Numerical simulation of the environmental impact of hydraulic fracturing of tight/shale gas reservoirs on near‐surface groundwater: Background, base cases, shallow reservoirs, short‐term gas, and water transport

Abstract

Hydrocarbon production from unconventional resources and the use of reservoir stimulation techniques, such as hydraulic fracturing, has grown explosively over the last decade. However, concerns have arisen that reservoir stimulation creates significant environmental threats through the creation of permeable pathways connecting the stimulated reservoir with shallower freshwater aquifers, thus resulting in the contamination of potable groundwater by escaping hydrocarbons or other reservoir fluids. This study investigates, by numerical simulation, gas and water transport between a shallow tight-gas reservoir and a shallower overlying freshwater aquifer following hydraulic fracturing operations, if such a connecting pathway has been created. We focus on two general failure scenarios: (1) communication between the reservoir and aquifer via a connecting fracture or fault and (2) communication via a deteriorated, preexisting nearby well. We conclude that the key factors driving short-term transport of gas include high permeability for the connecting pathway and the overall volume of the connecting feature. Production from the reservoir is likely to mitigate release through reduction of available free gas and lowering of reservoir pressure, and not producing may increase the potential for release. We also find that hydrostatic tight-gas reservoirs are unlikely to act as a continuing source of migrating gas, as gas contained within the newly formed hydraulic fracture is the primary source for potential contamination. Such incidents of gas escape are likely to be limited in duration and scope for hydrostatic reservoirs. Reliable field and laboratory data must be acquired to constrain the factors and determine the likelihood of these outcomes.Key Points:Short-term leakage fractured reservoirs requires high-permeability pathways Production strategy affects the likelihood and magnitude of gas release Gas release is likely short-term, without additional driving forces

Dynamic Assessment of Comprehensive Water Quality Considering the Release of Sediment Pollution

“…Meanwhile, the water quality shows the tendency of becoming better from upstream to downstream (dam) where the average level of the water quality is 2.31, and is better than others. The process of hydraulic transport agrees with this trend [37]. The water quality in the surface layer is better than that in the bottom layer because of sediment pollution, which influences water quality by releasing contaminants and consuming dissolved oxygen [38].…”

Salinity of deep groundwater in California: Water quantity, quality, and protection

“…If the horizontally propagated pressure increase is sufficiently large, upward water migration and groundwater contamination can occur through permeable vertical pathways, such as abandoned wells (33) or geologic faults (34). Upward migration of resident brine or fracturing fluids requires pressure gradients that can overcome gravity forces and is controlled by subsurface conditions and various fluid and porous media properties (34)(35)(36)(37). Salinity has been identified as a key variable controlling brine/saline water densities (38).…”

Hydraulic fracturing and the environment: risk assessment for groundwater contamination from well casing failure

“…As it is evidenced in multiple field studies for Marcellus Shale and other shale formations (e.g. Saiers and Barth 2012;Cohen et al 2013;Reagan et al 2015), it is not a valid assumption to model the formation underneath the aquifer as a fully-saturated medium. This medium is, in general, partially filled with water.…”

“…In a more recent study, the possibility of contaminant transport through a faulting system has been reported using a 2D single phase multicomponent model (Gassiat et al 2013). But that study did not investigate the risk characterization within a 3D modeling scope, and did not consider multiphase flow and transport, nor the associated capillary effects (Reagan et al 2015).…”