Monitoring CO2 migration in a shallow sand aquifer using 3D crosshole electrical resistivity tomography

Majken C Looms,Xianjin Yang,Karsten H Jensen,Rune N Lassen

doi:10.1016/j.ijggc.2015.09.005

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. 45kg of CO2 was injected over a 50-h period at 9.85m 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 5m and 10m depths. The gaseous and dissolved CO2 plumes are collocated and grow in tandem for the first 24h and their opposite effects resulted in a small bulk EC increase. After raising the injection rate from 10g/min to 20g/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.

Highlights

A primary risk with geological carbon sequestration is the leakage of CO2 into a fresh groundwater aquifer from a deep storage
Our electrical resistivity tomography (ERT) monitoring differs from Auken et al (2014) and Doetsch et al (2015) in that (1) we used crosshole ERT instead of surface ERT; (2) our pilot experiment had a shorter injection duration and a much smaller CO2 injection volume (45 kg in 2 days versus 1600 kg in 72 days); (3) our study focused on small changes during the early stage of gaseous and dissolved CO2 plume growth; (4) we had ground penetration radar (GPR) data to aid in identification of the likely effect from gaseous CO2
The primary factors that control the migration of a CO2 plume consist of buoyancy of gaseous CO2, sediment heterogeneity, groundwater flow and external pressure exerted by the injector

Summary

Introduction

A primary risk with geological carbon sequestration is the leakage of CO2 into a fresh groundwater aquifer from a deep storage. Electrical conductivity (EC) has become a proven and effective indicator for detection of dissolved CO2 in a vadose zone (Strazisar et al, 2009; Zhou et al, 2012) or in a shallow aquifer (Denchik et al, 2014; Trautz et al, 2013; Lamert et al, 2012; Auken et al, 2014) and for detection of supercritical CO2 in a deep saline formation (Kiessling et al, 2010; Bergmann et al, 2012; Carrigan et al, 2013)

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Conclusion