The shear-thinning technology is designed to enhance delivery of remedial amendments to low permeability (low-k) zones for which treatment is typically limited when standard amendment delivery processes are utilized. Improved distribution of injected fluids is achieved by exploiting the rheological properties of shear-thinning fluids (STFs) during injection and transport within a formation, such as cross-flow from high-k to low-k zones. The overall objective of this project was to demonstrate and validate the use of STFs for enhanced delivery of bioremediation amendments at a chlorinated solvent-impacted site and to develop guidance for their use at other sites.
The term “shear-thinning” is applied to fluids to describe their dynamic viscosity-reducing behavior when shear rates are increased. Shear-thinning fluids are non-Newtonian, meaning that their viscosities exhibit a temporary drop when the applied shear rate is increased. A viscosity-modifying shear force can be applied using methods as simple as mixing or shaking of the solution, or—in the context of subsurface delivery—by injecting the fluid through a well screen and into porous media. For the enhanced amendment delivery process, a non-toxic biodegradable polymer, such as xanthan gum, is added to the injection solution to form a non-Newtonian fluid with shear-thinning properties. The shear-thinning behavior causes a more significant viscosity reduction to the fluid flowing through the lower-k zones relative to the viscosity reduction of the fluid flowing in higher permeable zones, i.e., the fluid mobility in the higher-k zone is controlled. Therefore, the preferential flow through the more permeable zones is significantly reduced while the flow into the lower-k zone is increased. In addition, mobility reduction behind the viscous injection fluid front in a higher-k layer creates a transverse pressure gradient that drives cross-flow of viscous fluids into adjacent less permeable layers. These mechanisms result in an improvement in the sweep efficiency within a heterogeneous system. The remedial amendments added to the shear-thinning solution therefore can be delivered to low-k zones that otherwise would be bypassed.
Once injection stops, the injected fluid viscosity increases and creates a more stable zone for biodegradation reactions because the amendment-laden fluid with high viscosity is not as easily displaced by flow from upgradient groundwater. The persistence of the delivered amendment helps to minimize inefficiencies associated with supplying sufficient electron donor to reduce competing electron acceptors, and the appropriate conditions for promoting microbial growth and activity can be maintained over a longer period of time. Over time, the xanthan gum will degrade and is anticipated to act as a long-term carbon source as the treatment zone returns to pre-treatment hydraulic conditions.
The technology demonstration was performed using a combination of xanthan gum (shear-thinning polymer) and ethyl lactate (carbon substrate) to promote biological reductive dechlorination in a low-level trichloroethene (TCE) plume at Joint Base Lewis McChord. The formation consisted of mixed glacial till and outwash with considerable small-scale heterogeneity and preferential pathways.
An evaluation of project results yielded the following key conclusions:
- Relative to a comprehensive baseline injection using a water-only solution, the STF injection resulted in improved breakthrough and distribution characteristics in the majority of monitoring locations. The percentage of the treatment zone covered by the amendment after pumping just over 1 pore volume increased from 49% without the STF to 69% with the STF, representing an improvement in the sweep efficiency of 41%.
- The STF injection successfully distributed measurable levels of organic carbon to the treatment zone, including the majority of lower-k layers. A significant portion of the amendment persisted through the end of the 8-month performance monitoring period, with evidence for enhanced persistence in the lower-k zones relative to higher-k zones.
- The amendment resulted in the complete degradation of the parent compound (TCE) throughout the heterogeneous treatment zone, with no evidence of rebound.
Based on the results of this demonstration and other applications, this technology is most appropriate for aquifers with permeability contrasts less than 2 orders of magnitude and/or thin low-k layers (< 0.5 m) unless distribution to the center of the layer is unnecessary (e.g., interface treatment to reduce flux). This permeability contrast would be equivalent to silt layers present within a sand matrix, but not clay layers. Other recommendations include: (1) a default static viscosity of approximately 100 cP for STF fluids in the absence of supporting data; and (2) adjusting the STF injection rate based on a pre-determined maximum pressure limit.
There are no significant regulatory or end-user concerns with using this technology, primarily due to its similarity to an existing treatment technology (in situ bioremediation).
Costs for this technology are moderate on a per injection event basis because of its similarity to conventional bioremediation. Primary requirements are extra time for hydration of the polymer solution and the cost of the material itself. However, there is a potential for significant life-cycle cost savings due to fewer injection events (i.e., the viscous polymer is more persistent in the subsurface) and shorter remediation timeframes (i.e., more effective treatment as a result of improved substrate distribution).