Reports: ND956715-ND9: Design of Brine for Wettability Alteration and Improved Spontaneous Imbibition in Reservoir Nanopores

Venkat Ganesan, University of Texas, Austin

Project Report (Period: 2016 – 2017)

Objectives: Recently, significant interest has arisen in the use of brine solutions of carefully tuned salinity    to alter the wettablity of oil-wet rock surfaces. However, despite a number of experimental results, a clear understanding of the mechanisms underlying such phenomena is still lacking. Motivated by such issues, we proposed to develop and use a multiscale computational approach designed to unravel the fundamental mechanisms of wettablity alteration and dynamics of imbibition of brine solutions in combination with surfactants. For this purpose, we proposed to use of atomistic simulations to study the organization of water, oil and salt molecules near chemically realistic models of rock surfaces for different compositions and temperatures of the mixtures. We proposed to use such an approach to parametrize the surface-component interactions for a coarse-grained model. Such a coarse-grained representation will then be used to study phenomena such as kinetics of imbibition which are inaccessible to the length and time scales of atomistic simulations.

Progress in reporting period: In the reporting period, we have been primarily concerned with the development of the multiscale simulation approach for multicomponent systems containing surfactants, polymers etc. Indeed, such multicomponent systems (such as oil, water and surfactant mixtures) are typically phase separated, and coarse-graining approaches need to necessarily capture the interaction features and morphologies that results in such systems. While a number of coarse-grained simulation approaches do exist for study of morphology of multicomponent systems, such methods may not necessarily capture the interaction with solid surfaces such as may be encountered in pore walls.

Motivated by the above considerations, we have undertaken two related efforts:

(i)             Development of a multiscale approach of coarse-graining and fine-graining ideas designed to not only capture the morphological details of multicomponent polymer systems but also allows for the reintroduction of atomistic details to capture the interaction details which may be critical to determining the surface energy. For this purpose, we developed a coarse-grained approach which relies on coarse-graining only the intermolecular interactions. Such interactions were parametrized by a comparison of the long-range structural characteristics of the multicomponent system with the coarse-grained simulations. The coarse-grained simulation approach is then used to effect simulations of long length and time scales which can capture the morphological characteristics of the system. Subsequently, the finer scale details are reintroduced into the morphologies so generated and are followed by a shorter time scale simulation.

To test the efficacy of the above methodology, we have implemented the above approach on a simpler system of block copolymer solvated with salt. As a consequence of microphase separation, such systems display morphological characteristics at nanoscale. We implemented our above methodology to such a system to identify the differences in the ion coordination behavior arising from the nanoscale separation. In addition, we used such ideas to identify the impact of microphase separation on polymer dynamical characteristics.

As next steps, we plan to adopt the above simulation framework to study the characteristics of oil-water-surfactant systems.

(ii)            A second direction of our work has focused on using coarse-grained simulation tools to transport characteristics of phase separated systems (such as oil-water-surfactant mixtures). In this context, we have adopted the methodology of the dissipative particle dynamics approach, and supplemented such an approach with an ability to accommodate the physics arising from electrostatic interactions, such as in the presence of brine/salt. At this stage, we have tested the simulation approach to validate the equilibrium characteristics of mixtures of water, oil and salt.

As part of the next step, we plan to adapt the above approach to study the dynamical characteristics in mixtures of oil, water and salt.

Subsequent to the completion of the above-discussed steps, we plan to embark on a methodology which combines the developments of (ii) with the capabilities of (i). More explicitly, we plan to study the dynamics of multicomponent mixtures (using the approach of (ii)) in the midst of surfaces (mimicking rock surfaces). Therein the approach of (i) will be used to inform the interfacial interactions between oil/water and the surface and the specific role of brine concentrations. By correlating the dynamical characteristics with the interfacial interactions, we plan to bring new insights into the modulation of wettability and its impact on the dynamics of imbibition.