Development of a Methodology to Evaluate Potential Fluid Migration Pathways from Deep Shale Units to Surficial Aquifers in a Context of Shale Gas Production
Rivard, Christine; Lefebvre, René; Lavoie, Denis; Sejourne, Stephan; Duchesne, Mathieu J.; Ahad, Jason; Benoit, Nicolas; Wang, Baolin; Pugin, André; Lamontagne, Charles
Hydrocarbon production from Canadian shales is recent. Natural gas from shales is being produced in north-eastern British Columbia (Western Canada), while other shales with gas potential are currently being evaluated in different parts of Canada, including the Upper Ordovician Utica Shale in southern Quebec (Eastern Canada).
Hydrocarbon exploration and production operations involve risks to degrade groundwater quality and these hazards need to be assessed and minimized. Contamination risks are primarily related to the casing integrity and surface activities (e.g. truck traffic, water storage). They are, however, mainly related to engineering issues. On the other hand, some concerns have been raised with respect to groundwater contamination that could result from potential fracture or fault interconnections between the shale unit and surficial aquifers, which would allow frac fluids and methane to reach shallow aquifers. Even though groundwater resources are relatively well characterized in a few regions, there is currently no recognized method to evaluate the vulnerability or risks to aquifers resulting from activities carried out at great depths.
This contribution aims at describing the context and methodology currently being developed to evaluate the presence or absence of preferential pathways linking a fracked shale interval to surficial aquifers. The approach involves the use of geological, geophysical, hydrogeological and geochemical data. The Utica Shale in Quebec was targeted for this research because 1) no large-scale exploration and production has taken place so far, 2) this region is relatively populated and 3) groundwater represents an essential water supply for rural areas. A site previously explored by the industry was selected to provide a case study for hazard assessment based on actual geological constraints of a specific region. This contribution also describes the main geological conditions of the Utica Shale and overlying cap rock, critical elements for a thorough understanding of the subsurface geological framework that forms the cornerstone of safe and sustainable development. The study workflow includes: seismic surveys, borehole geophysics, drilling, hydraulic testing, groundwater and gas sampling, as well as 2-D geomechanical and 3-D hydrogeological models. The methodology and tools that are being developed will be general enough to serve as a model for the environmental assessment of other sites in Canada and elsewhere.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013