The effect of water flushing on CO2 concentration around injection well as identified
through laboratory study
Lanlan Jiang1,2 , Ziqiu Xue1, Hyuck Park1, Yongchen Song2
1
Research Institute of Innovative Technology for the Earth, Kizugawa City, Kyoto 619-0292, Japan
2
Key Laboratory of Ocean Energy Utilization and Energy Conservation of Ministry of Education,
Dalian University of Technology, Dalian, Liaoning116024, China
Abstract
After CO2 injection has ceased, a buffered fluid (brine) will be used to flush without exceeding
fracture pressure for the safety of well. To monitor the dynamic fluid flushing is useful for the
effectiveness and safety of CO2 geological storage. Dissolution trapping, dominantly the water
flushing process, is caused by dissolution of CO2 into water at interfaces and the transport of
dissolved CO2. The objective of this study was to investigate the effect of water flushing on CO2
concentration including displacement and the dissolution process on laboratory scale. The highresolution Magnetic Resonance Imaging (MRI) technique was used to visualize the process. The
core sample was packed with glass beads under the temperature and pressure of 40 and 8 MPa,
corresponding to a depth of 800 m. The average permeability is 13 Darcy and the average porosity
is 37%. The CO2 unsaturated water was injected upward into the supercritical CO2 saturated sample
with flow rate of 0.03 mL /min.
The transient intensity in MRI image during the water flushing was observed, reflecting the
transition from displacement to dissolution in the porous media (Fig.1). The flow before 5.50 PV
can be characterized by displacement with a compacted finger; the finger is nearly vertical,
indicating a strong effect of gravity. During this process, CO2 was easily displaced because of the
gravity force, viscous forces and capillary force. At the low flow rate, viscous forces could be
expected to be negligible compared. The gravity force suppressed the finger and relative uniform
advance occurred. In the study, total 28.5% residual saturation was obtained after displacement.
After 5.50PV, the mass transfer dominated by dissolution process. The movement of CO2
dissolution front from the inlet of water injection was visualized. Although the water displacement
front was relative flat, CO2 dissolution occurred along a specific path. Trapped CO2 dissolved into
water along the flow paths until the dissolution fronts breakthrough, and then other CO2 can be
dissolved. The injected water displaced the CO2 saturated water overtime.
With the small flow rate, the CO2 has sufficient resident time to travel through the porous
media and to have local equilibrium dissolution within pores of the flow paths. The concentration
distribution along the porous media was inhomogeneous. The CO2 concentration declined along the
porous media with injected pore volume (Fig. 2). The mean concentration decreased from 1.18 *102
to 1.33*10-4 kg/m3 for displacement and from 1.33*10-4 to 6.90 *10-4 kg/m3 for dissolution. The
results showed that little supercritical CO2 dissolved into water during the dissolution process. The
most of CO2 dissolved into water during displacement process because of the lower interface force
1
and larger interfacial area under supercritical condition. The maximum dissolved CO2 concentration
in water were below the solubility because of the non- equilibrium between displacement and
dissolution. After displacement, the CO2 concentration in water is closed to the maximum value at
the experiment conditions. And the dissolution rate for supercritical CO2 decreased from 10-6 to 10-7
Kg/m3.s.The total time for supercritical CO2 dissolution is short with 19.68 PV, meaning quickly
mass transfer of supercritical CO2. Most of important, the CO2 saturation decreased sharply from
28.5% to 3.44% during the dissolution process, which is useful for safety assessment in CO2
geological storage.
Displacement
1
i
0.28 1.24 1.71
2.19
2.67 3.15
X
s
3.62 4.11 5.50 PV
Sg
Y
0
Dissolution
11.66 12.04 12.32 12.70 12.99 13.28 13.66 14.04 15.09 PV
Fig .1 The intensity images during water flushing
0.030
CO2 Concentration/kg.m
-3
0.025
0.020
0.015
2.87PV
3.82PV
4.77PV
5.73PV
6.69PV
7.64PV
8.59PV
14.42PV
15.38PV
16.33PV
0.010
0.005
0.000
0
5
10
15
20
25
30
Position/mm
Fig.2 The concentration of CO2 during water flushing
2