Assessing potential impacts of shale gas development on shallow aquifers through upward fluid migration: A multi-disciplinary approach applied to the Utica Shale in eastern Canada

Abstract

Potential impacts of shale gas development on shallow aquifers has raised concerns, especially regarding groundwater contamination. The intermediate zone separating shallow aquifers from shale gas reservoirs plays a critical role in aquifer vulnerability to fluid upflow, but the assessment of such vulnerability is challenging due to the general paucity of data in this intermediate zone. The ultimate goal of the project reported here was to develop a holistic multi-geoscience methodology to assess potential impacts of unconventional hydrocarbon development on freshwater aquifers related to upward migration through natural pathways. The study area is located in the St. Lawrence Lowlands (southern Quebec, Canada), where limited oil and gas exploration and no shale gas production have taken place. A large set of data was collected over a ~500 km 2 area near a horizontal shale gas exploration well completed and fracked into the Utica Shale at a depth of 2 km. To investigate the intermediate zone integrity, this project integrated research results from multiple sources in order to obtain a better understanding of the system hydrodynamics, including geology, hydrogeology, deep and shallow geophysics, soil, rock and groundwater geochemistry, and geomechanics. The combined interpretation of the multidisciplinary dataset demonstrates that there is no evidence of, and a very limited potential for, upward fluid migration from the Utica Shale reservoir to the shallow aquifer. Microbial and thermogenic methane in groundwater of this region appear to come from the shallow, organic-rich, fractured sedimentary rocks making up the regional aquifer. Nonetheless, diluted brines present in a few shallow wells close to and downstream of a normal fault revealed that some upward groundwater migration occurs, but only over a few hundred meters from the surface based on the isotopic signature of methane. The 3 methodology developed should help support regulations related to shale gas development aiming to protect groundwater.

Continual long-term monitoring of methane in wells above the Utica Shale using total dissolved gas pressure probes

“…Surficial sediments in this region are usually thin (<10 m) and made up of reworked tills and near-shore sediments of the former Champlain Sea, except in a few areas where finegrained marine sediments have accumulated in local lows of the paleo-topography (Rivard et al 2018). Most residential wells in this region are completed into bedrock, with depths ranging from 30 to 80 m. Similarly, the monitoring wells that were drilled for the previous Saint-Édouard project (Bordeleau et al 2018a, b;Rivard et al 2019;Bordeleau et al 2019) were open-hole bedrock wells with depths between 30 and 149 m. These wells were only cased across unconsolidated units and to 2 m within the bedrock. Borehole geophysical logging revealed that the upper 60 m of bedrock, and especially the upper 30 m, contain substantial fractures (Crow and Ladevèze 2015), and is believed to be the active zone of groundwater flow (Ladevèze et al 2018).…”

“…Groundwater from wells from this region is typically of good quality and is generally of the Na-HCO 3 or Na-Cl type, although some shallower wells also had Ca-HCO 3 water (Bordeleau et al 2018b). Further details of the geology and hydrogeology of the study area are presented by Lavoie et al (2014) and Rivard et al (2018Rivard et al ( , 2019.…”

“…This work involved in situ continuous measurements of P TDG for >2 years and approximately bimonthly groundwater sampling to measure the dissolved gas composition over time, including methane and some higher alkane gases, to determine whether P TDG sensor measurements alone could represent a good proxy for methane concentrations in this region. This study used three monitoring wells from a previous project (Bordeleau et al 2018a, b;Rivard et al 2019) that studied potential impacts of shale gas activities on shallow groundwater in the Saint-Édouard area (eastern Canada). The shallow bedrock aquifer of this region is known to contain elevated methane concentrations (Bordeleau et al 2018b).…”

Modeling of methane migration from gas wellbores into shallow groundwater at basin scale

“…The presence of thermogenic methane in groundwater in the vicinity of oil and gas operations is most commonly associated with leakage from hydrocarbon wells (Jackson et al 2013;Darrah et al 2015;Rivard et al 2019). Multiple studies have shown that leaked methane from the deep subsurface migrates upward by the buoyancy resulting from density contrasts between gas and brine (Rice et al 2018a).…”

Reactive Transport Modeling of Natural Gas Molecular and Isotopic Evolution During Diffusive Transport in the Subsurface

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Studies evaluating hydrocarbon gas occurrence in shallow groundwater often include assessment of the isotopic composition of carbon (δ 13 C) and hydrogen (δ 2 H) in dissolved methane, which has been used to postulate the origin of the gas (Rivard et al, 2019;Rodrigues et al, 2019). Generally, gases formed through microbial action tend to be isotopically lighter than thermogenic gases, although there is considerable overlap in δ 13 C values between the two sources (Milkov & Etiope, 2018).

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