Baseline Fluid Geochemistry

Introduction

A program for permanent and direct monitoring of gases, for analyzing the composition of brines, and for determining the natural CO₂-flux at the ground surface was started at the CO₂SINK site. The gained data serve as a baseline for comparison with analyses, made later during and after injection of CO₂ and targeted to identify possible risk by leakage of the CO₂ storage reservoir at depth.

Instruments and methods used

To perform a regional and on-site survey on naturally occurring subsurface and surface gas concentrations and CO₂-flux rates at the Ketzin test site, special instruments are required for an accurate and reliable long-term field monitoring of CO₂ dissolved in ground waters as well as gases contained in well head space and soil.

Electrochemical Multi-Sensor Modules MSM (UIT Dresden) with an internal data logger (Figure 1) were judged to be most suitability for the monitoring of CO₂ dissolved in ground water wells. The system allows the unattended long term (several weeks) monitoring of pH, dissolved carbon dioxide, temperature, electrical conductivity and water level.

	Figure 1: Multisensormodul MSM for subsurface water well measurements (Umwelt und Ingenieurtechnik GmbH, Dresden)

For the surface CO₂-flux measurements a LI-8100 S, Li-COR Inc., Nebraska was chosen. The device is a portable, battery-operated system with a pressure and temperature compensated infrared CO₂/H₂O gas analyzer. The detector measures the change in the CO₂ concentration in the range of 0-3000 µmol/mol and H₂O concentration in the range of 0-80 mmol/mol, respectively. Additionally, the soil temperature and moisture are recorded simultaneously and become an integral part of the data set.

	Figure 2: LI-8100 automated soil CO2-flux System

A TVA-100B toxic vapor analyzer equipped with a flame ionization detector (FID) from Thermo Electron Corporation, is used to determine hydrocarbon gas concentrations in the soil. It measures combustible gases over a dynamic range from 0-50.000 ppm.

	Figure 3: TVA-100B vapor analyzer with FID

Field Monitoring at Ketzin

To gain adequate data on the natural local and seasonal background CO₂-flux variations, we carried out repeated measurements at 20 locations (the distance between each location is about 400m) at 60 cm depth and were thus able to obtain an accurate measure of spatial variability in the test area. These measurements are performed once every 4 weeks since January 2005. The hydrocarbon analyses are conducted at 10 sites at 2 meter depth in the soil since February 2006. Since January 2005, the electrochemical determinations of CO₂ in two water wells were repeated every 30 minutes. Additionally, a meteorological station is set up to monitor the local weather conditions (ambient temperature, pressure, rainfall, wind, moisture). These data are necessary to correlate and, if necessary, correct the geochemical data.

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