Diffusion of CO in single-step silica nanofluid for subsurface utilization: an experimental study.

This study investigates the role of single-step silica nanofluids as additives to increase CO absorption in polymeric solutions for proposed oilfield applications. Using pressure decay approach, the study investigates the applicability of single-step silica nanofluids for CO absorption in a high pressure-high temperature (HPHT) cell. Various parameters like nanoparticle size (30-120 nm) and concentration (0.1-1 wt%) were investigated to ascertain the absorption performance of the nanofluids and optimization their application in subsurface applications as carrier fluids for CO. The solutions under observation (deionized water and silica nanofluids) were pressurized under the desired pressure and temperature inside a stirring pot and the decline in pressure was continuously noted. To comprehensively cover the near-reservoir field conditions, the CO absorption was investigated in the pressure range of 5-10 MPa and at temperatures of 30-90 °C. While increasing the nanoparticle concentration (from 0.1 to 1 wt%) increased the CO absorption (evident by the sharper decline in pressure), increasing the nanoparticle size reduced the absorption capacity of the nanofluids as a lesser volume of decline in pressure was noted. Furthermore, increasing the temperature of the experimental investigation caused a major reduction (12-19%) in the pressure decay. However, it was also observed that higher pressure (> 7.5 MPa) was detrimental for CO absorption (due to its supercritical nature). Adding salt (sodium chloride, NaCl) was found to massively lower (up to 33%) while adding surfactant (sodium dodecyl sulfate, SDS) slightly increased the amount of CO absorption (in presence of salinity). Based on the observations of this study, the use of single-step silica nanofluids as CO carrier fluids is recommended for oilfield conditions where salinity is less than 4 wt%.