Monitoring CO2 migration in a shallow sand aquifer using 3D crosshole electrical resistivity tomography. (November 2015)

• Record Type: Journal Article
• Authors: Yang, Xianjin; Lassen, Rune N.; Jensen, Karsten H.; Looms, Majken C.
• Is Part Of: International journal of greenhouse gas control. Volume 42(2015:Nov.)
• Page Start: 534
• Page End: 544
• Publication Date: 2015-11
• Subjects: Monitoring -- Electrical resistivity tomography (ERT) -- Electrical conductivity (EC) -- Dissolved CO2 -- Gaseous CO2 -- Shallow aquifer -- Crosshole; Greenhouse gases -- Environmental aspects -- Periodicals; Air -- Purification -- Technological innovations -- Periodicals; Gaz à effet de serre -- Périodiques; Gaz à effet de serre -- Réduction -- Périodiques; Air -- Purification -- Technological innovations; Greenhouse gases -- Environmental aspects; Periodicals; 363.73874605
• DOI: 10.1016/j.ijggc.2015.09.005
• Languages: English
• ISSNs: 1750-5836

Abstract

Highlights: We monitored a controlled gaseous CO2 release in a shallow sand aquifer. Dissolved and gaseous CO2 had opposite effects on bulk electrical conductivity (EC). Conductive dissolved CO2 has a larger effect on ERT data than resistive gaseous CO2 . The bulk EC increase during the first 2 days of CO2 injection is smaller than 10%. Buoyancy, heterogeneity, groundwater flow and injector pressure control CO2 migration. Abstract: Three-dimensional (3D) crosshole electrical resistivity tomography (ERT) was used to monitor a pilot CO2 injection experiment at Vrøgum, western Denmark. The purpose was to evaluate the effectiveness of the ERT method for detection of small electrical conductivity (EC) changes during the first 2 days of CO2 injection in a shallow siliciclastic aquifer and to study the early-time behavior of a controlled small gaseous CO2 release. 45 kg of CO2 was injected over a 50-h period at 9.85 m depth. ERT data were collected using horizontal bipole-bipole (HBB) and vertical bipole-bipole (VBB) arrays. The combined HBB and VBB data sets were inverted using a difference inversion algorithm for cancellation of coherent noises and enhanced resolution of small changes. ERT detected the small bulk EC changes (<10%) from conductive dissolved CO2 and resistive gaseous CO2 . The primary factors that control the migration of a CO2 plume consist of buoyancy of gaseous CO2, local heterogeneity, groundwater flow and external pressure exerted by the injector. The CO2 plume at the Vrøgum site migrated mostly upward due to buoyancy and it also skewed toward northeastern region by overcoming local groundwater flow. The conductive eastern part is more porous and becomes the preferential pathway for the CO2 plume, which was trapped within the slightly more porous glacial sand layer between 5 m and 10 m depths. The gaseous and dissolved CO2 plumes are collocated and grow in tandem for the first 24 h and their opposite effects resulted in a small bulk EC increase. After raising the injection rate from 10 g/min to 20 g/min at the 24-h mark, the CO2 plume grew quickly. The bulk EC changes from ERT agreed partially with water sample EC and GPR data. The apparent disagreement between high CO2 gas saturation and prevailing positive bulk EC changes may be caused by limited and variable ERT resolution, low ERT sensitivity to resistive anomalies and uncalibrated CO2 gas saturation. ERT data show a broader CO2 plume while water sample EC had higher fine-scale variability. Our ERT electrode configuration can be optimized for more efficient data acquisition and better spatial resolution.