When reagents that stimulate biological or chemical destruction of contaminants can be mixed uniformly with target contaminants in the subsurface, remediation practitioners can have a high degree of confidence that the treatment will be reasonably effective. While this represents an enormous opportunity for the industry, the formidable challenge remains of ensuring that mixing and/or contact of biological or chemical treatment reagents with target contaminants occurs in a reasonable time frame in low-permeability or fractured geological settings.
This study demonstrated the use of permeability enhancement technology (i.e., environmental fracturing) to facilitate enhanced amendment delivery and distribution in low-permeability materials. The overall objective of this project was to compare the performance and cost benefits of hydraulic and hybrid pneumatic permeability enhancement for in situ treatment at low-permeability sites, as well as advanced monitoring techniques that can be used during implementation.
The goal of permeability enhancement technology is to increase bulk hydraulic conductivity and radius of amendment delivery to facilitate enhanced in situ remediation in low-permeability formations. A low- or high-viscosity fluid is introduced into a borehole at a rate and pressure high enough to overcome the in situ confining stress and the material strength of a geologic formation, resulting in the formation of a fracture. In high-viscosity permeability enhancement applications, sand can be injected simultaneously with a solid amendment such as zero-valent iron (ZVI) to maintain the integrity of the propagated fractures that can otherwise become restricted or collapsed entirely, particularly in plastic geologic materials. The emplaced fracture network typically results in an increase in hydraulic conductivity by about an order of magnitude and allows for more effective injections or extractions. Hydraulic permeability enhancement can be performed using almost any drilling technique, including direct-push.
Pneumatic permeability enhancement technology utilizes a gas at flow volumes exceeding the natural permeability of the formation to generate high enough pressures to overcome the in situ confining stress and the material strength of a formation such that fractures are formed. The result is the enhancement of existing fractures and planes of weakness (e.g., bedding planes) and the propagation of a dense fracture network surrounding the in situ delivery well. Once a geologic zone has been fractured, the injection of the amendment can be performed in an integrated process. For example, the amendment liquid or slurry can be blended into a nitrogen gas stream above ground and become atomized, and then introduced into the formation at relatively low pressures. The atomization apparatus is a down-hole injection assembly that consists of an injection nozzle with straddle packers that isolate and focus the injection to the target interval. Using this method, the amendment might be distributed 10 to 25 radial feet depending on site-specific conditions. As with hydraulic permeability enhancement, this fracture network extends the radius of influence for injection, thus enhancing in situ treatment. For this demonstration, a hybrid approach to pneumatic permeability enhancement was applied, where the nitrogen gas stream was used to generate the fracture network, followed by hydraulic delivery of the aqueous amendment.
Although much more sophisticated, tilt meters operate on the same principle as a carpenter’s level. Tilt meters contain two tilt sensors (on orthogonal axes) and precision electronics. As the tilt meter tilts, the gas bubble must move to maintain its alignment with the local gravity vector. The movement of the gas bubble within the conductive liquid causes a change in the total resistance between the electrodes. This resistance change is measured with a resistance bridge or voltage divider circuit to precisely detect the amount of tilt. Tilt data collected can be processed, analyzed, and converted into a dynamic, 3D graphical output that can be viewed in any perspective in space, and can be manipulated to view individual fracture configurations or the fracture network as a whole.
Electrical conductivity (EC) logging is utilized for high-resolution characterization of hydrostratigraphic conditions in unconsolidated media. Direct-push EC probes typically operate using a four-electrode Wenner array, passing current through the outer two electrodes and measuring voltage across the inner two electrodes. The sensors can collect 20 measurements per second and collect data at a vertical resolution of 0.05 foot. Clayey materials tend to have higher electrical conductivity and charge characteristics compared to sandy or gravelly soils. The high vertical resolution of the probe readings allows the user to identify fine-scale features such as low-permeability clay or silt lenses or sand stringers, which are important for transport of injected amendments in the subsurface.
Electrical resistivity tomography (ERT) is a geophysical visualization technique used to study hydrogeological characteristics of the subsurface. Resistivity, an inherent property of all materials, measures the degree to which a material resists the flow of an electrical current. As resistivity depends on chemical and physical properties such as saturation, concentration, and temperature, ERT can be used to monitor natural and anthropogenic processes responsible for changes in such properties. In the context of environmental engineering, ERT can aid in monitoring active remedial progress and provide insights into material emplacement and deformational processes, both of which are very relevant to in situ treatment technologies in general and the permeability enhancement technology in particular. In the context of this demonstration, ERT has the potential to aid in visualization of the 3D distribution of an injected fluid if the resistivity of that fluid is significantly different from the groundwater.
Despite the challenging subsurface conditions, more than 70% of the target injection volume was introduced into the subsurface via hydraulic permeability enhancement at the Lake City Army Ammunition Plant (LCAAP). Between 99 and 100% of the target injection volume was achieved within the treatment area at Marines Corps Base-Camp Pendleton, Site 1115 (MCB-CP) and the Grand Forks Air Force Base (GFAFB). No statistically significant changes in hydraulic conductivities were observed at demonstration sites where a sand proppant was not added, by design, including GFAFB and the hybrid pneumatic demonstration area at LCAAP. The lack of changes in hydraulic conductivities following hydraulic permeability enhancement at LCAAP is likely attributable to presence of voids, vertical and horizontal preferential pathways, and other uncertainties in the subsurface due to past disturbances within the hydraulic demonstration area. At MCB-CP, where a sand proppant was hydraulically emplaced, significant increases in hydraulic conductivities ranging between approximately three and 40 times were observed. At all three sites, significant changes in geochemistry and contaminant profile were observed at existing or new monitoring wells strategically placed within the anticipated radius of influence of the hydraulic initiation points. In addition, orders-of-magnitude higher injection rates and volumes were achieved using hydraulic permeability enhancement instead of conventional injection approaches. Note that at LCAAP, where a side-by-side comparison of hydraulic and hybrid pneumatic technologies was performed, the purely pneumatic approach to permeability enhancement could not be performed due to surfacing and, thus, fracture initiation was performed pneumatically while amendment delivery was achieved hydraulically (hybrid pneumatic technology). Despite the use of this more advanced hybrid approach, significantly higher total organic carbon (TOC) concentrations were observed in soil and groundwater within the hydraulic demonstration cell than in the pneumatic cell.
The accuracy and precision of tilt meter monitoring in predicting depth-discrete intervals where fractures were initiated and amendment was delivered were verified using soil confirmation sampling results. On the other hand, data collected indicated that while potentially useful in some applications, ERT was a partially effective geophysics monitoring tool for monitoring fracture initiation and subsequent amendment distribution. Data collected at GFAFB was inconclusive regarding evaluation of EC logging as a potentially applicable geophysics tool for fracture monitoring.
A cost comparison exercise was performed, which indicated that permeability enhancement techniques can be more or significantly more competitive than conventional injection techniques that are susceptible to unreasonably low injection rate and injection radius of influence, uncontrolled fracturing of the subsurface, and high reinjection frequency.
A variety of regulatory, procurement, and end-user issues may be encountered during permeability enhancement implementation. Regulatory issues may include overcoming the often-negative connotation associated with hydraulic fracturing for oil and gas and concerns regarding vertical migration of site contaminants as a potential unintended result of permeability enhancement. Procurement issues regarding permeability enhancement implementation generally center around the use of specialized equipment, chemistry, and technical knowledge that are only offered by few commercial vendors as well as the patented nature of certain permeability enhancement applications. End-user concerns with respect to the technologies include the use of nonstandard equipment required for implementation, hazards associated with high-pressure injections, and amendment surfacing. Note that in nearly all cases, proper planning and engineering controls can be used to mitigate concerns associated with field implementation of permeability enhancement technologies.