(159an) Transportation and Plugging Behavior of CO2-Responsive Nanogel for CO2 Conformance Control and Storage

Due to the size limitation, millimeter-sized particle gels can only penetrate in the fractures or channels (Sun, 2017, 2018a, 2018b), which is 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 for in-depth plugging. In this study, the novel CO2-responsive nano-particle gel is injected into sandstone cores to study the transportation behavior and whether it can form an in-depth plugging during CO₂ flooding.

Core flooding experiments were conducted using homogeneous sandstone cores. Five pressure sensors are installed along the core evenly to monitor the nanogel transportation. The nanogel was injected into the core until the third pressure sensor has a reading, then the brine is injected into the core until the fourth pressure sensor has a reading. After placing the nanogel 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 nanogel and supercritical CO2. The plugging behavior was tested by the following water flooding.

Method

1. Materials

1.1 CO2-responsive nanogel

CO2-responsive nanogel has a novel function that it can increase its swelling ratio under CO2 condition. Therefore, after placing the nanogel in the in-depth, the nanogel can swell more during the post CO2 flooding, hereby divert the CO2 to unswept area to increase oil recovery. In this experiment, the concentration of CO2-responsive nanogel is 2000 ppm and the nanogel solution is prepared with 1% NaCl brine.

• Sandstone rock

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

• Supercritical CO2

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

2. Experimental apparatus design

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 the inlet and outlet, respectively. The pressure data is transmitted to the computer. The whole system is heated to 45℃. The backpressure 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 backpressure. The nanogel and supercritical CO2 are injected using two accumulators.

3. Experimental procedures

3.1 Nanogel preparation

The nanogel concentration is 2000 ppm and the solvent is 1% NaCl brine. After dissolving the nanogel in the brine, the solution is put into a 65 ℃ oven for 24 hours to allow the nanogel to reach an equilibrium swelling ratio. The nanogel is filtered using 10 µm filter paper before use to remove any impurities.

• Core flooding procedures
• Measure the dry weight of the core. Vacuum the core and saturate the core with 1% NaCl brine;
• Measure core wet weight to determine the pore volume (PV);
• Set back pressure at 1100 psi;
• Measure sandstone core permeability with 1% NaCl brine using flowrates of 1, 2, 3, 4 mL/min;
• Inject 0.5 PV of 2000 ppm nanogel at 0.05 mL/min (1ft/day) flowrate (or inject nanogel until the second pressure sensor has a reading);
• Inject 0.25 PV of 1% NaCl brine at 0.05 mL/min flowrate (or inject brine until the third pressure sensor has a reading);
• Inject supercritical CO₂ using 4 mL/min at 65 ℃ until CO₂ breakthrough from the outlet;
• Keep the CO2 accumulator connected with the coreholder and soak the core in supercritical CO2 for 7 days;
• Inject water using 1, 2, 3, 4 mL/min flowrates to test the plugging efficiency.

4. Results

4.1 Absolute permeability measurement and homogeneity test

As shown in Figure 2, 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.

4.2 Nanogel injection process and post water flooding process

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

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

4.3 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 mL/min until the CO2 break through 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 nanogel increased, therefore, the nanogel stimulation under CO2 conditions 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 nanogel after CO2 stimulation.

4.4 Post waterflooding process

After soaking the core for 7 days, 1% NaCl brine was injected into the core at various flowrates. The pressure data during the post waterflooding was monitored and plotted in Figure 3. The backpressure was set at 1168 psi. The injection flowrates were 1, 2, 3, and 4 mL/min to compare with the brine injection pressure before nanogel treatment. The results indicate that the brine injection pressure increased after nanogel treatment, which revealed that the nanogel can provide sufficient plugging to brine after soaking in supercritical CO₂. As shown in Figure 3, the differential pressure for the first segment and the second segment was higher than the other segments. As described in the experimental design section, the nanogel was designed to be placed in the middle of the core. The differential pressure for each segment indicated that the nanogel was successfully delivered to the second segment. The higher differential pressure (P2-P3) of the second segments proved that the nanogel formed a plug in the in-depth. However, a differential pressure increment was also observed for the first segment, which was caused by the nanogel adsorption during nanogel transportation. The nanogel absorbed on the pore surface, thereby, decrease the pore throat size, which resulted in the pressure increase during post water injection. The differential pressure for the third and fourth segments before and after the nanogel treatment was almost the same, which indicated that the nanogel was not transporting to these two segments; therefore, the placement of the nanogel in the in-depth was successful. The residual resistant factor for each segment and the whole core was calculated and plotted in Figure 4. Segment 2 has the highest resistance to water flow, while the Frr for segments 3 and 4 was close to 1, which indicated that the nanogel was not penetrated segment 3 and 4 or did not form a sufficient plugging.

Conclusions

The CO2 responsive nano-gel can be injected into the in-depth during the post-water flooding. The CO2 flooding stimulated the nano-gel, which increased the nanoparticle size, therefore provide additional resistance to CO2 and water flow. The differential pressure of the post waterflooding indicated that the nano-gel could be delivered in-depth. After CO2 stimulation, the water residual resistance factor increased.

References

Sun, X. and Bai, B. (2017). Dehydration of polyacrylamide-based super-absorbent polymer swollen in different concentrations of brine under CO₂ conditions. Fuel, 210, 32-40.

Sun, X., Alhuraishawy, A.K., Bai, B., and Wei, M. (2018). Combining preformed particle gel and low salinity waterflooding to improve conformance control in fractured reservoirs. Fuel, 211, 501-512.

Sun, X., Suresh, S., Zhao, X., and Bai, B. (2018). Effect of CO₂ on the dehydration of polyacrylamide-based super-absorbent polymer used for water management. Fuel, 224, 628-636.