PNNL-13501 - Pacific Northwest National Laboratory

Effluent pH and electrical conductivity were monitored
continuously with a computer data acquisition and control
system. Total mass of the column was also monitored
and used to calculate the pore water saturation level
during the tests. After test termination, samples were
removed from the column and analyzed for major mineral
phases with powder x-ray diffraction and optical
microscopy.
The second set of experiments involved sequential
flooding of a 500 millidarcy Berea sandstone core with a
saturated Ca(OH)2 solution and pressurization with CO2.
The radial area of the sandstone was sealed with an
ultraviolet resin prior to emplacement in a pressurized
unsaturated flow column. The amount of pore water
injected and extracted during each cycle was recorded to
estimate the change in porosity from calcite precipitation.
Unsaturated hydraulic conductivity and water retention as
a function of capillary pressure were measured before and
after the CO2 injections with an ultracentrifugation
method (McGrail et al. 1999b). X-ray microtomography
was used to image pore structure changes as well.
Results and Accomplishments
Pressurized Unsaturated Flow Experiments
Figure 2 shows the results from the computer-monitored
test metrics. Initially, deionized water was injected into
the column at 0.25 mL/h to establish baseline conditions.
After 34 hours, a 1.5 M Ca(NO3)2·4H2O solution was
injected. However, calcite did not form in this solution
due to the formation of a Ca(NO3) + complex. After
55.5 hours, a saturated Ca(OH)2 solution was then
injected. Breakthrough was observed after 8 hours and
the electrical conductivity dropped dramatically,
indicative of CaCO3 precipitation. The calculated
equilibrium pH of a fluid saturated with respect to calcite
and quartz at a pCO2 of 1.8 bar is 5.8, which is close to
the observed effluent pH.
Percent Water Saturation
100
10
90
80
Quartz Sand
425-250 µm Particles
23°C, pCO = 26.7 psia
2
Ca(OH) 2
Breakthrough
9
70
60
DIW
0.25 mL/h
1.5 M Ca(NO 3 ) 2
0.25 mL/h
Sat Ca(OH) 2
0.25 mL/h
Sat Ca(OH) 2
6 mL/h 8
50
40
7
30
6
20
10
0
Ca(NO ) Breakthrough
3 2
θ
pH
Ω
5
4
0 10 20 30 40 50
Time, h
60 70 80 90 100
Figure 2. Computer-monitored test metrics from
pressurized unsaturated flow test
After 74 hours, the Ca(OH)2 influent solution was doped
with a cresol red plus thymol blue pH indicator, and the
flow rate increased to 6 mL/h. Water content in the
column increased to about 30% saturation but the effluent
pH remained near pH 6. Visual observations showed that
the blue color (indicating alkaline pH) remained within a
few millimeters of the CO2 injection port. Essentially all
of the calcite was formed within 10% or less of available
pore space in the column. Consequently, simple injection
of CO2 into a host rock would cause pore plugging
immediately around the injector and poor use of the
reservoir capacity for sequestering CO2.
To solve this problem, we developed a modified injection
strategy. The column was flooded with the saturated
Ca(OH)2 solution and then pressurized with CO2. The gas
supply was then turned off and the residual pore water
allowed to drain. A time-lapse series of photographs
shown in Figure 3 illustrates the procedure. The change
in pore water pH after CO2 injection was visible,
indicating more extensive use of the available pore
volume for CaCO3 precipitation. The column was then
reflooded with the Ca(OH)2 solution and the procedure
repeated a total of 15 times. As seen in Figure 3, rapid
neutralization of the pore water and concomitant
0 13 min 28 min 38 min 50 min
Figure 3. Time lapse photos of pulsed CO 2 injection experiment. Blue color is a cresol red plus thymol blue pH indicator dye,
with a critical pH for color transition of 8.3.
pH
log 10 Conductivity, µS/cm
Earth System Science 225
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