In recent years, hydraulic fracturing has led to a dramatic increase in the worldwide production of natural gas. In a typical hydraulic fracturing treatment, millions of gallons of water, sand and chemicals are injected into a reservoir to generate fractures in the reservoir that serve as pathways for fluid flow. Recent research has shown that both the effectiveness of fracturing treatments and the productivity of fractured reservoirs can be heavily influenced by the presence of pre-existing natural fracture networks. This work presents a fully implicit hydro-mechanical algorithm for modeling hydraulic fracturing in complex fracture networks using the two-dimensional discontinuous deformation analysis (DDA). Building upon previous studies coupling the DDA to fracture network flow, this work emphasizes various improvements made to stabilize the existing algorithms and facilitate their convergence. Additional emphasis is placed on validation of the model and on extending the model to the stochastic characterization of hydraulic fracturing in naturally fractured systems. To validate the coupled algorithm, the model was tested against two analytical solutions for hydraulic fracturing, one for the growth of a fixed-length fracture subject to constant fluid pressure, and the other for the growth of a viscosity-storage dominated fracture subject to a constant rate of fluid injection. Additionally, the model was used to reproduce the results of a hydraulic fracturing experiment performed using high-viscosity fracturing fluid in a homogeneous medium. Very good agreement was displayed in all cases, suggesting that the algorithm is suitable for simulating hydraulic fracturing in homogeneous media. Next, this work explores the relationship between the maximum tensile stress and Mohr-Coulomb fracture criteria used in the DDA and the critical stress intensity factor criteria from linear elastic fracture mechanics (LEFM). The relationship between the criteria is derived, and the ability of the model to capture the relationship is examined for both Mode I and Mode II fracturing. The model was then used to simulate the LEFM solution for a toughness-storage dominated bi-wing hydraulic fracture. Good agreement was found between the numerical and theoretical results, suggesting that the simpler maximum tensile stress criteria can serve as an acceptable substitute for the more rigorous LEFM criteria in studies of hydraulic fracturing. Finally, this work presents a method for modeling hydraulic fracturing in reservoirs characterized by pre-existing fracture networks. The ability of the algorithm to correctly model the interaction mechanism of intersecting fractures is demonstrated through comparison with experimental results, and the method is extended to the stochastic analysis of hydraulic fracturing in probabilistically characterized reservoirs. Ultimately, the method is applied to a case study of hydraulic fracturing in the Marcellus Shale, and the sensitivity of fracture propagation to variations in rock and fluid parameters is analyzed.

Le nord-ouest des Alpes est un domaine intraplaque présentant de très faibles déformations. C'est pourquoi il paraît délicat de déduire la probabilité d'occurrence d'un séisme de taille lithosphérique (magnitude supérieure à 7) à partir des observations de microsismicité. De telles observations sont en effet des processus superficiels et présentent peu ou pas de lien avec des processus profonds de plus grande ampleur. L'objectif est de déterminer le potentiel sismique d'un secteur au nord-ouest des Alpes en étudiant le champ de contrainte résultant d'un chargement gravitaire. Seuls les objets de taille lithosphérique, i.e. de l'ordre de la centaine de kilomètres sont pris en compte. Un modèle de contraintes à l'échelle 360 km par 400 km par 230 km d'épaisseur, centré sur la subduction fossile des Alpes de l'ouest et s'étendant jusqu'au nord de Strasbourg, est établi. L'étude des structures du nord-ouest alpin montre l'importance de l'orogène alpin qui se retrouve, enparticulier, dans les variations de profondeur des interfaces de la lithosphère. Une étude du champ de contrainte dans le socle a permis d'identifier une rotation des contraintes principales horizontales avec l'axe des Alpes. Bien que la valeur absolue des contraintes principales n'ait pas pu être déterminée, un rapport de valeur relative est calculé. Le résultat de la modélisation montre l'importance de la rhéologie dans le cas d'un chargement gravitaire. Si une rhéologie élastique est prise en compte, les directions de contrainte calculées sont totalement différentes des observations. Par contre, l'utilisation d'une rhéologie élasto-plastique combinée à l'utilisation d'une géométrie réaliste des interfaces lithosphériques permet d'obtenir des directions de contraintes cohérentes avec les données. / The north-west of the Alps is an intraplate domain with very slow deformations. So, it seems difficult to determine the probability of occurrence of a lithospheric earthquake (magnitude greater than 7) from microseismic observations. Such observations are superficial processes with little relation to deeper and bigger ones. The aim is to determine the seismic potential of a lithospheric sector north-west of the Alps, studying the stress field generated by a gravity driven model. This model is 360 km by 400 km by 230 km deep, centered on the west alpine fossil subduction and going up to the north of Strasbourg. The study of the north-west alpine structures shows the importance of the alpine orogen which generates variations in depth of the lithospheric interfaces. A study of the stress field in the basement shows a variation of principal stress directions along the strike of the Alpine chain. Even if the absolute magnitude of stresses could not be determined a relative magnitude ratio is calculated. Results underline the importance of rheology for a gravity driven model. If an elastic rheology is modeled, calculated stress directions do not match observations. However, using an elasto-plastic rheology with a realistic geometry of the lithospheric interfaces, we can obtain stress directions coherent with the data.

Understanding economic success of unconventional production from shales requires an explanation of the relationship between induced seismicity and hydraulic fracturing. This thesis deals with observing and analyzing synthetic and real microseismic monitoring data acquired during hydraulic fracturing. The thesis is based on observation and analyses of source mechanisms of induced microseismic events that have recently become regularly inverted and interpreted in the oil and gas industry. The results of analyses are interpreted with the geomechanical model of the relationship between hydraulic fracturing and induced seismicity. The study of source mechanisms starts with detailed analyses of spatial distribution of full moment tensor inversion stability. It was mapped based on synthetically computed condition numbers in the vicinity of different monitoring arrays including dense arrays at the surface and sparse arrays with sensors in the boreholes. Stability of inversion was tested under several conditions, mainly dependency on size and geometry of monitoring array and level of noise in the data. In this part of the thesis it is shown that dense surface arrays may provide very stable inversion of source mechanisms which may be interpreted. The study shows that an increasing percentage of non-shear...