Abstract
Understanding the transport of hydraulic fracturing (HF) fluid that is injected into the deep subsurface for shale gas extraction is important to ensure that shallow drinking water aquifers are not contaminated. Pressure gradients, permeable pathways such as faults or improperly abandoned wellbores, and the density contrast of the HF fluid to the surrounding brine could encourage upward HF fluid migration. In contrast, very low shale permeability and well production may work to keep HF fluid at depth and remove it from the subsurface. Single-phase flow and transport simulations are performed to quantify how much HF fluid is removed via the wellbore and how much reaches overlying aquifers. If a permeable pathway connects the shale reservoir to the overlying drinking water aquifer, the pressure transient due to injection and the density contrast allows rapid upward plume migration at early times, but well production reverses the direction of flow and removes a large amount of HF fluid from the subsurface. We present estimates of HF fluid migration to shallow aquifers during the first 1,000 years and show that the pressure transient from well operations should be included in subsequent numerical models while buoyancy may be neglected depending on depth and permeability.
1. INTRODUCTION
Hydraulic fracturing, often referred to as fracking, poses potential long-term risks to drinking water resources. One of the main concerns is that chemically treated hydraulic fracturing (HF) fluid and/or highly saline brine could migrate upwards from a shale gas unit and enter shallow drinking water aquifers. The HF fluid chemicals are used as viscosity reducers, biocides, surfactants, scaling inhibitors, and for other purposes [1]; while many of the additives are biodegradable and/or have low toxicity, approximately one third of the chemicals studied by Stringfellow et al. [2] do not have toxicity data, some are toxic, and some are known or suspected carcinogens. Of the 2-5 million gallons of HF fluid that is injected, 5-50% returns to the surface as flowback water [1], some unknown volume moves into the small pores of the shale via capillary imbibition, and the remaining volume is free to move within the subsurface, possibly towards drinking water aquifers. Public concern has prompted the EPA to study the environmental impacts of hydraulic fracturing, which include aquifer contamination via subsurface leakage [3], while other studies suggest that aquifer contamination via a subsurface pathway is virtually impossible or would occur over extremely long timescales (>106 years) [4,5].
