Rich Hall

Rich did a research project investigating the effects of hydraulic fractures on air sparging. He conducted field tests, which involved injection of air at constant pressure while measuring the transient flow rate, and he also did theoretical analyses of the sparging process using T2VOC. His thesis is available here.

Introduction from Rich's Thesis

Air sparging has become a popular alternative to conventional pump-and-treat methods of remediating contaminated sites during the past 20 years. It is applied by injecting air in the vicinity of contaminants in the saturated zone. As air rises through the saturated zone contaminants partition into the vapor phase and then flow into the vadose zone. Soil vapor extraction wells are typically installed above the air sparging wells to remove contaminant-laden gas from the vadose zone (Kerfoot, 1992). Another benefit of air sparging is an enhanced rate of biodegradation because oxygen partitioning from the injected air into the aqueous phase will stimulate aerobic microbes (Johnson, 1998).

The conditions conducive to successful air sparging application are limited. Air sparging is effective when used to remediate high volatility compounds, whereas it can be ineffective for remediation of low volatility compounds (Liban, 2001). Air sparging effectiveness can also be limited in low-permeability formations where air injection rates are small. The distribution of air in the subsurface may be limited by formation heterogeneity (McCray and Falta, 1996).

Laboratory investigations have shown that air flow in porous media during air sparging typical occurs through continuous channels (Ji et al, 1993). The size and spacing of channels is an important factor in the contaminant mass transfer rate from the aqueous phase to the gaseous phase. This is because the contaminant must diffuse through water to reach the phase interface at a channel (Falta, 2000).

Hydraulic fracturing involves pumping fluid into a geologic formation at a rate great enough to cause pressures that will initiate a fracture. Hydraulic fracturing was originally developed in the petroleum industry, and has been used for more than 60 years to stimulate the productivity of oil and gas wells (Gidley, 1989). It is possible to create hydraulic fractures by only injecting fluid; however, granular material termed proppant is typically suspended in the fracturing fluid during injection (Murdoch et al, 1994). The proppant holds the fracture open after injection is completed. A permeable proppant increases hydraulic fracture transmissivity by increasing the thickness, or aperture, of the fracture.

The beneficial effects produced using hydraulic fracturing techniques have led to environmental remediation applications (Murdoch and Slack, 2002). The ability to introduce proppants makes it possible to design hydraulic fractures for a wide variety of applications. For example, granular graphite can be used as a proppant to create disk-shaped electrodes for electro-kinetic remediation systems (Murdoch, 1997). Hydraulic fractures can also be filled with reactive solids that increase the in situ degradation rate of contaminants (Murdoch and Slack, 1992). Biodegradation can also be accelerated by filling fractures with oxygen-releasing compounds or nutrients (Vesper et al, 1994b).

The most common environmental application of hydraulic fractures involves increasing well performance by using well-sorted quartz sand to create transmissive layers in low permeability formations. Quartz-sand-filled fractures typically increase well production by 1.5 to 10 times (Murdoch, 1994, Bradner, 2003), although productivity increases of up to two orders of magnitude have been observed (Vesper et al, 1994a).

Creating hydraulic fractures in air injection wells could potentially expand the applicability of air sparging to encompass low permeability materials. Hydraulic fractures have been installed in air sparging wells in a few cases; however, field tests for characterizing air sparging are limited. The effects of hydraulic fractures on sparging well performance or subsurface air flow are unclear.

Objective

The objective of this investigation is to develop methods for quantifying and comparing the performance of air sparging wells. These methods will be developed to characterize the effects of sand-filled hydraulic fractures on air sparging operations. Of particular interest is comparing the performance of sparging wells that intersect hydraulic fractures to the performance of conventional air sparging wells.

Approach

In order to meet the objectives of this study, a total of eight wells with screens approximately 14 m below ground surface were installed using three different completion methods. The wells are in a granitoid gneiss saprolite at the Simpson Station field site in Pendleton, South Carolina. Three field sparging tests were developed to characterize well performance and formation properties. The three tests are all variations of a pressure step test, involving some number of injection periods at constant pressures while monitoring mass flow.

The multi-phase flow simulator T2VOC (Falta et al, 1995) was also utilized as an investigative tool during this research. Models were calibrated to Simpson Station field conditions with PEST-ASP by reproducing field sparging test results. The calibrated models were used to characterize gas saturation patterns and sparging radius of influence of the wells at the field site. Some model parameters were then varied to characterize well performance under different formation conditions.

Last Updated: August 28, 2007 -- Questions or comments, contact Larry Murdoch.
School of Environment, Department of Geological Sciences
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