(559v) Using CO2-Responsive Nanogel to Improve CO2 Flooding Sweep Efficiency and CO2 Storage Efficiency

Using CO₂-Responsive
Nano-gel to Improve CO₂ Flooding Sweep Efficiency and CO₂
Storage Efficiency

Introduction

Due to the size limitation,
millimeter-sized particle gels can only penetrate in the fractures or channels,
which are mainly focused on the near wellbore problems. However, the in-depth
plugging cannot be achieved by conventional millimeter-sized particle gel
treatment. Therefore, the nano-particle gels are
developed based on the demand of the in-depth plugging. In this study, a novel nano-gel with CO2 responsive property is proposed for the
first time. The CO2-responsive nano-gel can be
stimulated by CO2 to increase particle size; therefore to provide additional
plugging to CO2 flooding sweep efficiency and CO2 storage efficiency.

            The novel CO₂-responsive
nano-particle gel was examined using sandstone cores
in order to study the transportation behavior and whether it can form an
in-depth plugging during CO₂ flooding. Six cores were used in
this experiment and were divided into two groups: one is the group with CO₂
and another is the group without CO₂. The permeability of the core
is 40, 130, and 370 md, respectively. The
plugging efficiency of the group without CO₂ was measured directly
after nano-gel injection, while the
plugging efficiency of the other group was measured after CO₂
stimulation in the high-pressure vessels using brine. Transportation
test of nano-gel was conducted using homogeneous
sandstone cores. Five pressure sensors are installed along the core evenly in
order to monitor the nano-gel transportation. The nano-gel was injected into the core until
the third pressure sensor has reading, then the brine
is injected into the core until the fourth pressure sensor has reading. After
placing the nano-gel in the middle of the
core, supercritical CO₂ was injected into the core until CO₂
breakthrough from the outlet. The core was then soaked in supercritical CO₂
for 7 days to allow the interaction between the nano-gel and supercritical CO₂.
The plugging behavior
was tested by
the following water flooding

1. Materials

1.1 CO₂-responsive nano-gel

CO₂-responsive
nano-gel has a novel function that it can increase
its swelling ratio under CO₂ condition. Therefore, after placing the
nano-gel in the in-depth, the nano-gel
can swell more during the post CO₂flooding,
hereby divert the CO₂ to unswept area to
increase oil recovery. In this experiment, the concentration of CO₂-responsive
nano-gel is 2,000 ppm and the nano-gel
solution is prepared with 1% NaCl brine.

1.2  Sandstone rock

The
sandstone rock used in this study has a permeability
around 100 md. The diameter of the core is 1.5 inch
and the length is 1 ft.

1.3  Supercritical CO₂

The
CO₂ used in this experiment is in supercritical CO₂
condition. The CO₂ is pressurized in an accumulator which is wrapped
with a heating pad, therefore, the pressure and the temperature is higher than
the CO₂ critical point to achieve supercritical condition.

2.      Experimental apparatus design

The
Figure 1 is the experimental apparatus design. The sandstone core is put into a
coreholder and secured by adding confining pressure.
Three pressure sensors are installed along the coreholder
evenly and two pressure sensors at inlet and outlet, respectively. The pressure
data is transmitted to the computer. The whole system is heat to 45¡æ. The back pressure regulator (BPR) is used to
increase the whole system pressure above CO₂ supercritical point.
Keeping both temperature and pressure above the supercritical point can ensure
that the CO₂ flow through the system is in the supercritical
condition. The nitrogen gas source is used to provide back pressure. The nano-gel and supercritical CO₂ are injected
using two accumulators.

Figure
1 Experimental apparatus design

3.      Experimental procedures

3.1
Nano-gel preparation

The nano-gel concentration
is 2,000 ppm and the solvent is 1% NaCl brine. After
dissolve the nano-gel in the brine, the solution is
put into 65 ¡æ
oven for 24 hours to allow the nano-gel reach an
equilibrium swelling ratio. The nano-gel is filtered
using 10 µm filter paper before use to remove any impurities.

3.2 
Core
flooding procedures

1)      Measure
the dry weight of the core. Vacuum the core and saturate the core with 1% NaCl
brine;

2)      Measure
core wet weight to determine the pore volume (PV);

3)      Set
back pressure at 1100 psi;

4)      Measure
sandstone core permeability with 1% NaCl brine using
flowrates of 1, 2, 3, 4 cc/min;

5)      Inject
0.5 PV of 2000 ppm nano-gel at 0.05
cc/min (1ft/day) flowrate (or inject nano-gel
until the second pressure sensor has reading);

6)      Inject
0.25 PV of 1% NaCl brine at 0.05 cc/min flowrate (or inject brine until the third
pressure sensor has a reading);

7)      Inject
supercritical CO₂ using 4 cc/min at 65 ¡æ until CO₂
breakthrough from the outlet;

8)      Keep
the CO₂ accumulator connected with the coreholder
and soak the core in supercritical CO₂ for 7 days;

9)      Inject
water using 1, 2, 3, 4 cc/min flowrates to test the plugging efficiency.

4. Results

4.1
CO₂-responsive nano-gel plugging
efficiency

As shown in Figure 2, nano-particle gel has the best plugging performance in low
permeability cores after CO₂ stimulation. After CO₂
stimulation, the residual resistant factors (Frr) of
most of the cores were improved, which represents that the novel CO₂-responsive
nano-gel plugging efficiency can be
improved in supercritical CO₂ condition. The plugging efficiency of nano-particle gel to brine is plotted in Figure 4.5. The
results show that the maximum plugging efficiency is achieved in 40 md and it decreases with the increase of rock
permeability. After CO₂ stimulation, the plugging efficiency to
brine is more than 90% in the 40 md core.

Figure 2 Plugging efficiency of nano-particle gels to brine

4.2
CO₂-responsive nano-gel transportation in
sandstone rock

4.2.1
Absolute permeability measurement and homogeneity test

As shown in Figure 3, the
differential pressure for each segment is almost the same, which indicates that
this sandstone core is nearly homogenous. The water absolute permeability for
this sandstone core is 98 md.

Figure
3 Absolute permeability measurement results

4.2.2 Nano-gel injection process and post water flooding process

0.43 pore volume (PV) of nano-gel was injected at a constant flowrate of 0.05 cc/min. As shown in the figure below, the
differential pressure for each segment is almost the same throughout the nano-gel injection process, which indicates that the 0.43
PV of nano-gel injection did not result in an
efficient plugging before CO₂ stimulation. Therefore, the nano-gel can transport into the in-depth without forming a
surface plug or near wellbore plug.

After placing the nano-gel in the core, a 0.25 PV
brine was injected into the core in order to displace the nano-gel
near the inlet and push the nano-gel moving towards
the in-depth. 

4.2.3
CO₂ injection and soaking process

The supercritical CO₂
was pressurized and heated in an accumulator. The CO₂ was injected
into the core at a constant flowrate of 4 cc/min until
the CO₂ breakthrough the core. The injection was terminated once the
CO₂ production was observed. Since the supercritical CO₂
broken through the core, the contact area between the CO₂ and the nano-gel increased, therefore, the nano-gel
stimulation under CO₂ condition was more efficient. The core was
then soaked in the supercritical CO₂ by keep connecting the coreholder with the CO₂ accumulator for 7 days.
The post waterflooding will be conducted to test the
plugging efficiency of the nano-gel after CO₂
stimulation.

4.2.4
Post waterflooding process

After soaking the core for 7 days,
1% NaCl brine was injected into the core at various
flowrates. The results indicate that the brine injection pressure increased
after nano-gel treatment, which revealed that the nano-gel can provide a sufficient plugging to brine after
soaking in supercritical CO₂. The residual resistant factor for each
segment and the whole core was calculated and plotted in Figure 4. As shown in
the figure, the segment 2 has the highest resistance to water flow, while the Frr for segment 3 and 4 was close to 1, which indicated
that the nano-gel was not penetrate into segment 3
and 4 or did not form a sufficient plugging.

Figure
4 Water residual resistant factor for each segment
after nano-gel treatment

Acknowledgement

The
authors would like to express their gratitude for the financial support
provided by the DOE under contract DE-FE0024558.