Testing efficiency of storage in the subsurface: frio brine pilot experiment Page: 3 of 6
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Well Construction and Permitting
The experimental well has been permitted by the Texas Commission on Environmental Quality, Underground
Injection Control Division, as a Class 5 experimental well with concurrence of the Texas Railroad Commission.
However, to assure conformance of injection, well engineering has been adapted from Class 1 techniques using a
design developed by Sandia Technologies LLC. Detailed characterization of the area of review was provided in a
report to accompany the Class 5 application , cement was injected behind casing for the entire well, and
stakeholders were informed through public meetings. The observation well originally had a typical oil-production
well construction, with no cement behind casing between the top of the production interval (2,417 m) and the base
of surface casing (621 m). To control behind-casing leakage and allow isolation of the injection zone for
observation, we have done remedial cement squeezes behind casing to attempt to limit annular flow. The injection
will test the success of this remediation and the methods for measuring potential leakage. We also completed a
report assessing environmental impacts of the experiment for National Energy Policy Act (NEPA) review .
Modeling using TOUGH2 has helped us identify significant areas of uncertainty that will be resolved by field
testing. TOUGH2 is a general-purpose simulator for multiphase flows in porous and fractured media ; here we
use a fluid-property model for supercritical CO2 . One key uncertainty is to determine the appropriate conceptual
model for two-phase behavior of CO2 and brine within a heterogeneous permeability system at field scale. We focus
here on two aspects: how CO2 will move into the pore system under pressure from injection and how it will drain out
of the pore system under gravity. An introduced immiscible fluid has a widely observed tendency to minimize the
area wetted. As injection proceeds, the introduced fluid may preferentially move into areas that have already been
wetted by that fluid and by-pass areas that have not been wetted, a process described as fingering. Analysis 
suggests that hydrodynamic causes of fingering may be millimeter to meter scale and therefore not significant for
field-scale problems; however, stratigraphic heterogeneity may result in significant channeling of flow and
bypassing of unwetted rock. To date, modeling done on this project has discretized the rock volume into
stratigraphically defined cells averaging 1 m in thickness (figure 2), an interpretation that seems appropriate for
heterogeneity observed in core and on logs in massive sandstones that will be major flow units. Models show that
CO2 moves into the rock volume with a relatively smooth front at a rate proportional to zone permeability (figure 3,
Case 1). To test the validity of this conceptualization, observation of saturations using Schlumberger's RST tool will
document the shape and evolution of the plume as it moves through the rock volume and past the observation well.
During the injection period (modeled as 14 days), flow is dominated by pressure at the well and is dominantly
radial out from the injection well. At the end of injection, the pressure gradient from the well to the rock volume
declines, and gravity becomes a significant force, moving CO2 upward. Gravitationally driven flow occurs both bed-
parallel within these steeply dipping units, resulting in elongation of high-permeability plumes, and upward.
Because highest permeability in the upper part of the Frio "C" is in the reworked sandstone at the top, both factors
lead to maximum spreading of the plume within the upper layers. Some of the CO2 that enters each pore will be
trapped by capillary forces and left behind as residual saturation (SGR). Data compiled from the literature 
suggest that the appropriate SGR for Frio conditions could be 30 percent. Comparing base Case 1, SGR =.05, with
Case 2, SGR =.3 (figures 3 and 4), shows the significance of residual saturation in controlling the fate of CO2. The
Case 2 high SGR results in more retention of CO2 at the injection well, in turn resulting in slower breakthrough to the
observation well. The largest change is observed after injection, when, within a year, a higher SGR results in near
immobilization of the CO2 plume close to the injection site (figure 3). If residual saturation is less effective, a much
larger proportion of CO2 remains mobile and moves away from the injection site to form a more extensive plume. In
either case, modeling suggests that CO2 will ultimately dissolve into the pore water and pressure will decline back
toward initial conditions.
Assessment of Results
Matching detailed data collected at the injection site over a short (less than 1-year) period to model results can
confirm the validity of conceptual models or suggest improvements in numerical inputs and assumptions.
Breakthrough to the observation well will be measured using diverse techniques, including saturation measurements
using the wireline RST tool, fluid real-time fluid sampling, and detection of natural and introduced tracers in the
injected CO2. Changes in saturation at the observation well measured over time will refine our understanding of the
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Hovorka, Susan D.; Doughty, Christine & Holtz, Mark. Testing efficiency of storage in the subsurface: frio brine pilot experiment, article, June 30, 2004; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc782320/m1/3/: accessed March 22, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.