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Chapter 5: Underground geological storage 237 be undertaken by sampling for CO2 or tracers in soil gas and near surface water-bearing horizons (from existing water wells or new observation wells). Surface CO2 fluxes may be directly measurable by techniques such as infrared spectroscopy (Miles et al., 2005; Pickles, 2005; Shuler and Tang, 2005). 5.6.3.2 Indirect techniques for monitoring CO2 migration Indirect techniques for measuring CO2 distribution in the subsurface include a variety of seismic and non-seismic geophysical and geochemical techniques (Benson et al., 2004; Arts and Winthaegen, 2005; Hoversten and Gasperikova, 2005). Seismic techniques basically measure the velocity and energy absorption of waves, generated artificially or naturally, through rocks. The transmission is modified by the nature of the rock and its contained fluids. In general, energy waves are generated artificially by explosions or ground vibration. Wave generators and sensors may be on the surface (conventional seismic) or modified with the sensors in wells within the subsurface and the source on the surface (vertical seismic profiling). It is also possible to place both sensors and sources in the subsurface to transmit the wave pulses horizontally through the reservoir (inter-well or cross-well tomography). By taking a series of surveys over time, it is possible to trace the distribution of the CO2 in the reservoir, assuming the free-phase CO2 volume at the site is sufficiently high to identify from the processed data. A baseline survey with no CO2 present provides the basis against which comparisons can be made. It would appear that relatively low volumes of free-phase CO2 (approximately 5% or more) may be identified by these seismic techniques; at present, attempts are being made to quantify the amount of CO2 in the pore space of the rocks and the distribution within the reservoir (Hoversten et al., 2003). A number of techniques have been actively tested at Weyburn (Section 5.6.3.3), including time-lapse surface three-dimensional seismic (both 3- and 9- component), at one-year intervals (baseline and baseline plus one and two years), vertical seismic profiling and cross-well (horizontal and vertical) tomography between pairs of wells. For deep accumulations of CO2 in the subsurface, where CO2 density approaches the density of fluids in the storage formation, the sensitivity of surface seismic profiles would suggest that resolution on the order of 2500–10,000 t of free- phase CO2 can be identified (Myer et al., 2003; White et al., 2004; Arts et al., 2005). At Weyburn, areas with low injection rates (<2% hydrocarbon pore volume) demonstrate little or no visible seismic response. In areas with high injection rates (3– 13% hydrocarbon pore volume), significant seismic anomalies are observed. Work at Sleipner shows that the CO2 plume comprises several distinct layers of CO2, each up to about 10 m thick. These are mostly beneath the strict limit of seismic resolution, but amplitude studies suggest that layer thicknesses as low as 1 m can be mapped (Arts et al., 2005; Chadwick et al., 2005). Seismic resolution will decrease with depth and certain other rock-related properties, so the above discussion of resolution will not apply uniformly in all storage scenarios. One possible way of increasing the accuracy of surveys over time is to create a permanent array of sensors or even sensors and energy sources (US Patent 6813566), to eliminate the problems associated with surveying locations for sensors and energy sources. For CO2 that has migrated even shallower in the subsurface, its gas-like properties will vastly increase the detection limit; hence, even smaller threshold levels of resolution are expected. To date, no quantitative studies have been performed to establish precise detection levels. However, the high compressibility of CO2 gas, combined with its low density, indicate that much lower levels of detection should be possible. The use of passive seismic (microseismic) techniques also has potential value. Passive seismic monitoring detects microseismic events induced in the reservoir by dynamic responses to the modification of pore pressures or the reactivation or creation of small fractures. These discrete microearthquakes, with magnitudes on the order of -4 to 0 on the Richter scale (Wilson and Monea, 2005), are picked up by static arrays of sensors, often cemented into abandoned wells. These microseismic events are extremely small, but monitoring the microseismic events may allow the tracking of pressure changes and, possibly, the movement of gas in the reservoir or saline formation. Non-seismic geophysical techniques include the use of electrical and electromagnetic and self-potential techniques (Benson et al., 2004; Hoversten and Gasperikova, 2005). In addition, gravity techniques (ground or air-based) can be used to determine the migration of the CO2 plume in the subsurface. Finally, tiltmeters or remote methods (geospatial surveys from aircraft or satellites) for measuring ground distortion may be used in some environments to assess subsurface movement of the plume. Tiltmeters and other techniques are most applicable in areas where natural variations in the surface, such as frost heave or wetting-drying cycles, do not mask the changes that occur from pressure changes. Gravity measurements will respond to changes in the subsurface brought on by density changes caused by the displacement of one fluid by another of different density (e.g., CO2 replacing water). Gravity is used with numerical modelling to infer those changes in density that best fit the observed data. The estimations of Benson et al. (2004) suggest that gravity will not have the same level of resolution as seismic, with minimum levels of CO2 needed for detection on the order of several hundred thousand tonnes (an order of magnitude greater than seismic). This may be adequate for plume movement, but not for the early definition of possible leaks. A seabed gravity survey was acquired at Sleipner in 2002 and a repeat survey is planned for 2005. Results from these surveys have not yet been published. Electrical and electromagnetic techniques measure the conducting of the subsurface. Conductivity changes created by a change in the fluid, particularly the displacement of high conductivity saline waters with low-conductive CO2, can be detected by electrical or electromagnetic surveys. In addition to traditional electrical or electromagnetic techniques, the self- potential the natural electrical potential of the Earth can be measured to determine plume migration. The injection of CO2 will enhance fluid flow in the rock. This flow can produce anPDF Image | CARBON DIOXIDE CAPTURE AND STORAGE
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