Experimental Evaluation of a Recrosslinkable CO2-Resistant Micro-Sized Preformed Particle Gel for CO2 Sweep Efficiency Improvement in Reservoirs with Super-K Channels

Abstract

A recrosslinkable CO₂-resistant branched preformed particle gel (CO₂-BRPPG) was developed for controlling CO₂ injection conformance, particularly in reservoirs with super-permeable channels. Previous work focused on a millimeter-sized CO₂-BRPPG in open fractures, but its performance in high-permeability channels with pore throat networks remained unexplored. This study used a sandpack model to evaluate a micro-sized CO₂-BRPPG under varying conditions of salinity, gel concentration, and pH. At ambient conditions, the equilibrium swelling ratio (ESR) of the gel reached 76 times its original size. This ratio decreased with increasing salinity but remained stable at low pH values, demonstrating the gel's resilience in acidic environments. Rheological tests revealed shear-thinning behavior, with gel strength improving as salinity increased (the storage modulus rose from 113 Pa in 1% NaCl to 145 Pa in 10% NaCl). Injectivity tests showed that lower gel concentrations reduced the injection pressure, offering flexibility in deep injection treatments. Gels with higher swelling ratios had lower injection pressures due to increased strength and reduced deformability. The gel maintained stable plugging performance during two water-alternating-CO₂ cycles, but a decline was observed in the third cycle. It also demonstrated a high CO₂ breakthrough pressure of 177 psi in high salinity conditions (10% NaCl). The permeability reduction for water and CO₂ was influenced by gel concentration and salinity, with higher salinity increasing the permeability reduction and higher gel concentrations decreasing it. These findings underscore the effectiveness of the CO₂-BRPPG in improving CO₂ sweep efficiency and managing CO₂ sequestration in reservoirs with high permeability.

Keywords: CO2-EOR; CO2-resistant particle gel; conformance control; enhanced oil recovery (EOR); recrosslinkable preformed particle gel (RPPG); super-permeable channels; water alternating gas (WAG).

Figure 1

(A) A micro-size RPPG is injected to fill the pore space within the high-permeability channel. (B) RPPG is associated with forming a bulk, continuous-phase gel.

Figure 2

(a) Effect of salinity on particle size. (b) Effect of pH on particle size.

Figure 3

Viscosity as a function of the shear rate of the CO₂-BRPPG suspension at different suspension concentrations.

Figure 4

(a) The linear viscoelastic region of the CO₂-BRPPG demonstrates consistent modulus values within this range. (b) The effect of salinity on gel strength.

Figure 5

(a) Swelling ratio of the CO₂-BRPPG before and after exposure to CO₂ at pressures of 500 psi, 850 psi (dense phase), and 1200 psi (supercritical CO₂). (b) Effect of salinity on the swelling ratio of the CO₂-BRPPG under 850 psi CO₂ pressure.

Figure 6

Set of pictures showing the CO₂-BRPPG after exposure to CO₂ at 850 psi for different salinities.

Figure 7

SEM picture for the gel before (a) and after (b) exposure to supercritical CO₂ for 3 days.

Figure 8

Profile of the injection pressure on the CO₂-BRPPG microgel at different salinity and concentration values.

Figure 9

(a) Relationship of CO₂-BRPPG concentration and stable injection pressure gradient. (b) The CO₂-BRPPG suspension salinity and stable injection pressure gradient.

Figure 10

Differential pressure profile of water-alternating-CO₂ injection in a sandpack treated with 3000 ppm CO₂-BRPPG.

Figure 11

Differential pressure of the first CO₂ injection cycle.

Figure 12

Differential pressure of the first brine injection cycle.

Figure 13

(a,b): CO₂ breakthrough pressure for different gel concentrations and salinity conditions.

Figure 14

The residual resistance factor for both CO₂ and water at different flow rates (1, 1.25, 1.5, and 1.75 mL/min) and gel concentrations (1500 ppm, 3000 ppm, 5000 ppm, and 7000 ppm).

Figure 15

The relationship between gel concentration and RDPR reduction for CO₂ and water.

Figure 16

The residual resistance factor for both CO₂ and water at different flow rates (1, 1.25, 1.5, and 1.75 mL/min) under varying salinity conditions with a constant gel concentration of 5000 ppm.

Figure 17

The relationship between NaCl concentration and RDPR for CO₂ and water.

Figure 18

(a) shows the CO₂-BRPPG bulk gel that formed after free-radical polymerization, (b) shows the CO₂-BRPPG ground to a 170/230 mesh size, (c) shows a microscopic picture of dry CO₂-BRPPG, and (d) shows a swelling particle gel.

Figure 19

High-pressure vessel and loaded sample.

Figure 20

Schematic of the experimental setup for CO₂/brine flooding.