Engineered in situ treatment processes, such as in situ bioremediation, are being employed at many Department of Defense (DoD) installations to remediate contaminants such as volatile organic compounds (VOCs) in soil and groundwater. Many of these treatment processes involve the addition of biological and/or chemical amendments into subsurface aquifers. Several delivery techniques have been developed to facilitate and increase subsurface contact between treatment materials and contaminants, including hydraulic fracturing. Although fracture-based delivery strategies are being used to increase subsurface distribution of delivered treatment materials (i.e., amendments) within tight formations, demonstrating the actual achieved distribution using conventional borehole drilling methods can be ineffective, cost prohibitive, and accompanied by high uncertainty. In particular, emplaced fractures may lead to complicated three-dimensional geometries, which can be difficult to characterize using one-dimensional (e.g., wellbore) sampling approaches. This project focused on improving the ability to develop conceptual amendment delivery models for in situ bioremediation and is based on the premise that geophysical imaging of amendment emplacement via hydraulic fracturing can reduce uncertainty in design and performance monitoring phases, thereby increasing efficacy and cost effectiveness of the remedial treatment.
The objectives of this project were to (1) assess the utility of time-lapse geophysical methods for monitoring fracture development and the ability of fractures to distribute the remedial amendment and (2) use that information together with conventional soil core and groundwater monitoring data to quantify the cost-benefit of using geophysical methods to develop an optimal delivery strategy prior to full-scale remedial action. Spill Site 7 (SS7), the location of a trichloroethene (TCE) plume at F.E. Warren Air Force Base (FEW), was used for comparison of the fracturing results for this demonstration. A remedial action (RA) involving hydraulic fracturing and in situ bioremediation was conducted at SS7 in 2009. A particular benefit of using SS7 as a comparison for this demonstration is that an average radius of influence (ROI) has been estimated for SS7 using conventional (wellbore-based) approaches and full-scale RA costs have been documented. Therefore, the ROI can be used to assess the potential cost-benefit of using geophysical imaging technologies for field-scale implementation.
This project involved in situ bioremediation via hydraulic fracturing and the use of geophysical imaging to monitor fracture emplacement and amendment distribution. Seismic and electrical monitoring methods were the primary techniques selected for this project based on a conceptual model of geophysical detection limits of fracture and remediation processes. Laboratory studies were first conducted to confirm the conceptual models. Then a pilot study was conducted at SS7 to introduce the amendment (Hydrogen Release Compound® [HRC®]) with guar and proppant into the subsurface using hydraulic fracturing. Time-lapse electrical resistivity tomography (ERT) and seismic datasets were acquired before, during, and after fracture emplacement using both surface and crosshole-based configurations. Several seismic methods were tested, including continuous active source seismic monitoring (CASSM), which was the first-of-kind deployment for remediation monitoring. Other traditional datasets (such as groundwater contaminant concentrations, drilling pressure and injection monitoring, surface displacement and tilt meter measurements, and drill-back validation holes) were collected and used to constrain and validate the interpretation of geophysical measurements in terms of fracture and associated system responses.
The seismic and electrical monitoring methods were found to be useful in imaging fracture propagation and amendment distribution during the project. In particular, crosshole high frequency and CASSM seismic data were useful for quantifying the number and distribution of emplaced fractures, while the crosshole electrical data provided information about the distribution of amendments within and away from the fractures. A novel orbital S-wave crosswell source was also tested but technical limitations encountered during processing limited the utility of the datasets. Analysis of the geophysical results, both individually and in combination, was performed to determine the size of the area impacted by the fractures. After 1 year of post-fracture monitoring, quarterly geophysics datasets indicated a minimum fracture ROI of 9 meters (m) (29.5 feet [ft]) with a 0.3 m (1.0 ft) vertical impact zone and an amendment distribution ROI of 5.2 m (17 ft) with a 0.8 m (2.5 ft) vertical zone distribution. The geophysical imaging also led to an updated conceptual model of fracture and amendment emplacement for the site. For example, the geophysical data suggested that the fractures and distributed amendment did not emanate radially from the fracture initiation point but were offset; the fracture radius was larger than expected, but the amendment distribution radius was smaller. Additionally, the geophysics confirmed that fractures cannot always be successfully emplaced in the subsurface, as was expected based on drilling indications.
The geophysically-obtained ROIs were used for subsequent cost-benefit analysis. To evaluate the performance, the observed ROIs were compared to a standard design ROI of 6.1 m (20 ft), which was borrowed from the analogue SS7 site at FEW. The cost-benefit analysis suggests that use of geophysical methods could lead to a 20% reduction in fracture initiation points when compared to the standard design, meeting one of the key technical objectives for this project. Future deployment of similar geophysically-based fracture assessments could follow a similar series of steps: (a) a pilot fracture installation with imaging, (b) determination of geophysically-informed fracture ROI (both vertical and lateral), and (c) design of full-scale fracture treatment using determined fracture ROI constraints.
Several issues could potentially impact the use of geophysical data to provide high-quality data needed to design a full-scale remedial action. The first issue is the thoughtful design of the pilot study site, which should take into consideration the propagation characteristics of the different geophysical signals, the expected fracture distribution, and the costs of drilling wellbores for geophysical data acquisition. This project found that the fracture and amendment distribution where wellbore placement resulted in good geophysical signal coverage could be estimated with high confidence, but that certainty was low where geophysical coverage was poor or absent. Another consideration is the role of heterogeneity on fracture propagation characteristics. This project was designed to test and image induced fracture characteristics in two different lithologies. The project was unsuccessful at installing fractures in one of the lithologies. Although these results highlight the value of geophysical monitoring for understanding fracture distribution as a function of heterogeneity, the geology should be carefully considered during pilot study and full-scale design of fracture-based treatment. Finally, procurement of the CASSM geophysical method also presents an implementation issue. The CASSM system was developed at Lawrence Berkeley National Laboratory and is not commercially available unlike the other geophysical methods tested. However, the fabrication of the CASSM system is not onerous, and it could be built in the future by geophysical service providers.