Abstract

In our previous paper (SPE-190281-MS), we presented results from a suite of multiscale experiments to understand interactions occurring across crude oil/brine/carbonate rock interfaces with different brine compositions. A new atomic to molecular scale mechanism was proposed based on changes in adhesion energies at different length- and time-scales to explain SmartWater effects for improved oil recovery (IOR) in carbonates. It was also understood that SmartWater effect is due to three distinct but interrelated physico-chemical mechanisms, involving changes to the colloidal interaction forces, surface roughening due to dissolution and re-precipitation, and removal of pre-adsorbed organic-ionic ad-layers (termed ‘flakes’) from the rock surface.

In the present study, we carried out surface forces apparatus (SFA) experiments to understand SmartWater IOR mechanisms at elevated temperatures and pressures (up to 150°C and 2,200 psi) representative of realistic reservoir conditions. The results of earlier SFA measurements at elevated temperature showed a significant dependence of SmartWater effect on temperature, while the dependence of pressure still remained unexplored. To overcome this major shortcoming and fill the missing gap in existing knowledge, a unique High Pressure-High Temperature Surface Forces Apparatus (HPHT-SFA) has been designed with the same surface visualization capabilities as regular SFA (nm normally and μm laterally).

The calcite thickness and roughness changes measured using the HPHT-SFA at elevated pressures showed a significant difference between SmartWater flooding versus high salinity water (HSW) flooding. During SmartWater flooding, a high rate of removal of organic-ad layer from the aged calcite surface (manifested by a substantial decrease in the layer thickness) and an unexpected degree of smoothening of calcite (i.e., decrease in the difference between the maximum and minimum thicknesses of calcite) were observed. The change in maximum thickness (i.e., thickness of flakes removed) was found to be around 100 nm, consistent with measurements at atmospheric pressure. The rate of flake removal from carbonate surface with SmartWater, however, was aggravated at high pressures when compared to that observed at atmospheric conditions. Another set of experiments revealed that under high pressures HSW flooding was not able to remove organic flakes from aged calcite surface, in contrast to analogous results obtained at ambient pressure. These findings suggest that not only temperature has strong effect governing the restructuring of the calcite surface, but also the pressure plays an important role affecting the kinetics of organic layer detachment from the calcite surface.

This study presents first ever results obtained from the newly designed HPHT Surface Forces Apparatus to demonstrate the importance of elevated pressures on crude oil/brine/rock interactions in SmartWater flooding. The novel findings obtained at reservoir temperature and pressure conditions are of practical significance to provide a better understanding of SmartWater flooding IOR mechanisms and subsequently guide the optimization of SmartWater flooding processes in carbonate reservoirs.

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