Investigation of acid pre-flushing and pH-sensitive microgel injection in fractured carbonate rocks for conformance control purposes

Abstract

Waterflooding in fractured reservoirs is a challenging task due to the presence of high conductive flow pathways such as fractures. Much of the injected water passes through fractures without sweeping the oil in the low permeable area, which results in an early breakthrough. Implementing deep conformance control techniques can be a remedy for this early water breakthrough. pH-sensitive microgel injection is a conformance control method in which the dependency of microgel viscosity to pH guarantees easy injection of these microgels into formations at low pH environments. Because of the geochemical reactions among rock minerals, microgels, and a pre-flushing acid, the microgel pH increases; therefore, these microgels swell and block high conductive fractures. In this study, a designed visual cell containing rock samples is implemented to observe rock–microgel interactions during a pH-sensitive microgel flooding into a fractured carbonate medium. First, the dependency of fracture aperture changes to the acid pre-flush flow rate is examined. Then, we investigate the effect of pH-sensitive microgel concentration on its resistance to block fractures during post-water flooding by studying the gel failure mechanisms (e.g., adhesive separation, cohesive failure). Finally, the effect of an initial aperture of fracture is examined on microgel washout when water injection is resumed. The results showed that both decreasing the acid flow rate and lowering the initial aperture could increase the rate of aperture changes. Moreover, the microgel solution with a concentration of 1 wt% showed the highest resistance (98.2 psi/ft) against post-water injection. Additionally, this microgel concentration had the highest permeability reduction factor. Meanwhile, the smaller initial aperture of fracture contributed to a higher microgel resistance.

Review of Microgels for Enhanced Oil Recovery: Properties and Cases of Application

“…With increasing pH, the carboxylic groups of polyacrylate networks are hydrolyzed, and the charged groups repel each other. As a result, polymers chains are stretched and microgels swell [ 86 ]. To guarantee microgel penetration into the deep reservoir zone on the first stage of the reservoir treatment, pre-flush of formation by acids is recommended [ 86 ].…”

“…As a result, polymers chains are stretched and microgels swell [ 86 ]. To guarantee microgel penetration into the deep reservoir zone on the first stage of the reservoir treatment, pre-flush of formation by acids is recommended [ 86 ]. The experiments showed that acetic acid is better as a pre-flush agent [ 86 , 87 ].…”

“…To guarantee microgel penetration into the deep reservoir zone on the first stage of the reservoir treatment, pre-flush of formation by acids is recommended [ 86 ]. The experiments showed that acetic acid is better as a pre-flush agent [ 86 , 87 ]. The second stage is microgel injection, and the last stage of treatment is post water flooding.…”

Geochemical modelling of acid injection in high salinity sandstones at reservoir conditions

“…The dissolution of cement fractures resulting from injecting strong acids before microgel flooding was also investigated (Patterson, 2014). Experimental results achieved by Teimouri et al (2020) showed that the rate of fracture aperture changes is a function of acid injection flow rate. Koochakzadeh et al (2021b) conducted both batch and flooding experiments to characterize the geochemical performance of acid pre-flushing into carbonate rocks showing acetic acid as the prefered preflushing fluid to maintain a low-pH environment.…”

Evaluation of 3D printed microfluidic networks to study fluid flow in rocks

“…In a study, a 500-lm rock piece, mounted between two polydimethylsiloxane plates, was used as a microfluidic device [21]. Recently, Teimouri et al [22] designed and manufactured a transparent cell consisting of two plexiglass plates around a carbonate rock sample to investigate the conformance control during pH-sensitive polymer flooding in a fractured media. The transparent sealing rubber enabled them to photograph the rock surface easily during the experiments.…”