The further exploration and development of Pabst Gas Field with faulted sand reservoirs require an understanding of the properties and roles of faults, particularly Low Throw near Vertical Faults (LTNVFs), in gas migration and accumulation at a reservoir scale. This study presents numerical modeling of gas migration and accumulation processes in Pabst Field. Based on studies of the reservoirs, structure, faults, and fluid properties of the field, reservoir scale modeling was performed to determine the gas supply style and the fault properties by means of hundreds of iterations in which the fault properties and gas supply pattern were modified to match the gas distribution obtained from modeling with the gas distribution inferred from seismic data constrained by well data and production data. This study finds that in the main three sand reservoirs of Pabst Field the overlying younger sands cut down into the underlying older sands, so that partial connections between the three sands allow gas communication among the sands. Meanwhile, three fault families break up the three sands into numerous compartments. A primary fault and large synthetic and antithetic faults act as gas migration pathways: the synthetic and antithetic faults are inlets for gas flow and the primary fault is an outlet, and LTNVFs act as barriers to gas flow. Modeling requires fault properties in the field to change while the field is formed. The porosity and permeability of the faults in Pabst Field are 10% and 0.1 md, respectively, during gas charging of the sand reservoirs. But when there is no gas charging and large gas columns are maintained, the porosity and permeability of the faults decrease to 6% and 0.001 md, respectively. Pabst Field probably has an impulse gas charge history. Fault opening and closing, gas charge and recharge, and replacement of gas by formation water may occur. A combination of stratigraphy, structure, overpressure and gas charge rate control gas migration style, gas charge history, and gas distribution in the field. The significance of the study is that this improved numerical approach for modeling gas migration into and through specifically faulted sand reservoirs fills the gap between basin modeling and production modeling.

The further exploration and development of Pabst Gas Field with faulted sand reservoirs require an understanding of the properties and roles of faults, particularly Low Throw near Vertical Faults (LTNVFs), in gas migration and accumulation at a reservoir scale. This study presents numerical modeling of gas migration and accumulation processes in Pabst Field. Based on studies of the reservoirs, structure, faults, and fluid properties of the field, reservoir scale modeling was performed to determine the gas supply style and the fault properties by means of hundreds of iterations in which the fault properties and gas supply pattern were modified to match the gas distribution obtained from modeling with the gas distribution inferred from seismic data constrained by well data and production data. This study finds that in the main three sand reservoirs of Pabst Field the overlying younger sands cut down into the underlying older sands, so that partial connections between the three sands allow gas communication among the sands. Meanwhile, three fault families break up the three sands into numerous compartments. A primary fault and large synthetic and antithetic faults act as gas migration pathways: the synthetic and antithetic faults are inlets for gas flow and the primary fault is an outlet, and LTNVFs act as barriers to gas flow. Modeling requires fault properties in the field to change while the field is formed. The porosity and permeability of the faults in Pabst Field are 10% and 0.1 md, respectively, during gas charging of the sand reservoirs. But when there is no gas charging and large gas columns are maintained, the porosity and permeability of the faults decrease to 6% and 0.001 md, respectively. Pabst Field probably has an impulse gas charge history. Fault opening and closing, gas charge and recharge, and replacement of gas by formation water may occur. A combination of stratigraphy, structure, overpressure and gas charge rate control gas migration style, gas charge history, and gas distribution in the field. The significance of the study is that this improved numerical approach for modeling gas migration into and through specifically faulted sand reservoirs fills the gap between basin modeling and production modeling.

Geological disposal is the preferred option for the long term management of British intermediate level radioactive waste. The disposal site is currently being identified, with possible geological environments including fractured crystalline rocks and low permeability rocks such as clay. The selection of the host rock will have an impact on the design of the waste repository. This thesis investigates the ways the behaviour of repository borne gas can be affected by the repository design and the selection of the host rock. Commercially available TOUGH2 package is used to model the resaturation of the disposal facility, along with gas migration out of the repository and towards the ground surface in a generic geology. A facility located in fractured rock is estimated to resaturate within 6.5 years of its closure. The resaturation time is found to be strongly dependent on the presence and properties of a low permeability liner around the disposal vaults. The inflowing water starts gas generation processes within the repository; gas initially accumulates within the facility, but it is estimated to find its way into the host rock approximately 450 years after the facility has been closed. A maximum outflow rate is reached after approximately 1,000 years. The flow of gas migrating through the host rock is strongly affected by site-specific features. In the case of a uniform crystalline rock, gas is found to break through at the surface after 29,000 years. For a disposal site with a very slow groundwater flow rate, the resaturation phase may take several decades and gas outflow will occur much later. It is estimated that, in very low permeability environments, gas breakthrough may not occur before 100,000 years.

Hydrate Ridge is a peanut shape bathymetric high located about 80 km west of Newport, Oregon on the Pacific continental margin, within the Cascadia subduction zone’s accretionary wedge. The ridge's two topographic highs (S. and N. Hydrate Ridge) are characterized by gas vents and seeps that were observed with previous ODP initiatives. In 2008, we acquired a 3D seismic reflection data set using the P-Cable acquisition system to characterize the subsurface fluid migration pathways that feed the seafloor vent at S. Hydrate Ridge. The new high-resolution data reveal a complex 3D structure of localized faulting within the gas hydrate stability zone (GHSZ). We interpret two groups of fault-related migration pathways. The first group is defined by regularly- and widely-spaced (100-150 m) faults that extend greater than 300ms TWT (~ 250 m) below seafloor and coincide with the regional thrust fault orientations of the Oregon margin. The deep extent of these faults makes them potential conduits for deeply sourced methane and may include thermogenic methane, which was found with shallow drilling during ODP Leg 204. As a fluid pathway these faults may complement the previously identified sand-rich, gas-filled stratigraphic horizon, Horizon A, which is a major gas migration pathway to the summit of S. Hydrate Ridge. The second group of faults is characterized by irregularly but closely spaced (~ 50 m), shallow fractures (extending < 160ms TWT below seafloor, ~ 115 m) found almost exclusively in the GHSZ directly beneath the seafloor vent at the summit of S. Hydrate Ridge. These faults form a closely-spaced network of fractures that provide multiple migration pathways for free gas entering the GHSZ to migrate vertically to the seafloor. We speculate that the faults are the product of hydraulic fracturing due to near-lithostatic gas pressures at the base of the GHSZ. These fractures may fill with hydrate and develop a lower permeability, which will lead to a buildup of gas pressures below the GHSZ. This may lead to a vertical propagation of new fractures to release the overpressure, which results in the high concentration of shallow fractures within the GHSZ seen in the 2008 data. / text

Xenoliths preserve evidence of magma-crust interactions in magmatic reservoirs and conduits. They reveal processes of partial melting of country rock, and disintegration into magma. Widespread evidence for frothy xenoliths in volcanic deposits exists, and these evidently indicate processes of gas liberation, bubble nucleation and bubble growth. This report focuses on textural analysis of frothy sandstone xenoliths from Krakatau in Indonesia, Cerro Negro in Nicaragua, Cerro Quemado in El Salvador and from Gran Canaria, Canary Islands, and involves attempts to experimentally reproduce xenolith textures. To achieve this, magmatic conditions acting upon country rock in volcanoes are simulated by subjecting sandstones to elevated temperature and pressure in closed system-autoclaves. Subsequent decompression imitates magma ascent following xenolith entrainment, and is largely responsible for the formation of frothy xenolith textures. The experiments show a range of successive features, such as partial melting, gas-pressure build up, bubble nucleation, growth and development of bubble networks. The experiments closely reproduced textures of natural xenoliths and help to assess the controlling P-T parameters that encourage efficient bubble growth. Conditions proved ideal between 850˚C and 870˚C and pressure release from 1 kbar. Such conditions limit bubble overprinting by secondary crystallization and melt infilling. Country rock lithology proved vital regarding gas pressure build-up and resulting bubble nucleation during decompression. In particular, increased water content and relict crystals in the melt produced appear to ease and promote gas liberation by enabling early and effective bubble nucleation. Moreover, experiments confirm a decisive role for bubble coalescence. These results attest to the great potential of country rock to develop interconnected bubble networks upon magma contact, exsolving large amounts of crustal volatiles into the magma. Volatile input involves a change in magma viscosity and thus an accompanied change in disruptive behaviour, and may hence be responsible for increased potential to cause explosive volcanic eruptions. Moreover, H2O and CO2 vapour are severe greenhouse gases, which seems to be added to the atmosphere from crustal rocks via recycling by volcanic activity, and may have yet underappreciated effects on Earth’s climate.

This contribution aims to publicize the efforts made in the identification of gas hydrates in the Uruguayan continental shelf, analyze the most outstanding aspects related to its energy potential, as well as include this topic in other areas of knowledge for a comprehensive understanding of the subject. The hydrates, crystalline solid formed mainly by water and natural gas, are reservoirs of carbon that occur naturally in the continents in permafrost areas, and at sea, in the offshore basins of continental margins. They contain more than twice the total carbon in the world, surpassing the conventional hydrocarbon reserves. Principal energy programs foresee its commercial exploitation by 2015. International research programs include not only the energy aspect, but studying such systems considering their participation in the global carbon cycle, climate change and benthic communities associated with them. In our country, several seismic surveys showed evidence of the presence of gas hydrates in continental shelf and the surrounding area. The first survey was carried out by Brazil in the south of the Brazilian continental shelf, ANCAP then showed the continuity of the hydrate layer on the Uruguayan continental shelf and estimated the gas potential of the mineralized layer (87 TCF). Finally, the BGR survey verified the existence of seismic evidence of gas hydrates layer and the presence of free gas below these. The typical seismic response of gas hydrate and free gas is the BSR (Bottom Simulating Reflector) and is interpreted as a positive intensity reflection, followed by a negative intensity, showing the wave passage from a high acoustic impedance zone to a low acoustic impedance zone.

A one-kilometer-diameter carbonate/hydrate mound in Mississippi Canyon Block 118 has been chosen to be the site of a multi-sensor, multi-discipline sea-floor observatory. Several surveys have been carried out in preparation for installing the observatory. The resulting data set permits discussing the mound’s structure in some detail. Samples from the water column and intact hydrate outcrops show gas associated with the mound to be thermogenic. Lithologic and bio-geochemical studies have been done on sediment samples from gravity and box cores. Pore-fluid analyses carried out on these cores reveal that microbial sulfate reduction, anaerobic methane oxidation, and methanogenesis are important processes in the upper sediment. These microbial processes control the diffusive flux of methane into the overlying water column. The activity of microbes is also focused within patches near active vents. This is primarily dependent upon an active flux of hydrocarbon-rich fluids. The geochemical evidence suggests that the fluid flux waxes and wanes over time and that the microbial activity is sensitive to such change. Swath bathymetry by AUV combined with sea-floor video provides sub-meter resolution of features on the surface of the mound. Seismic reflection profiling with source-signature processing resolves layer thicknesses within the upper 200-300m of sediment to about a meter. Exploration-scale 3-D seismic imaging shows that a network of faults connects the mound to a salt diapir a few hundred meters below. Analyses of gases from fluid vents and hydrate outcrops imply that the faults act as migration conduits for hydrocarbons from a deep, hot reservoir. Source-signature-processed seismic traces provide normal-incidence reflection coefficients at 30,000 locations over the mound. Picking reflection horizons at each location allows a 3-D model of the mound’s interior to be constructed. This model provides a basis for understanding the movement of fluids within the mound.

xvii, 171 p. : ill. (some col.) / Crystals influence the migration of gas through magma. At low concentrations, they increase the bulk fluid properties, especially viscosity. At concentrations close to maximum packing, crystals form a rigid framework and magma cannot erupt. However, erupted pyroclasts with crystal contents close to the packing concentration are common at mafic volcanoes that exhibit Strombolian behavior. In this dissertation, I study the influence of solid particles on gas migration. I apply my results to Stromboli volcano, Italy, type locality of the normal Strombolian eruptive style, where gas moves through an essentially stagnant magma with crystallinity ∼50%. Specifically, I investigate the effect of crystals on flow regime, gas content (Chapter II), bubble concentration (number densities), bubble shapes, bubble sizes (Chapter III), and bubble rise velocities (gas flux) (Chapter IV). I find that gas-liquid flow regimes are not applicable at high particle concentrations and should be replaced by new, three-phase (gas-liquid-solid) regimes and that degassing efficiency increases with particle concentration (Chapter II). In Chapter III, I show that crystals modify bubble populations by trapping small bubbles and causing large bubbles to split into smaller ones and by modifying bubble shapes. In Chapter IV, I model Stromboli's crystal-rich magma as a network of capillary tubes and show that bubble rise velocities are significantly slower than free rise velocities in the absence of particles. In each chapter, I use analogue experiments to study the effect of different liquid and solid properties on gas migration in viscous liquids. I then apply my analogue results to magmatic conditions using simple parameterizations and/or numerical modeling or by comparing the results directly to observations made on crystal-rich volcanic rocks. Chapter V proposes a mechanism for Strombolian eruptions and gas migration through the crystalrich magma in which the effect of crystals is included. This model replaces the current twophase "slug" model, which cannot account for the high crystallinity observed at Stromboli. There are three appendices in this dissertation: a preliminary study of the influence of particles on gas expansion, image analysis methods, and the numerical code developed in Chapter IV. This dissertation includes previously published and unpublished co-authored material. / Committee in charge: Katharine Cashman, Chairperson; Alan Rempel, Member; Mark Reed, Member; Raghuveer Parthasarathy, Outside Member

With the widening and increasing use of nuclear energy, it is very important to design and build long-term deep geological repositories (DGRs) to manage radioactive waste. The disposal of nuclear waste in deep rock formations is currently being investigated in several countries (e.g., Canada, China, France, Germany, India, Japan and Switzerland). In Canada, a repository for low and intermediate level radioactive waste is being proposed in Ontario’s sedimentary rock formations. During the post-closure phase of a repository, significant quantities of gas will be generated from several processes, such as corrosion of metal containers or microbial degradation of organic waste. The gas pressure could influence the engineered barrier system and host rock and might disturb the pressure-head gradients and groundwater flows near the repository. An increasing gas pressure could also cause damage to the host rock by inducing the development of micro-/macro-cracks. This will further cause perturbation to the hydrogeological properties of the host rock such as desiccation of the porous media, change in degree of saturation and hydraulic conductivity. In this regard, gas generation and migration may affect the stability or integrity of the integrate barriers and threaten the biosphere through the transmitting gaseous radionuclides as long-term contaminants. Thus, from the safety perspective of DGRs, gas generation and migration should be considered in their design and construction. The understanding and modelling of gas migration within the host rock (natural barrier) and the associated potential impacts on the integrity of the natural barrier are important for the safety assessment of a DGR. Therefore, the key objectives of this Ph.D. study include (i) the development of a simulator for coupled modelling of gas migration in the host rock of a DGR for nuclear waste; and (ii) the numerical investigation of gas migration in the host rock of a DGR for nuclear waste in Ontario by using the developed simulator. Firstly, a new thermo-hydro-mechanical-chemical (THMC) simulator (TOUGHREACT-COMSOL) has been developed to address these objectives. This simulator results from the coupling of the well-established numerical codes, TOUGHREACT and COMSOL. A series of mathematical models, which include an elastoplastic-damage model have been developed and then implemented into the simulator. Then, the predictive ability of the simulator is validated against laboratory and field tests on gas migration in host rocks. The validation results have shown that the developed simulator can predict well the gas migration in host rocks. This agreement between the predicted results and the experimental data indicates that the developed simulator can reasonably predict gas migration in DGR systems. The new simulator is used to predict gas migration and its effects in a potential DGR site in Ontario. Valuable results regarding gas migration in a potential DGR located in Ontario have been obtained. The research conducted in this Ph.D. study will provide a useful tool and information for the understanding and prediction of gas migration and its effect in a DGR, particularly in Ontario.

Improving methods of hydrocarbon production and developing new techniques for the creation of natural gas storage facilities are critically important for the petroleum industry. This dissertation focuses on two key topics: (1) mechanisms of sand production from petroleum reservoirs and (2) mechanical characterization of caverns created in carbonate rock formations for natural gas storage. Sand production is the migration of solid particles together with the hydrocarbons when extracted from petroleum reservoirs. It usually occurs from wells in sandstone formations that fail in response to stress changes caused by hydrocarbon withdrawal. Sand production is generally undesirable since it causes a variety of problems ranging from significant safety risks during high-rate gas production, to the erosion of downhole equipment and surface facilities. It is widely accepted that a better understanding of the mechanics of poorly-consolidated formations is required to manage sand production; which, in turn, enables the cost effective production of gas and oil resources. In this work, a series of large-scale laboratory experiments was conducted in fully saturated, cohesionless sand layers to model the behavior of a petroleum reservoir near a wellbore. We directly observed several key characteristics of the sand production phenomenon including the formations of a stable cavity around the wellbore and a sub-radial flow channel at the upper surface of the tested layer. The flow channel is a first-order feature that appears to be a major part of the sand production mechanism. The channel cross section is orders of magnitude larger than the particle size, and once formed, the channel becomes the dominant conduit for fluid flow and particle transport. The flow channel developed in all of our experiments, and in all experiments, sand production continued from the developing channel after the cavity around the borehole stabilized. Our laboratory results constitute a well constrained data set that can be used to test and calibrate numerical models employed by the petroleum industry for predicting the sand production phenomenon. Although important for practical applications, real field cases are typically much less constrained. We used scaling considerations to develop a simple analytical model, constrained by our experimental results. We also simulated the behavior of a sand layer around a wellbore using two- and three-dimensional discrete element methods. It appears that the main sand production features observed in the laboratory experiments, can indeed be reproduced by means of discrete element modeling. Numerical results indicate that the cavity surface of repose is a key factor in the sand production mechanism. In particular, the sand particles on this surface are not significantly constrained. This lack of confinement reduces the flow velocity required to remove a particle, by many orders of magnitude. Also, the mechanism of channel development in the upper fraction of the sample can be attributed to subsidence of the formation due to lateral extension when an unconstrained cavity slope appears near the wellbore. This is substantiated by the erosion process and continued production of particles from the flow channel. The notion of the existence of this surface channel has the potential to scale up to natural reservoirs and can give insights into real-world sand production issues. It indicates a mechanism explaining why the production of particles does not cease in many petroleum reservoirs. Although the radial character of the fluid flow eventually stops sand production from the cavity near the wellbore, the production of particles still may continue from the propagating surface (interface) flow channel. The second topic of the thesis addresses factors affecting the geometry and, hence, the mechanical stability of caverns excavated in carbonate rock formations for natural gas storage. Storage facilities are required to store gas when supply exceeds demand during the winter months. In many places (such as New England or the Great Lakes region) where no salt domes are available to create gas storage caverns, it is possible to create cavities in limestone employing the acid injection method. In this method, carbonate rock is dissolved, while CO₂ and calcium chloride brine appear as products of the carbonate dissolution reactions. Driven by the density difference, CO₂ rises towards the ceiling whereas the brine sinks to the bottom of the cavern. A zone of mixed CO₂ , acid, and brine forms near the source of acid injection, whereas the brine sinks to the bottom of the cavern. Characterization of the cavern shape is required to understand stress changes during the cavity excavation, which can destabilize the cavern. It is also important to determine the location of the mixture-brine interface to select the place of acid injection. In this work, we propose to characterize the geometry of the cavern and the location of the mixture-brine interface by generating pressure waves in a pipe extending into the cavern, and measuring the reflected waves at various locations in another adjacent pipe. Conventional governing equations describe fluid transients in pipes loaded only by internal pressure (such as in the water hammer effect). To model the pressure wave propagation for realistic geometries, we derived new governing equations for pressure transients in pipes subjected to changes in both internal and external (confining) pressures. This is important because the internal pressure (used in the measurement) is changing in response to the perturbation of the external pressure when the pipe is contained in the cavern filled with fluids. If the pressure in the cavern is perturbed, the perturbation creates an internal pressure wave in the submerged pipe that has a signature of the cavern geometry. We showed that the classic equations are included in our formulation as a particular case, but they have limited validity for some practically important combinations of the controlling parameters. We linearized the governing equations and formulated appropriate boundary and initial conditions. Using a finite element method, we solved the obtained boundary value problem for a system of pipes and a cavern filled with various characteristic fluids such as aqueous acid, calcium chloride brine, and supercritical CO₂ . We found that the pressure waves of moderate amplitudes would create measurable pressure pulses in the submerged pipe. Furthermore, we determined the wavelengths required for resolving the cavern diameter from the pressure history. Our results suggest that the pressure transients technique can indeed be used for characterizing the geometry of gas storage caverns and locations of fluid interfaces in the acid injection method.