Shallow Aquifer Vulnerability From Subsurface Fluid Injection at a Proposed Shale Gas Hydraulic Fracturing Site

Abstract

Groundwater flow resulting from a proposed hydraulic fracturing (fracking) operation was numerically modeled using 91 scenarios. Scenarios were chosen to be a combination of hydrogeological factors that a priori would control the long‐term migration of fracking fluids to the shallow subsurface. These factors were induced fracture extent, cross‐basin groundwater flow, deep low hydraulic conductivity strata, deep high hydraulic conductivity strata, fault hydraulic conductivity, and overpressure. The study considered the Bowland Basin, northwest England, with fracking of the Bowland Shale at ∼2,000 m depth and the shallow aquifer being the Sherwood Sandstone at ∼300–500 m depth. Of the 91 scenarios, 73 scenarios resulted in tracked particles not reaching the shallow aquifer within 10,000 years and 18 resulted in travel times less than 10,000 years. Four factors proved to have a statistically significant impact on reducing travel time to the aquifer: increased induced fracture extent, absence of deep high hydraulic conductivity strata, relatively low fault hydraulic conductivity, and magnitude of overpressure. Modeling suggests that high hydraulic conductivity formations can be more effective barriers to vertical flow than low hydraulic conductivity formations. Furthermore, low hydraulic conductivity faults can result in subsurface pressure compartmentalization, reducing horizontal groundwater flow, and encouraging vertical fluid migration. The modeled worst‐case scenario, using unlikely geology and induced fracture lengths, maximum values for strata hydraulic conductivity and with conservative tracer behavior had a particle travel time of 130 years to the base of the shallow aquifer. This study has identified hydrogeological factors which lead to aquifer vulnerability from shale exploitation.

Permeability of rock discontinuities and faults in the Triassic Sherwood Sandstone Group (UK): insights for management of fluvio-aeolian aquifers worldwide

“…In fact, core-plug and well-test scales substantially coincide (K well-test /K core-plug 1.1-1.3) at such depth in the Triassic fluvial deposits of the St Bees Sandstone of the eastern Irish Sea Basin and in the Stuttgart Formation of the North German Basin (Förster et al 2006;Medici et al 2018). Nevertheless, subhorizontal (0-15°) bedding plane fractures up to~1 km depth contribute to enhance flow anisotropy favouring flow in directions parallel to the beds, leading to aquifer anisotropy that impedes rise of contaminants related to shale gas extraction at such elevated depths (Cai and Ofterdinger 2014;Wilson et al 2017).…”

“…In contrast, residual flow through bedding-plane discontinuities at greater depths (~150-1,500 mBGL) favours horizontal fluxes of contaminants and retards upward migration associated with steeply inclined faults (Wilson et al 2017). Yet, both core-plug-scale and welltest-scale data are required jointly to effectively quantify the impact of geological heterogeneities on flow.…”

Compartmentalisation and groundwater–surface water interactions in a prospective shale gas basin: Assessment using variance analysis and multivariate statistics on water quality data

“…Wilson et al (2017) showed that compartmentalisation may increase the risk of injected fluids migrating to near‐surface aquifers. In the study region, the Woodsfold fault is considered to compartmentalise the central and southern Fylde (Ove Arup and Partners Ltd., 2014a; Wilson et al, 2019).…”

Identifying groundwater compartmentalisation for hydraulic fracturing risk assessments

“…was because the modelled low-permeability faults acted to structurally compartmentalise groundwater in the basin, discouraging lateral flow and encouraging upward flow through strata in the presence of a vertical hydraulic head gradient. 14…”

“…[8][9][10][11][12][13] However, more recent numerical modelling found that for fluid injection situated between faults (a more likely development scenario because operators aim to avoid fluid injection into faults), it was the scenarios with low-permeability faults that resulted in the highest risk to the shallow aquifer. 14…”