The chemical agent was developed using a combination of two monomers that were polymerized together. The base of the agent was 90% water, with the remaining percentage being mostly acrylamide along with the second polymer, ACID. The polymers were neutralized using sodium hydroxide to alter the pH for initiation. The initiator used was ammonium per sulfate (APS) and the entire structure was crosslinked with an organic crosslinker, MBA. Different formulations were tested to reach the optimum strength and performance based on CO2 static tests. Following this, the optimum formulation was tested under dynamic CO2 flooding tests.
More than 20 different batches of polymer with different formulations were developed and tested. It was found that increasing the ACID polymer concentration resulted in an increase in strength, however the gelation time and crosslinking time was shortened significantly as well. This can impact the injectivity of the crosslinked polymer. Also, increasing the ACID polymer concentration beyond 7500 ppm resulted in rapid hydrolysis of acrylamide due to creation of carboxylic groups. Based on this, the concentration of ACID was maintained at a range of 1000 – 3000 ppm. The salinity of the water used to formulate the crosslinked polymer had minor impact on the gel performance up to 130,000 ppm of NaCl. When divalent cations were introduced in the form of magnesium chloride, the gel could only function up to 33000 ppm. Overall, the optimized crosslinked polymer managed to divert the flow of CO2 for more than 18 continuous hours of injection at pressures up to 3200 psi. After the experiments, analysis of the crosslinked polymer showed minor degradation, which was evident from the SEM analysis of the polymer chains.
This research managed to develop and optimize a pH activated conformance control chemical agent for CO2 conformance and diversion. This can be extremely useful in CO2 enhanced oil recovery applications and in CO2 storage in underground reservoirs. The research showed that the polymer had excellent salinity, pH, and pressure resistance even at elevated temperatures.