Naturally fractured reservoirs (NFRs) hold a significant amount of the world's hydrocarbon reserves. Compared to conventional reservoirs, NFRs exhibit a higher degree of heterogeneity and complexity created by fractures. The importance of fractures in production of oil and gas is not limited to naturally fractured reservoirs. The economic exploitation of unconventional reservoirs, which is increasingly a major source of short- and long-term energy in the United States, hinges in part on effective stimulation of low-permeability rock through multi-stage hydraulic fracturing of horizontal wells. Accurate modeling and simulation of fractured media is still challenging owing to permeability anisotropies and contrasts. Non-physical abstractions inherent in conventional dual porosity and dual permeability models make these methods inadequate for solving different fluid-flow problems in fractured reservoirs. Also, recent approaches for discrete fracture modeling may require large computational times and hence the oil industry has not widely used such approaches, even though they give more accurate representations of fractured reservoirs than dual continuum models. We developed an embedded discrete fracture model (EDFM) for an in-house fully-implicit compositional reservoir simulator. EDFM borrows the dual-medium concept from conventional dual continuum models and also incorporates the effect of each fracture explicitly. In contrast to dual continuum models, fractures have arbitrary orientations and can be oblique or vertical, honoring the complexity and heterogeneity of a typical fractured reservoir. EDFM employs a structured grid to remediate challenges associated with unstructured gridding required for other discrete fracture models. Also, the EDFM approach can be easily incorporated in existing finite difference reservoir simulators. The accuracy of the EDFM approach was confirmed by comparing the results with analytical solutions and fine-grid, explicit-fracture simulations. Comparison of our results using the EDFM approach with fine-grid simulations showed that accurate results can be achieved using moderate grid refinements. This was further verified in a mesh sensitivity study that the EDFM approach with moderate grid refinement can obtain a converged solution. Hence, EDFM offers a computationally-efficient approach for simulating fluid flow in NFRs. Furthermore, several case studies presented in this study demonstrate the applicability, robustness, and efficiency of the EDFM approach for modeling fluid flow in fractured porous media. Another advantage of EDFM is its extensibility for various applications by incorporating different physics in the model. In order to examine the effect of pressure-dependent fracture properties on production, we incorporated the dynamic behavior of fractures into EDFM by employing empirical fracture deformation models. Our simulations showed that fracture deformation, caused by effective stress changes, substantially affects pressure depletion and hydrocarbon recovery. Based on the examples presented in this study, implementation of fracture geomechanical effects in EDFM did not degrade the computational performance of EDFM. Many unconventional reservoirs comprise well-developed natural fracture networks with multiple orientations and complex hydraulic fracture patterns suggested by microseismic data. We developed a coupled dual continuum and discrete fracture model to efficiently simulate production from these reservoirs. Large-scale hydraulic fractures were modeled explicitly using the EDFM approach and numerous small-scale natural fractures were modeled using a dual continuum approach. The transport parameters for dual continuum modeling of numerous natural fractures were derived by upscaling the EDFM equations. Comparison of the results using the coupled model with that of using the EDFM approach to represent all natural and hydraulic fractures explicitly showed that reasonably accurate results can be obtained at much lower computational cost by using the coupled approach with moderate grid refinements. / text

Evolution of neural networks, or neuroevolution, has been a successful approach to many low-level control problems such as pole balancing, vehicle control, and collision warning. However, certain types of problems — such as those involving strategic decision-making — have remained difficult to solve. This dissertation proposes the hypothesis that such problems are difficult because they are fractured: The correct action varies discontinuously as the agent moves from state to state. To evaluate this hypothesis, a method for measuring fracture using the concept of function variation of optimal policies is proposed. This metric is used to evaluate a popular neuroevolution algorithm, NEAT, empirically on a set of fractured problems. The results show that (1) NEAT does not usually perform well on such problems, and (2) the reason is that NEAT does not usually generate local decision regions, which would be useful in constructing a fractured decision boundary. To address this issue, two neuroevolution algorithms that model local decision regions are proposed: RBF-NEAT, which biases structural search by adding basis-function nodes, and Cascade-NEAT, which constrains structural search by constructing cascaded topologies. These algorithms are compared to NEAT on a set of fractured problems, demonstrating that this approach can improve performance significantly. A meta-level algorithm, SNAP-NEAT, is then developed to combine the strengths of NEAT, RBF-NEAT, and Cascade-NEAT. An evaluation in a set of benchmark problems shows that it is possible to achieve good performance even when it is not known a priori whether a problem is fractured or not. A final empirical comparison of these methods demonstrates that they can scale up to real-world tasks like keepaway and half-field soccer. These results shed new light on why constructive neuroevolution algorithms have difficulty in certain domains and illustrate how bias and constraint can be used to improve performance. Thus, this dissertation shows how neuroevolution can be scaled up from learning low-level control to learning strategic decision-making problems. / text

The transport and fate of contaminants in fractured sedimentary rock aquifers depends strongly on the nature and distribution of the fracture network. The current standard practice of using only vertical coreholes to characterize bedrock aquifers can result in significantly biased data that is insufficient for fracture orientation analysis. This study involves the addition of two inclined coreholes to supplement existing data from eleven vertical coreholes at a contaminated site in Guelph, Ontario to reduce the effects of this bias. A suite of high-resolution, depth discrete data collection methods including core logging, borehole geophysics, and hydraulic testing were conducted to determine fracture orientation and spacing as well as hydraulic aperture distributions. The results of the orientation analysis demonstrate that the inclined coreholes were more effective at sampling high-angled fractures than the vertical coreholes and were necessary to identify all three of the dominant fracture sets on the site. The fracture network properties from this study can be used as input parameters for static and dynamic discrete fracture network models to assess current and future risks to municipal supply wells.

The objective of this thesis is to constrain the fundamental hydrogeological processes of a large crystalline fractured rock watershed in the Canadian Shield. The fundamental hydrogeological processes of groundwater recharge, flow and discharge are examined individually as well as holistically using a revised conceptual model. The study area is the topographically-subdued Tay River watershed in eastern Ontario where a thin veneer of soil overlies Precambrian crystalline rocks and Paleozoic sediments. Spatial scales from local-scale (100s m2 to 1 km2) to watershed-scale (>100 km2) are examined. Recharge processes are defined using hydrogeological characterization, numerical simulation and isotopic, thermal and hydraulic responses to a snowmelt event. Soil thickness and bedrock transmissivity are highly heterogeneous at the local scale. Cold, 2H depleted snowmelt locally recharged the bedrock aquifer to depths of at least 20 m within two days. This rapid recharge process is localized to areas where the soil is very thin whereas slow recharge is likely widespread. The impact of lineaments on groundwater flow at the watershed-scale is examined using geomatic analysis, hydrogeological characterization, numerical simulation and fracture mapping. Lineaments are interpreted as structural features because the two principal lineament sets are oriented parallel to fracture and fault orientations. The fractured bedrock underlying lineaments generally consists of poorly connected zones of reduced permeability suggesting lineament can be barriers to recharge and flow in this setting. Natural conservative, radioactive, and thermal tracers are integrated with streamflow measurements and a steady-state advective model to delimit the discharge locations and quantify the discharge flux to lakes, wetlands, creeks and the Tay River. The groundwater discharge rate to most surface water bodies is low. Groundwater discharge is distributed across the watershed rather than localized around lineaments or zones of exposed brittle fractures. In the revised conceptual model, recharge is considered two separate processes, groundwater flow is compartmentalized and the discharge flux is considerably lower than porous media watersheds. This thesis provides a better understanding of fundamental hydrogeological processes in a large crystalline fractured rock watershed which impacts the sustainability of water resources and ecology. / Thesis (Ph.D, Civil Engineering) -- Queen's University, 2009-07-13 14:51:25.244

Groundwater is an important resource that is relied on by approximately half of the world’s population for drinking water supply. Source water protection efforts rely on an understanding of flow and contaminant transport processes in aquifers. Bedrock aquifers are considered to be particularly vulnerable to contamination if the overburden cover is thin or inadequate. The objective of this study is to further the understanding of modeling, sampling, and the potential for anthropogenic contamination in fractured bedrock aquifers. Two numerical modeling studies were conducted to examine geochemical groundwater sampling using multi-level piezometers and the role of discretization in a discrete fracture radial transport scenario. Additionally, two field investigations were performed to study the variability of bacterial counts in pumped groundwater samples and the potential for anthropogenic contamination in a bedrock aquifer having variable overburden cover in a semi-urban setting. Results from the numerical modeling showed that choosing sand pack and screen materials similar in hydraulic conductivity to each other and the fractures intersecting the borehole can significantly reduce the required purge volume. Spatiotemporal discretization was found to be a crucial component of the numerical modeling of solute transport and verification of the solution domain using an analytical or semi-analytical solution is needed. Results from the field investigations showed fecal indicator bacterial concentrations typically decrease on the order of one to two orders of magnitude from the onset of pumping. A multi-sample approach that includes collection at early-time during the purging is recommended when sampling fecal indicator bacteria for the purpose of assessing drinking water quality. Surface contaminants in areas with thin or inadequate overburden cover can migrate quickly and deeply into the bedrock aquifer via complex fracture networks that act as preferential pathways. While the presence of fecal indicator bacteria in groundwater samples signifies a possible health risk through human consumption, it was the suite of pharmaceuticals and personal care products that allowed the identification of septic systems and agriculture as the dominant sources of contamination. Land-use planning and source water protection initiatives need to recognize the sensitivity of fractured bedrock aquifers to contamination. / Thesis (Ph.D, Civil Engineering) -- Queen's University, 2011-02-28 17:27:54.806

An analytical model is presented for the analysis of pulse interference tests conducted in a fractured porous medium with connection to a free surface boundary at the water table. The solution is applicable to open borehole pulse interference tests due to the accommodation of multiple horizontal fractures intersecting each wellbore and a connection from the uppermost horizontal fracture to a free surface boundary. The solution is derived using the Laplace transform method and evaluation of the solution is performed by numerical inversion using the Talbot algorithm. Based on an informal sensitivity analysis, unique values for transmissivity, storativity, specific storage, vertical hydraulic conductivity and specific yield are predicted over a range of realistic values for these parameters. The analytical model is used to analyze slug tests and pulse interference tests conducted in a fractured gneiss formation. The results of these tests are compared to long-term pumping tests to identify the effect of measurement scale on transmissivity, storativity, vertical hydraulic conductivity and specific yield obtained in a fractured rock setting. Scale artefacts relating to measurement or analysis methods are minimized through the use of consistent well configurations in each of the applied hydraulic testing methods. The geometric mean estimates of transmissivity and storativity are shown to vary by less than an order of magnitude from local-scale tests to long-term pumping tests. The geometric mean specific yield result from a series of pulse interference tests that samples both highly fractured and poorly fractured portions of the rock formation approximates the long-term pumping test estimate of specific yield. The geometric mean result for vertical hydraulic conductivity decreases by approximately 1.5 orders of magnitude from the slug test to pumping test scale; however, pulse interference tests conducted on highly fractured portions of the formation produce vertical hydraulic conductivity estimates that are within a half order of magnitude of the long-term pumping test results. This suggests that the performance of pulse interference tests on a highly fractured portion of a rock formation may be a less time-intensive alternative to large-scale pumping tests in the determination of vertical hydraulic conductivity. / Thesis (Master, Civil Engineering) -- Queen's University, 2011-06-24 19:29:52.743

No description available.

<p>The aim of this study was to test whether one could use the same methods as used for sampling heavy metals and apply them to radioactive elements. Furthermore a sampling protocol was developed, the first of its kind, for the sampling of radioactive elements in fractured rock aquifers. This was achieved by initially examining local as well as international manuals and methods. The aforementioned was done in conjunction with a literature review of the movement of radioactive elements in these fractured rock aquifers. Beaufort West was utilised as a study area and the geology, hydrogeology and topography was outlined. Background radioactivity was generally acceptable except for two samples which were anomalously high. Taking cognisance of the methods used, as well as those previously applied in the area and abroad, a sampling protocol for radioactive elements in fractured rock aquifers was developed and attached as an appendix. In conclusion it was suggested that multiple methods be tested on one well in order to check whether similar results would occur. This would thus determine the best applicable methods. Also it was proposed that a new method, called DGT sampling, be applied in order to gain a time weighted average of the heavy metals and radioactive elements in groundwater. It could also be clearly seen, by comparing historical data and the current data, that the methods used for sampling heavy metal can be applied to radioactivity.</p>

Large amounts of hydrocarbon reserves are trapped in fractured reservoirs where fluid flux is far more rapid along fractures than through the porous matrix, even though the volume of the pore space may be a hundred times greater than the volume of the fractures. These are considered extremely challenging in terms of accurate recovery prediction because of their complexity and heterogeneity. Conventional reservoir simulators are generally not suited to naturally fractured reservoirs’ production history simulation, especially when production processes are associated with large pressure and temperature changes that lead to large redistribution of effective stresses, causing natural fracture aperture alterations. In this case, all the effective processes, i.e. hydraulic, thermal and geomechanical, should be considered simultaneously to explain and evaluate the behavior of stress-sensitive reservoirs over the production period. This is called thermo-hydro-mechanical (THM) coupling. In this study, a fully coupled thermo-hydro-mechanical approach is developed to simulate the physical behavior of fractures in a plane strain thermo-poroelastic medium. A hybrid numerical method, which implements both the finite difference method (FDM) and the displacement discontinuity method (DDM), is established to study the pressure, temperature, deformation and stress variations of fractures and surrounding rocks during production processes. This method is straightforward and can be implemented in conventional reservoir simulators to update fracture conductivity as it uses the same grid block as the reservoir grids and requires only discretization of fractures. The hybrid model is then verified with couple of analytical solutions for the fracture aperture variation under different conditions. This model is implemented for some examples to present the behavior of fracture network as well as its surrounding rock under thermal injection and production. The results of this work clearly show the importance of rate, aspect ratio (i.e. geometry) and the coupling effects among fracture flow rate and aperture changes arising from coupled stress, pressure and temperature changes. The outcomes of this approach can be used to study the behavior of hydraulic injection for induced fracturing and promoting of shearing such as hydraulic fracturing of shale gas or shale oil reservoirs as well as massive waste disposal in the porous carbonate rocks. Furthermore, implementation of this technique should be able to lead to a better understanding of induced seismicity in injection projects of all kinds, whether it is for waste water disposal, or for the extraction of geothermal energy.

Isotopic methods have been developed over the last 10 years as a method for determining chemical interactions of chlorinated solvents. These methods are especially promising for. This study attempts to employ and develop compound specific isotopic analyses of TCE and cDCE, along with chemical data, to characterize the degradation of TCE in a fractured bedrock aquifers. The Smithville site is a contaminated field site with extremely high levels of TCE contamination that is currently undergoing monitored remediation. From December 2008 until April 2010 extended samples were collected from the site to provide additional data analyses including isotopic data. The redox conditions at the site are anoxic to reducing, with sulfate reduction and methanogenesis as dominant terminal electron accepting processes. Redox data indicates that well electrochemical conditions are highly variable within the site, including areas near the source zone that not very reducing. Documented changes in groundwater conditions to much more reducing environments indicate that oxidation of organic matter is occurring at the Smithville site in select wells. Chemical analyses of TCE, DCE, VC, ethene and ethane are employed determine whether reductive dechlorination was occurring at the site. Results of field testing indicate that many wells on site, especially in the proximity of the source zone, dechlorination products were found. The isotopic data had a high range in both carbon and chlorine isotopes. Chlorine isotopic data ranges from a δ37Cl(TCE) of 1.39 to 4.69, a δ37Cl(cDCE) of 3.57 to 13.86, a δ13C(TCE) of -28.9 to -20.7, and a δ13C(cDCE) of -26.5 to -11.82. The range in values indicate varying degrees of degradation throughout the site, with the same wells grouping together. Combined chemical, redox and isotopic data shows that degradation seems to be a removal process for TCE at the Smithville site. Concentrations of chemicals created as a result of TCE degradation verify degradation, especially in wells 15S9, R7 and 17S9. Historically production of DCE in significant amounts, above 1.0 ppb, was observed to only occur after 2003. In addition to this, DCE data shows that the percentage of DCE made up of cDCE is above 96%. This indicates that microbes most likely mediate the processes that formed DCE from TCE. The linear regression of the delta-delta plot for isotopic TCE data shows line that is likely a direct function of the carbon and chlorine isotopic fractionation imparted upon the original TCE released. The slope found is consistent with data collected from other studies though cannot be applied to determining the process directly given the range of variability in isotopic field data.