Geology topics

C. Gregoire

Publications and source records attributed to C. Gregoire.

3 recordsLinked to original sources

USGS2006

Use of borehole radar reflection logging to monitor steam-enhanced remediation in fractured limestone--Results of numerical modelling and a field experiment

Ground penetrating radar is an efficient geophysical method for the detection and location of fractures and fracture zones in electrically resistive rocks. In this study, the use of down-hole (borehole) radar reflection logs to monitor the injection of steam in fractured rocks was tested as part of a field-scale, steam-enhanced remediation pilot study conducted at a fractured limestone quarry contaminated with chlorinated hydrocarbons at the former Loring Air Force Base, Limestone, Maine, USA. In support of the pilot study, borehole radar reflection logs were collected three times (before, during, and near the end of steam injection) using broadband 100 MHz electric dipole antennas. Numerical modelling was performed to predict the effect of heating on radar-frequency electromagnetic (EM) wave velocity, attenuation, and fracture reflectivity. The modelling results indicate that EM wave velocity and attenuation change substantially if heating increases the electrical conductivity of the limestone matrix. Furthermore, the net effect of heat-induced variations in fracture-fluid dielectric properties on average medium velocity is insignificant because the expected total fracture porosity is low. In contrast, changes in fracture fluid electrical conductivity can have a significant effect on EM wave attenuation and fracture reflectivity. Total replacement of water by steam in a fracture decreases fracture reflectivity of a factor of 10 and induces a change in reflected wave polarity. Based on the numerical modelling results, a reflection amplitude analysis method was developed to delineate fractures where steam has displaced water. Radar reflection logs collected during the three acquisition periods were analysed in the frequency domain to determine if steam had replaced water in the fractures (after normalizing the logs to compensate for differences in antenna performance between logging runs). Analysis of the radar reflection logs from a borehole where the temperature increased substantially during the steam injection experiment shows an increase in attenuation and a decrease in reflectivity in the vicinity of the borehole. Results of applying the reflection amplitude analysis method developed for this study indicate that steam did not totally replace the water in most of the fractures. The observed decreases in reflectivity were consistent with an increase in fracture-water temperature, rather than the presence of steam. A limiting assumption of the reflection amplitude analysis method is the requirement for complete displacement of water in a fracture by steam.

Maine↗

USGS2006

Use of borehole radar tomography to monitor steam injection in fractured limestone

Borehole radar tomography was used as part of a pilot study to monitor steam‐enhanced remediation of a fractured limestone contaminated with volatile organic compounds at the former Loring Air Force Base, Maine, USA. Radar tomography data were collected using 100‐MHz electric‐dipole antennae before and during steam injection to evaluate whether cross‐hole radar methods could detect changes in medium properties resulting from the steam injection. Cross‐hole levelrun profiles, in which transmitting and receiving antennae are positioned at a common depth, were made before and after the collection of each full tomography data set to check the stability of the radar instruments. Before tomographic inversion, the levelrun profiles were used to calibrate the radar tomography data to compensate for changes in traveltime and antenna power caused by instrument drift. Observed changes in cross‐hole radar traveltime and attenuation before and during steam injection were small. Slowness‐ and attenuation‐difference tomograms indicate small increases in radar slowness and attenuation at depths greater than about 22 m below the surface, consistent with increases in water temperature observed in the boreholes used for the tomography. Based on theoretical modelling results, increases in slowness and attenuation are interpreted as delineating zones where steam injection heating increased the electrical conductivity of the limestone matrix and fluid. The results of this study show the potential of cross‐hole radar tomography methods to monitor the effects of steam‐induced heating in fractured rock environments.

Maine↗

USGS2005

Steam injection pilot study in a contaminated fractured limestone (Maine, USA): Modeling and analysis of borehole radar reflection data

Steam-enhanced remediation (SER) has been successfully used to remove DNAPL and LNAPL contaminants in porous media. Between August and November 2002, SER was tested in fractured limestone at the former Loring Air Force Base, in Maine, USA. During the SER investigation, the U.S. Geological Survey conducted a series of borehole radar surveys to evaluate the effectiveness of radar methods for monitoring the movement of steam and heat through the fractured limestone. The data were collected before steam injection, 10 days after the beginning of injection, and at the end of injection. In this paper, reflection-mode borehole radar data from wells JBW-7816 and JBW-7817A are presented and discussed. Theoretical modeling was performed to predict the variation of fracture reflectivity owed to heating, to show displacement of water and to assess the effect of SER at the site. Analysis of the radar profile data indicates some variations resulting from heating (increase of continuity of reflectors, attenuation of deeper reflections) but no substantial variation of traveltimes. Spectral content analysis of several individual reflections surrounding the boreholes was used to investigate the replacement of water by steam in the fractures. Observed decrease in radar reflectivity was too small to be explained by a replacement of water by steam, except for two high-amplitude reflectors, which disappeared near the end of the injection; moreover, no change of polarity, consistent with steam replacing water, was observed. The decrease of amplitude was greater for reflectors near well JBW-7817A and is explained by a greater heating around this well.

Maine↗