Oil recovery rates in mature fields can be increased by using conformance control techniques. Conformance control using polymeric gels makes the sweeping of a reservoir more uniform, which allows the recovery of oil from less permeable regions, reducing excessive water co-production [1]. Blocking the zones preferably using polymeric gels causes the injected fluids to travel through previously unscanned areas [1]. Due to the low cost and ease of preparation of the systems, polymers based on polyacrylamide are the most used in the preparation of blocking gels. Polyethyleneimine (PEI) is currently the most investigated organic compound for crosslinking systems, mainly because it is ecologically correct and possesses a high nucleophilic character [2]. The incorporation of carbon nanomaterials such as oxidized carbon nanotubes (OCNT) and graphene oxide (GO) in hydrogel matrices has been shown to be important in obtaining polyacrylamide nanocomposite hydrogels with improved properties. In this work, polyacrylamide nanocomposite hydrogels were obtained in saline solutions with ionic strength μ = 0.6 from the incorporation of 1.0% wt. of OCNT and GO in relation to PAM. PEI was used as a crosslinker in the formation of the three-dimensional structure of the gel.
The gelling solutions obtained from the 9:3 PAM/PEI mass ratio (P3) with the addition of OCNT and GO, showed a slight increase in viscosity (Fig. 1a) in relation to the P3 reference, as a result of the physicochemical interactions promoted between the nanofillers and macromolecules. Fig. 1b shows that after 40 days of storage in an oven at 70 ºC, the G’ modules are superior to the G” modules in the entire frequency range analyzed, revealing the dominance of the elastic character of the gels. The P3-OCNT and P3-GO gels have lower G' values compared to the P3 gel, which can be explained by the fact that the interaction of PEI with the functional groups present on the surface of oxidized carbon nanomaterials [3] a smaller amount of active sites on the PEI available for PAM crosslinking, which can avoid excessive crosslinking and make the gels more stable [2].

Fig. 1: a) Viscosity as a function of shear rate for the gels obtained; b) Log. of G′ and G″ modules as a function of angular frequency; c) Percentage of syneresis of the gels obtained.

This behavior is also evidenced in the syneresis results presented for these gels in Fig. 1c, where it is possible note that the addition of nanofillers considerably reduces the percentage of water loss from the gel structure. Therefore, our group considers the use of carbon nanomaterials very promising for application in nanocomposite hydrogels for advanced oil recovery.

References:
[1] Bai, B. et al., Pet. Explor. Dev. 42, 525–532 (2015).
[2] Pereira, K.A.B. et al., ACS Omega 5, 4759−4769 (2020).
[3] Chen, B. et al., J. Mater. Chem. 21, 7736 (2011).