Key Reports:

State of the art review of CO2 Storage Site Selection and Characterisation Methods

3.3.3 The Frio Brine Pilot Experiments

The Frio Brine Pilot Experiment was designed to test storage performance of a typical subsurface environment in an area where large-volume sources and sinks are abundant, near Houston, Texas, USA (Fig. 3-3). The experiment site had two wells, a down-dip injector and a dedicated observation , which is 30 m up-dip of the injector. A relatively small volume of pure CO2Carbon dioxide (1,600 tonnes) was injected over a 10-day period into a high-permeability brine-bearing sandstone at 1500 m depth.

Reservoir simulations were carried out at each stage of the pilot using TOUGH2. Initial modelling was carried out to help design the experiment using probabilistic realisations constrained by predicted ranges of fluid properties and rock heterogeneity. As site-specific data became available, they were incorporated into the model. Model results were used to select the field site and the placement of the new injection well relative to the existing well (used as an observation ), to define the perforation and injection zone, and to optimize the volume and duration of injection. Simulated saturations of CO2Carbon dioxide were used to select appropriate monitoring tools.

Numerical simulation also showed that significant amounts of CO2Carbon dioxide would be trapped during the post-injection stage as the relative permeability to gas would decrease over time (two-phase trapping) (Hovorka et al., 2006). In addition, the modelling work has helped to identify significant areas of uncertainty that need to be resolved by field testing. Another usage of the flow model was to consider the heterogeneities that exist in the formation. Models show that CO2Carbon dioxide moves into the rock volume with a relatively smooth front at a rate proportional to zone permeability (Hovorka et al., 2005).

E. Fig . 3-3

Fig. 3-3: Frio experimental site setting showing geologic context near South Liberty Salt Dome and detail of injection well location in a gridded reservoir model made using seismic data of the fault block (Hovorka et al., 2006).

In the period between September and October 2006, the Frio-II brine pilot injected about 380 tonnes of CO2Carbon dioxide into the Blue sand of the Frio Formation. This 5-day injection was at the same site as the Frio-I pilot, but 150 m deeper (1657 m). The fluvial Blue sand is 17m thick, has a dip of 18°, approximately 30% porosity and permeability of 1 to over 4 Darcies. This small-scale pilot test was used to calibrate models and techniques, at an intermediate scale between core/logs and surface seismic methods, for extrapolation to the larger scales. An accurate model of the plume extent and spatial distribution in a small-scale pilot adds confidence to the model estimates of the key properties such as residual CO2Carbon dioxide saturation and the associated reservoir CO2Carbon dioxide storage capacity. Monitoring provides constraints to improve the model accuracy, but often the only quantitative measurements available are CO2Carbon dioxide 'breakthough' time and downhole P/T at an observation with the addition of sparsely conducted well logs. Daley et al. 2011 integrated the cross-well continuous active-source seismic monitoring (CASSM) data with the reservoir model to obtain an improved model of the CO2Carbon dioxide plume and the reservoir properties.