is greater in fluid-saturated rocks than in dry rocks. I study the influence of fluid saturation on P-wave attenuation through synthetic modelling and compare the attenuation signature to that of dry fractured rocks. The results of the analysis show that the relaxation time strongly controls the frequency range over which attenuation occurs. The magnitude of the induced attenuation increases with polar angle and also away from the fracture strike direction. The attenuation exhibits elliptical variations with azimuth which are also well fitted with a cos2θ function. The magnitude of the attenuation anisotropy is higher in the case of the fluid-saturated rocks. All of these properties of the numerical model are in agreement with the results of empirical experiments in the laboratory. The same crack density can result from many small cracks, from a few large cracks, or from an equal number of cracks of various sizes with varying thicknesses in the same volume of background material. This makes it difficult to distinguish between the anisotropy caused by micro-cracks and that caused by macro-cracks. I study the effects of fracture thickness or aperture on P-wave scattering attenuation through seismic physical modelling, and find that the induced attenuation has a direct relationship with the fracture thickness or aperture. This result indicates the potential of using P-wave attenuation to get information which might be useful in examining the effects of voids in the rocks, and also provides a basis for further future theoretical development to distinguish the effects caused by thin micro cracks and large open fractures. Finally, I study the effects of two types of fluid saturation (brine and CO2 in the supercritical state) on P-wave attenuation through synthetic modelling, with particular attention to varying CO2 saturation using the CO2 properties at the Sleipner gas Field in the North Sea. The presence of CO2 causes more attenuation in the numerical model output than when the rock is saturated with only brine. The induced attenuation increases with decreasing percentage of CO2 saturation and has a maximum magnitude at 10 % CO2 saturation. Further work is needed to quantify the additional effect of fractures on these results.

In fractured geologic media, flow and contaminant transport are predominantly controlled by the fractures, their distribution and connectivity. The accurate characterization of fractured geologic medium, imaging of fracture patterns and their connectivity have been a challenge for decades. Given the complexities of fractured networks in the subsurface and Dense Non Aqueous Phase Liquid (DNAPL) contamination, in this thesis, transient hydraulic tomography (THT), a recently developed tool for characterizing aquifer heterogeneity is evaluated under laboratory conditions to delineate discrete fractures. Laboratory experiments and modeling studies are also conducted to understand TCE plume behavior. A dolomite rock sample, which is 91.5 cm in length, 60.5 cm in height and 5 cm thick, was fractured in the laboratory to perform the experiments. After the fractured block was enclosed in a flow cell, flow-through and pumping tests were conducted to characterize the fractured rock block. The data from the pumping tests were then analyzed using the SSLE code developed by Zhu and Yeh [2005] and transient hydraulic tomography (THT) was conducted to image the fracture pattern and their connectivity through the delineation of K and Ss distributions (the tomograms). Synthetic pumping tests, identical in configuration to the laboratory ones were also conducted using HydroGeoSphere (HGS) [Therrien et al, 2009] in a synthetic replica of the fractured block to compare the observed and simulated drawdowns. Then synthetic THT analysis was performed utilizing the synthetic pumping test data to compare the tomograms obtained from the THT analysis of synthetic and laboratory pumping tests. Results suggest that the THT analysis of multiple laboratory pumping tests captured the fracture pattern and their connectivity quite well and they became more vivid with the additional pumping tests. The estimated high hydraulic conductivity (K) and low specific storage (Ss) zones clearly show the fractures and their connectivity. The pattern of K and Ss tomograms obtained from the analyses of synthetic and laboratory pumping tests were similar. Estimated K and Ss values for the fractures and the matrix may not exactly replicate the actual K and Ss values for the fractured rock, but the model also provides uncertainty estimates associated with the resulting K and Ss tomograms. In this study, two cases of transient hydraulic tomography (THT) analysis of the laboratory pumping tests were performed by changing the location of 2nd and 3rd pumping tests among the three to examine if there is any significant impact of these pumped location on the pattern of resulting hydraulic conductivity (K) and specific storage (Ss). The initial pumping test was the same for two cases. Results show that the patterns of estimated K and Ss tomograms obtained from these two cases are similar, although the pumped locations (2nd and 3rd tests among the three) utilized for the inversion were different for two cases suggesting that the location of these later pumping tests does not significantly impact the estimates for this fractured rock block. However, the initial test should be selected carefully as that seems to set the pattern of the tomograms. The estimated K and Ss tomograms were validated by predicting five independent pumping tests conducted in the fractured rock block. These five pumping tests were not included during the construction of the K and Ss tomograms. For most of the independent pumping tests, good correspondence between the simulated and observed drawdown was achieved. The study indicates that, it is possible to delineate discrete fractures, their pattern and connectivity by carefully applying of THT analysis of multiple pumping tests based on the inverse code SSLE [Zhu and Yeh, 2005]. In addition, hydraulic tomography seems to be a cost effective tool for characterizing fractured rock since it does not require the detailed information on fracture geometry parameters such as aperture, trace length, orientation, spatial distribution, and connectivity, which are difficult to quantify. These parameters are usually unavailable between boreholes. Therefore, THT appears to be a promising approach in delineating fractures and their connectivity in subsurface. However, it is still at the early stage as the study was conducted in the laboratory under controlled conditions. Small scale field experiments need to be conducted to validate THT as a tool for the characterization of hydraulic parameters of fractured rocks. Upon completion of the hydraulic characterization, several conservative tracer tests were conducted using bromide (Br-) as a conservative tracer to aid in the design of TCE dissolution experiment. Once the tracer experiments were completed, a known volume of pure phase TCE was injected at a known location in the flow cell to create a well-defined source zone. A constant hydraulic gradient was maintained by fixing the hydraulic heads at the two head tanks to induce steady groundwater flow through the flow cell. Water samples were obtained at a down gradient monitoring port for 3 months to obtain a long-term breakthrough curve of TCE in the aqueous phase. The purpose of this experiment was to study TCE dissolution behaviour in the fractured rock sample. Then HydroGeoSphere (HGS) was used to model the aqueous phase TCE transport using two separate approaches: 1) the Discrete Fracture Network modeling approach and 2) the stochastic continuum approach, to investigate whether they can capture the dissolution behavior. Both approaches were able to capture the pattern of the breakthrough curve in the fractured rock. The discrete fracture approach captured the observed TCE plume and the dissolution behavior quite well. On the other hand, the stochastic continuum approach, in which the fractured rock block was treated as porous medium having a heterogeneous K field obtained from THT analysis, also appeared to be promising in capturing the aqueous phase transport of TCE. Despite some early time deviation, the simulated breakthrough curve captured the overall observed concentration profile. However, the stochastic continuum approach seems to be more cost effective as it does not require detailed information about fracture aperture and their spatial distribution which are difficult if not impossible to obtain between boreholes. Note that, the studies were conducted based on a laboratory experiment conducted in a controlled environment. The experimental block was well characterized and the geometry of the experimental block as well as the flow through the system was well understood from the hydraulic and tracer experiments. Thus small scale field experiment is required to support this conclusion.

Much of southern Ontario relies on groundwater in fractured rock aquifers as a municipal drinking water source, thus the vulnerability of these sources is of importance from public health, economic, and environmental perspectives. Aquifer vulnerability assessments serve as visual communication tools useful in efficiently allocating resources for the establishment of new drinking water sources, hydrogeological characterization, and source water protection planning decisions. Examples of current vulnerability assessments include: DRASTIC, GOD, EPIK, AVI, COP and ISI. These vulnerability assessment methods either fail to quantitatively incorporate characteristics of fractured rock and preferential pathways, or they account for only heavily karstified areas; none are suited to the fractured rock formations in Ontario. The goal of this work is to incorporate fractured rock characteristics in a new aquifer vulnerability assessment method using readily attainable quantitative data to produce an inexpensive and straightforward regional aquifer vulnerability map highlighting hydrogeological areas that are more fundamentally prone to contamination than others. This proposed method is applied to the Acton-Georgetown study area in southern Ontario, along with the AVI and DRASTIC methods for comparison. The AVI and DRASTIC vulnerability assessments yield very different results from each other, and the proposed method demonstrates the heavy influence that fractured rock has on the vulnerability of the study area. The heterogeneity of variables used in some of the methods created difficulty in the interpolation of point data, rendering the use of generalized spatial data more valuable. These results and the corresponding limitations and recommendations for future improvements are discussed in light of these conclusions. / Thesis / Master of Applied Science (MASc)

Many hydraulically fractured shale gas horizontal wells in the Barnett shale have been observed to exhibit transient linear behavior. This transient linear behavior is characterized by a one-half slope on a log-log plot of rate against time. This transient linear flow regime is believed to be caused by transient drainage of low permeability matrix blocks into adjoining fractures. This transient flow regime is the only flow regime available for analysis in many wells. The hydraulically fractured shale gas reservoir system was described in this work by a linear dual porosity model. This consisted of a bounded rectangular reservoir with slab matrix blocks draining into adjoining fractures and subsequently to a horizontal well in the centre. The horizontal well fully penetrates the rectangular reservoir. Convergence skin is incorporated into the linear model to account for the presence of the horizontal wellbore. Five flow regions were identified with this model. Region 1 is due to transient flow only in the fractures. Region 2 is bilinear flow and occurs when the matrix drainage begins simultaneously with the transient flow in the fractures. Region 3 is the response for a homogeneous reservoir. Region 4 is dominated by transient matrix drainage and is the transient flow regime of interest. Region 5 is the boundary dominated transient response. New working equations were developed and presented for analysis of Regions 1 to 4. No equation was presented for Region 5 as it requires a combination of material balance and productivity index equations beyond the scope of this work. It is concluded that the transient linear region observed in field data occurs in Region 4 – drainage of the matrix. A procedure is presented for analysis. The only parameter that can be determined with available data is the matrix drainage area, Acm. It was also demonstrated in this work that the effect of skin under constant rate and constant bottomhole pressure conditions is not similar for a linear reservoir. The constant rate case is the usual parallel lines with an offset but the constant bottomhole pressure shows a gradual diminishing effect of skin. A new analytical equation was presented to describe the constant bottomhole pressure effect of skin in a linear reservoir. It was also demonstrated that different shape factor formulations (Warren and Root, Zimmerman and Kazemi) result in similar Region 4 transient linear response provided that the appropriate f(s) modifications consistent with lAc calculations are conducted. It was also demonstrated that different matrix geometry exhibit the same Region 4 transient linear response when the area-volume ratios are similar.

Opening-mode fractures—joints and veins—are widespread structures in sedimentary rocks even in slightly deformed and flat-lying sequences. Understanding the growth and connectivity of fractures in low permeability sandstone reservoirs is essential for optimal hydrocarbon exploitation. In a linear elastic fracture mechanics framework, it is generally assumed that fractures widen in aperture while they propagate in length or height. However, it is also conceivable that a phase of proportional aperture to length or height growth is followed by a phase of aperture growth with relatively slow or arrested tip propagation. Slow propagation relative to aperture opening can occur by non-elastic deformation processes or if the material elastic properties change over time. Fracture propagation in length or height can be halted by material strength heterogeneities. To test for concurrent length versus aperture growth of these fractures, I reconstructed the crack-seal opening history for multiple cement bridges sampled at different distances from the tip of three opening-mode fractures in Travis Peak Sandstone of the SFOT-1 well, East Texas. Crack-seal cement bridges have been interpreted to form by repeated incremental fracture opening and subsequent precipitation of quartz cement that bridges the fractures. Crack-seal cement textures were imaged using a scanning electron microscope with a cathodoluminescence detector, and the number and thickness of crack-seal cement increments determined. Trends in crack-seal increments number and thickness are consistent with fast initial fracture propagation relative to aperture growth, followed by a stage of slow propagation and pronounced aperture growth. Cumulative fracture opening displacement based on palinspastic reconstruction of two cement bridges was compared to analytical solutions for a stationary and a propagating fracture aperture as a function of position relative to the fracture tip in an elastic medium. Based on this comparison, I conclude that the crack-seal cement record reflects largely the phase of dominant aperture growth and subcritical fracture propagation under constant loading stress. / text

A thermal conductive heating (TCH) pilot test was conducted at the Naval Air Warfare Center (NAWC) in West Trenton, New Jersey in 2009 in collaboration with TerraTherm, Inc., the Naval Facilities Engineering Services Center and the United States Geological Survey. The NAWC site was historically used as a jet engine testing facility from the mid-1950s to the late 1990s. During this time, the subsurface was contaminated with trichloroethylene (TCE) which was a common solvent used at the facility. The pilot test consisted of 15 heater/extraction wells installed to a depth of 16.8 m in weathered mudstone and operated for 102 days. Rock core samples were taken pre- and post-remediation to measure the initial TCE concentrations and evaluate the effect the TCH pilot test had. The data collected during the pilot test was used to create a two-dimensional (2D) finite difference model using TMVOC. TMVOC is part of the TOUGH 2 family of codes and is a numerical model that is capable of simulating multiphase flow, heat transfer and transport of volatile organic compounds in three-dimensional heterogenous porous media or fractured rock. The 2D model was used as a screening model to investigate TCE removal from the rock matrix when heating for 100 days with a similar heating pattern to what was employed at the NAWC site. The numerical domain incorporated three primary fractures with competent bedrock in between. As the test pilot was conducted in the weathered bedrock zone, a sensitivity analysis was first completed on the matrix permeability to help to match the TCE removal from the pilot test. The pilot test had a 63.5% removal of TCE from the study area compared to 67% from the baseline model. A limited sensitivity analysis was completed which investigated how the matrix porosity and rate of energy application would have on the success of TCE removal from the rock matrix. It revealed that the TCE removal increases with increased matrix porosity and increased rate of energy application. / Thesis (Master, Civil Engineering) -- Queen's University, 2013-01-02 22:32:05.274

Compressional and shear wave velocity and attenuation measurements were obtained in the laboratory from 50 mm diameter, cylindrical, limestone core samples over a confining pressure range of 5 – 60 MPa. Normal and tangential fracture compliance values, as a function of confining pressure, were calculated for a single fracture cut perpendicular to the long axis of the core. The ratio of the normal to tangential compliance was approximately 0.4 and was independent of the applied stress. Values of normal and tangential fracture compliance calculated were of the order 10^-14 m/Pa, and decreased with an increase in confining pressure. Both Q^-1</sup>_P and Q^-1</sup>_S1/Qs were shown to be small for these samples. A borehole test site was constructed in a Carboniferous limestone quarry, at Tytherington, situated north of Bristol, UK. This quarry was chosen because the rock type was fairly homogeneous and the fractures could be mapped in the quarry walls as well as down three, 40 m vertical boreholes drilled in-line in the quarry floor. Wireline logs were obtained in all the holes and a seismic crosshole survey was carried out between the two outermost boreholes. An estimate of in-situ normal fracture compliance, Z_N, was obtained from the log and crosshole data, in 4 different ways, using effective medium theories as well as the displacement discontinuity theory. An additional estimate of Z_N was obtained from a separate borehole test site constructed in fractured Devonian meta-sediments at Reskajeage, Cornwall, UK. These fractures were much larger in size than those observed at Tytherington quarry. From the above field and laboratory measurements, fracture compliance was shown to increase approximately linearly with the size of the fractures. In addition, a study of crosshole seismic attenuation was performed at Tytherington quarry. Q was found to be frequency dependent. This frequency dependence was interpreted as being due to scattering rather than intrinsic attenuation.

[pt] O presente trabalho apresenta aplicações das técnicas de Relaxação Dinâmica e Análise Limite ao estudo da estabilidade de maciços rochosos fraturados. O maciço é modelado como um meio descontínuo formado por blocos rígidos com deformação concentrada nas juntas. A técnica de Relaxação Dinâmica é usada para a solução do problema de equilíbrio resultante, através do programa BLOCO. As expressões desenvolvidas para a matriz de rigidez tangente, usando o modelo de Barton & Bandis, foram implementadas no programa BLOCO. Exemplos para a validação do algoritmo são apresentados. A partir do trabalho de Faria (1992), foi implementado um procedimento automatizado e otimizado para a solução do problema de Análise Limite em um meio formado por blocos rígidos. O procedimento desenvolvido permitiu a solução de problemas de porte relatados na literatura. / [en] This work presents some applications of the Dynamic Relaxation and Limit Analysis techniques, to the study of the stability of fractured rock masses. Rock mass is modeled as a discontinuum formed by rigid blocks with deformable joints. Dynamic Relaxation was applied to solve the resulting equilibrium problem, using the program BLOCO. Expressions obtained for tangent stiffness matrix, derived from Barton & Bandis model, were implemented into the BLOCO program. In order to extend Faria`s (1992) work, an automatic and optimized procedure, to solve the Limit Analysis problem of a media formed by rigid blocks was implemented. The developed procedure was applied to the study of relatively large dimensions problems, reported in the literature. / [es] Este trabajo presenta aplicaciones de las técnicas de Relajación Dinámica y Análisis Límite al estudio de la estabilidad de macizos rocosos fracturados. EL macizo es modelado como un medio discontinuo formado por bloques rígidos con deformación concentrada en las juntas. La técnica de Relajación Dinámica se utiliza para resolver el problema de equilíbrio resultante, a través del programa BLOQUE. Las expresiones desarrolladas para la matriz de rígidez tangente, usando el modelo de Barton & Bandis, se implementaron en el programa BLOQUE. Se presentan algunos ejemplos para la evaluación del algoritmo. A partir del trabajo de Faria (1992), fue implementado un procedimiento automatizado y optimizado para la solución del problema de Análisis Límite en un medio formado por bloques rígidos. El procedimiento desarrollado permitió resolver problemas de porte relatados en la literatura.

Diversos métodos geofísicos têm sido empregados para a identificação de zonas fraturadas em rochas cristalinas a fim de locar pontos de perfuração de poços tubulares. Os métodos geoelétricos têm tido um papel importante na identificação destas zonas fraturadas, no entanto, em áreas urbanas sofrem com a falta de espaço e interferências diversas, o que limita suas aplicações. O presente trabalho visou correlacionar as anomalias geoelétricas típicas de zonas fraturadas em rochas cristalinas, com medidas de emanação natural de radônio a fim de estabelecer parâmetros que possam ser aplicados em áreas onde os fatores externos impeçam a execução dos métodos geoelétricos, uma vez que os fatores que interferem na aplicação desses métodos não interferem na emanação do radônio. Os resultados encontrados nas áreas investigadas mostraram uma boa correlação entre as anomalias geoelétricas e as anomalias de emanação de radônio, bem como foram boas as correlações com os resultados de vazão dos poços existentes nas áreas investigadas. A metodologia aqui proposta alcançou os objetivos podendo ser uma excelente ferramenta para prospecção de água subterrânea, tanto como método auxiliar, quanto como ferramenta principal de prospecção de aqüíferos em áreas onde outros métodos não possam ser aplicados devido a interferências diversas tais como: tubulações metálicas, fiações elétricas, entre outras. Estes tipos de interferências são bastante comuns em áreas urbanas, justamente onde a perfuração de poços é mais requerida. / A several geophysical methods have been used for the identification of shear zones in crystalline rocks in order to mark points to water well drilling. The geoelectrical methods have had an important role on the identification of these shear zones. However, in urban areas they suffer with the lack from space and diverse interferences, what it limits the application of these ones. The present research aimed to correlate the typical electrical anomalies of shear zones in crystalline rocks, with measures of natural emanation of radon, in order to establish parameters that can be applied in areas where the external factors hinder the execution of the geoelectrical methods, because the factors that intervene with the application of these methods do not intervene with the emanation of the radon. The results found in the investigated areas had shown a good correlation with the electrical anomalies and the radon emanation anomalies, as well as had been good the correlations with the results of yield of water wells on the investigated areas. The methodology proposal reached the objectives being able to be an excellent tool for groundwater prospection, as a auxiliary method auxiliary, as a main tool of prospection of fractured aquifers in areas where other methods cannot be applied due several interferences, as metallic pipes, electric wirings, and others. These types of interferences are sufficiently common in urban areas, exactly where the water well drilling its more necessary.

#DEFAULT