Transport of EOR Surfactant in Reservoirs: Impact of Polymer on Apparent Surfactant Inaccessible Pore Volume

Designing chemical EOR processes requires reservoir simulations that need to be backed by a good understanding of the mechanisms at play when injecting surfactant-based solutions in porous media. One of the main challenges is that laboratory coreflood tests often show early surfactant breakthroughs that cannot be easily history matched. Indeed, contrary to polymer macromolecules, smaller surfactant molecules are not supposed to experience the inaccessible pore volume (IPV) effect. The study's aim was to determine if, in surfactant-polymer flooding, the polymer could influence the transport of the surfactant in such a way that it would not be able to invade a fraction of the pore space. To that end, two multi-steps coreflood tests were performed with cores of outcrop rock in conditions representative of a reference field case. In the first test, the surfactant was injected without polymer and then, after a brine injection flush, with polymer. In the second test, the surfactant was directly injected with polymer. For both tests, in order to bypass the adsorption effect, the surfactant injected volumes at breakthrough were determined on rocks having their surface already fully saturated by surfactant. Namely, a first surfactant slug was injected in order to fulfill maximum rock adsorption capacity, then, immediately after, a second at a higher concentration of which the breakthrough was potentially influenced by IPV only. The polymer IPV were estimated by the conventional two-slugs method. In the first test, the result showed that, without polymer, the surfactant accessed all of the pore volume of the core while, in presence of polymer, the surfactant could not access about 2% of the pore volume, which corresponded to the polymer IPV. In the second test, the surfactant was not able to access 12% of the pore volume, which also corresponded to the polymer IPV. These outcomes stand as evidence that the presence of polymer impacts the transport of surfactant, leading it to experience an "apparent" surfactant IPV effect equal to the polymer's one. This suggests that interactions between polymer and surfactant molecules take place at the pore level. This study illustrates that surfactant transport properties in reservoirs can be more complex than conventionally accounted for in dynamic reservoir simulation. As history-matching of the coreflood essays is needed to build a representative dataset for surfactant-based EOR processes, improvements of the simulation software appear required for cases where IPV cannot be neglected.