Pore network-scale visualization of the effect of brine composition on sweep efficiency and speed of oil recovery from carbonates using a photolithography-based calcite microfluidic model

A novel photolithography-based technique was developed to fabricate a quasi-2D heterogeneous calcite micro-model of representative elementary volume size. The effect of brine-chemistry on the mobilization of capillarity and heterogeneity trapped oil after high salinity water injection was evaluated by using diluted seawater, and seawater modified with calcium, sulphate, and silica nanoparticles. Preliminary brine screening was performed based on modified contact angle experiments under dynamic salinity alteration. The main findings are that the chemical composition of brine impacts both the ultimate oil recovery and its speed. The highest and fastest oil recovery was obtained with diluted seawater and seawater augmented with nanoparticles. We also found that the ex-situ contact angle results, indicative of wettability alteration, can be predictive of each brine performance at the pore network-scale. A slow recovery process, from 7 days up to 12 days, without any oil banking, was observed with all the brines. Due to the time-dependent nature of the wettability alteration process, mere injection of brines, even several pore-volumes and the viscous force exerted by flooding, were not sufficient to result in any additional oil production. Oil production was obtained only during the shut-in period via enhanced spontaneous imbibition of brine into the pores and throats. This highlights that a sufficient soaking time (at least for laboratory scale experiments) would be necessary to assess suitability of brines and determine accurately the incremental oil recovery by low salinity waterflooding (LSWF).

Automated summary

The research found that the chemical composition of brine impacts both the ultimate oil recovery and its speed. The highest and fastest oil recovery was obtained with diluted seawater and seawater augmented with nanoparticles. In addition, ex-situ contact angle results can predict the performance of each brine at the pore network-scale.