Multi-scale hydraulic characterization of stimulated fractured crystalline rock at Grimsel test site

In-situ Stimulation and Circulation (ISC), Swiss Alps, hydraulic fracturing, hermo-hydro-mechanical (THM) behavior

Fluid flow in sparse fracture systems, prior to and after fault slip

Many geothermal pilot projects worldwide have been forced to shutdown in the last fifteen years after triggering earthquakes by fluid injections into critically-stressed faults. This reveals that our capacity to control the migration of injected fluids is still poor and shows the importance to better understand how faults control flow to improve the safety of hydraulic stimulations. Little is known also about how deep fluid injections affect heat transport and, ultimately, the performance of enhanced geothermal systems. Here, I investigate how downscaled controlled hydraulic stimulations performed at the Grimsel Rock Laboratory, Switzerland, enhanced subsurface flow and heat transport in a highly-monitored reservoir analog. The installation of 15 boreholes to monitor fluid pressure, temperature and rock deformation in a previously unexplored area of the Grimsel Rock Laboratory, from 2015 to 2016, provided a unique opportunity to carry out experiments and observe flow and heat transport in situ at unprecedented resolution. I first investigate how the internal structure of shallow crustal faults affects flow and pore-fluid diffusion. Starting at borehole scale, I systematically test the permeability of the host rock, single fracture and fracture networks and show how brittle damage induces a power-law decay in permeability with lateral distance from faults. Building on this knowledge, I investigate cross-hole scale flow interactions to understand the nature of spatial dimensions amenable to flow. By applying a generalized flow model, I show that fractional flow dimensions systematically emerge, converging to a dimension of $n$=1.3, due to a pressure diffusion slowdown. This slowdown is inconsistent with normal diffusion and suggests that anomalous diffusion is an important flow process in shallow crustal faults, and that their hydrologic structure is fractal. For the faults tested, I obtain a fractal dimension of $d_f$=2.23, which may reflect network-scale flow channeling. Characterizing the hydrological state of these faults after stimulation shows that their permeability and internal flow structure sustained significant, multiscale changes. Locally, changes in permeability measured in stimulation wells, where high-pressure injections took place, were found to be inversely proportional to the initial permeability and scale positively with seismic magnitudes and rupture areas. This suggests a relation between permeability enhancement and induced seismicity. Several locations in the near field became also less permeable. This shows, for the first time, that permeability reduction may be an overlooked but important near-field process following stimulation. Globally, stimulated faults became more permeable and better connected hydraulically compared to their pre-injection state. Faults with poor to moderate natural connectivity experienced the largest increase in connectivity. Hence, I propose that such faults constitute better stimulation targets. Finally, heat injection experiments confirmed that the heterogeneous flow structure of faults strongly affects subsurface heat transport. Thermal breakthroughs were observed at multiple locations, yet confined to the most permeable faults. These breakthroughs were found to be better explained by heat transport through a parallel plate fracture, instead of tube-like channels. Comparing then pre- and post-stimulation thermal transfer time distribution showed that high-pressure fluid injections enhanced, but also delayed, advective heat transport. The thermal response to shear stimulation of shallow crustal faults appears, therefore, to be more complex than previously described

Quantification of the regional groundwater flux to a northern peatland complex, Schefferville, Québec, Canada: results from a water budget and numerical simulations

Peatlands are significant soil carbon and freshwater reservoirs at the global scale, and thus play a key role in the global carbon and water cycles. Peatland inception and development are intrinsically related to optimal environmental conditions, including an adequate hydro-morphological setting that promotes the occurrence of waterlogged soils and low topographic gradients. This thesis investigates the hydrology of 0.2 km2 northern peatland complex, located in the region of Schefferville, Québec, Canada, in which a significant area has developed over a relatively steep topographic gradient. The objective of this thesis is to quantify and characterize the spatiotemporal flux of regional groundwater to the peatland local flow system in an attempt to evaluate the relative importance of this flux compared with other water inputs. A three-dimensional groundwater flow model, the finite-difference U.S. Geological Survey MODFLOW code, is used to simulate the peatland and characterize the groundwater flow system. The model is parameterized with data measured from June 17th to September 4th, 2009, including continuous meteorological measurements from an automatic weather station, 14 observation wells, and two 90° V-notch weirs gauging stream discharge. This study establishes that, over the measuring period, the regional groundwater influx accounted for 27% ± 0.4% of the total water inputs, the remainder 73% ± 0.4% consisting of precipitation (54% ± 0.4%), surface water inflow (14% ± 0.4%), and change in storage (5% ± 0.4%). Consequently, it is the main conclusion of this work that regional groundwater inflows were, volumetrically, the second most important source of water to the peatland local flow system.Les tourbières représentent d'importants réservoirs naturels riches en carbone et en eau douce à l'échelle globale. Ils jouent par conséquent un rôle majeur à la fois dans le cycle du carbone et le cycle hydrologique. L'origine et le développement des tourbières sont liés à l'existence de circonstances environnementales optimales. Il s'agit essentiellement d'un contexte hydro-morphologique adéquat favorisant l'occurrence de sols saturés en eau sur des terrains relativement plats. Ce document présente les résultats d'une étude scientifique conduite dans un complexe tourbier nordique, situé dans la région de Schefferville, Québec, Canada. Ce terrain représente un cas particulier peu observé dans la nature : une large partie de l'accumulation de matière organique a eu lieu sur un gradient topographique prononcé. L'objectif de cette étude est de quantifier et caractériser le flux spatiotemporel du système de circulation de l'eau souterraine régional à la nappe d'aquifère du système tourbier local. Un modèle numérique de circulation de l'eau souterraine est utilisé afin de simuler le système tourbier et caractériser son régime hydraulique. Basé sur le code informatique du modèle MODFLOW, développé par le Service de Géologie des Etats Unis (McDonald and Harbaugh 1988), le modèle tridimensionnel est utilisé afin de simuler la circulation des eaux sou terraine. Le modèle incorpore une série de données météorologiques et hydrométriques en tant que paramètres d'entrée. Mesurées entre le 17 juin et le 4 septembre 2009, ces données proviennent d'une station météo installée pour les besoins de l'étude sur le terrain même, ainsi que 14 puits d'observations et 2 barrages pour mesure le débit. Cette étude établit que, sur l'ensemble de la période d'étude, le flux de l eau souterraine du système régional a représenté 27% des entrées en eau. Les 73% restant se divisent entre 54% pour la précipitation, 14% pour l influx de surface et 5% pour le changement de stockage hydraulique. En conséquence, la conclusion majeure de cette étude est que le flux régional d'eau souterraine est, d'un point de vue volumétrique, le deuxième flux le plus important dans le system tourbier nordique

Analysis of thermal dilution experiments with Distributed Temperature Sensing for fractured rock characterization

International audienceThermal dilution experiments with Fiber-Optic Distributed Temperature Sensing (FO-DTS) were conducted at the In-situ Stimulation and Circulation (ISC) rock laboratory, at the Grimsel Test Site (GTS) in Switzerland. The experiment consists in replacing the total volume of a borehole with a warmer water and monitoring the rate of temperature increase and decrease during the heating and cooling periods, respectively. The changes in temperature monitored in depth and time under various hydraulic conditions and in different boreholes, are used to investigate the information provided by thermal dilution experiments in terms of groundwater flow and thermal properties in low-permeable fractured crystalline rock. The data analysis, and the use of analytical and numerical solutions for reproducing this data in the context of pure diffusion and advection-diffusion scenarios, lead to the following improvements and conclusions. (i) The formation thermal conductivity is estimated along the borehole by inverting the data collected under ambient conditions with a simple analytical solution. The estimated values are consistent with laboratory estimates. The method presents the advantage of requiring much shorter experiments than existing methods based on standard active-line-source (ALS) experiments, i.e., several hours versus the traditional 1-2 days. (ii) Hydraulically active fractures connecting boreholes are detected from experiments conducted under cross-borehole forced hydraulic conditions. (iii) The formation thermal conductivity and fracture flow velocity have a distinguished impact on the temperature anomalies for some ranges of these property values, implying that both properties can be estimated from well-parametrized experiments

Thirty years of tailings seepage history from tailings & mine waste

This paper explores thirty and more years of the development of methods and approaches to the evaluation, analysis, and quantification of seepage into, through, and from tailings impoundment to subsurface soil and rock and the subsequent impact thereof on the environment. This is done by way of a survey of technical papers that have been published in the proceedings of the conferences on Tailings & Mine Waste. The story told in this paper is of the movement from simple professional understanding of the geology through the development of flow nets, computer codes, 3D modeling of seepage and geochemical interaction, an understanding of acid drainage and the many ways to deal with it, and hence the current ability of mines to understand, predict, and control potential impact of tailings seepage to the environment. [All papers were considered for technical and language appropriateness by the organizing committee.]Non UBCUnreviewedOthe

Joint-analysis of single and cross-borehole hydraulic pressure transients to characterize the hydraulic properties and connectivity of fractured networks

ISSN:1029-7006ISSN:1607-796

Diagnostic pressure analysis from the in-situ decameter-scale HF experiment

ISSN:1029-7006ISSN:1607-796