Fluid-injection processes can induce earthquakes by increasing pore pressure and/or shear stress on faults. Natural processes, including transformation of organic material (kerogen) into hydrocarbon, can similarly cause fluid overpressure. Here we document examples where earthquakes induced by hydraulic fracturing are strongly clustered within areas characterized by pore-pressure gradient in excess of 15 kPa/m. By contrast, induced earthquakes are virtually absent in the same formations elsewhere. Monte Carlo analysis indicates that there is negligible probability that this spatial correlation developed by chance. A detailed analysis was undertaken within a region in Alberta, Canada where uniquely comprehensive data characterize dynamic interactions between seismicity and well completions. Seismicity is strongly clustered in space and time, exhibiting spatially varying persistence and activation threshold. The largest event (ML 4.4) can be reconciled with a previously postulated upper bound on magnitude, only if the cumulative effect of multiple treatment stages is considered. Induced seismicity from hydraulic fracturing reveals contrasting signatures of fault activation by stress effects and fluid diffusion. Patterns of seismicity indicate that stress changes during operations can activate fault slip to an offset distance of > 1 km, whereas pressurization by hydraulic fracturing into a fault yields episodic seismicity that can persist for months.
Abstract New constraints on the pattern of crustal flow in SE Tibet are obtained from joint analysis of receiver functions and Rayleigh wave dispersion with a newly deployed seismic array. The crust in the Sichuan‐Yunnan Diamond Block has an average thickness of ~45 km and gradually thins toward the Indo‐China Block to the west and the Yangtze Block to the east. High V P / V S ratios are detected to the west of the Xiaojiang fault, but not in the Yangtze Block to the east. The S wave velocity profile reveals that intra‐crustal low‐velocity zones (IC‐LVZs) are strongly heterogeneous, with two LVZs in the middle and mid‐lower crust, respectively, in marked contrast to previous observations of a single LVZ. Combined with other observations, the two IC‐LVZs are interpreted as isolated channels of crustal flow at different depths beneath SE Tibet, resulting in the observed complex pattern of radial anisotropy and further elucidating patterns of flow and deformation.
En septembre 1998, un des auteurs (WW) a decouvert un indice important de mineralisation en emeraude dans la region du lac Finlayson, dans le secteur sud-est du Yukon. L'indice de Regal Ridge est situe dans les roches metavolcaniques deformees de facon complexe dans le socle de Yukon-Tanana, pres du contact avec un pluton granitique mis en place au cretace moyen. Les cristaux d'emeraude se sont developpes ou des veines de quartz recoupent des niveaux micaces de l'unite schisteuse tardidevonienne de Fire Lake, mafique et a faible pendage. Au moins huit veines semblables ont ete reperees. Dans la plupart des cas, les veines sont entourees d'une masse de petits cristaux enchevetres de tourmaline foncee. Leurs sont associes localement de petites quantites de scheelite, et, dans les veines elles-memes, des sulfures. Une zone de sulfures epars coinciderait avec la zone a tourmaline, marquee par des produits d'oxydation ochres. Des cristaux de beryl vert atteignent 4 cm en longueur dans les zones a tourmaline et, dans certains cas, les veines de quartz. Certains des plus petits cristaux, et des portions des plus gros, ont une qualite gemme. La teneur en Cr (moyenne 3208 ppm) en fait le chromophore principal. Les donnees sur les inclusions fluides indiquent que la phase fluide responsable de la mineralisation avait une salinite maximale equivalente a 3% de NaCl. La composition isotopique de l'oxygene de l'emeraude est tres variable (entre 12.3 et 14.8‰), mais sans variation comparable dans les valeurs de 8D correspondantes (-57.3 et -59.8‰, respectivement), ce qui suppose une phase fluide isotopiquement homogene qui a amorce un echange isotopique avec l'encaissant sans toutefois atteindre l'equilibre. Les valeurs de δ 1 8 O du quartz et de la tourmaline des veines de quartz indiquent une temperature de formation d'environ 365 et 498°C. A la lumiere des donnees isochores des inclusions fluides, ces temperatures correspondraient a une pression entre 1.0 et 2.5 kbar, et donc une profondeur entre 3 et 7.7 km. La proximite du granite fait penser qu'il etait la source du beryllium, quoique sa teneur en Be est assez faible (entre 12 et 13.2 ppm). La source du Cr est le schiste (520 ppm Cr). Un âge 4 0 Ar/ 3 9 Ar d'un echantillon de mica du schiste, 109 Ma, pourrait temoigner de l'âge d'un rechauffement lie a la mineralisation, ou bien un refroidissement suivant la mise en place du granite, ou les deux.
Abstract Hydraulic fracturing increases the drainage area and effective permeability of unconventional oil and gas reservoirs by creating a fracture network or stimulated rock volume (SRV) within the reservoir rock. The dimensions of the SRV and its permeability are the key parameters that enhance the unconventional reservoirs' performance after the hydraulic fracture operation. Simulation of the SRV to obtain its dimension and permeability can be used to determine the optimum hydraulic fracture treatment parameters and production. In this study, finite element analysis is employed to determine the SRV characteristics based on field data from a hydraulic fracturing job in a horizontal well penetrating the Glauconite formation in Hoadley field, Alberta, Canada. The dimensions of the SRV are calibrated from the microseismic data. The fracture propagation pressure of the finite element model is matched to the field value by altering the permeability of the SRV. The matched model is used to obtain the in-situ stress changes and the pressure drop within the SRV. The SRV permeability and the pressure drop are used to calculate the aperture, the number, and the spacing of the fractures within the SRV using a semi-analytical approach. The final outputs can be used to optimize the future hydraulic fracture design at the Hoadley field or at other fields that have similar geomechanical properties. It could also be used to predict the reservoir production after the hydraulic fracturing and to provide estimates of changes in the in-situ stresses around the stimulated horizontal wellbore.
