Reliable quantification of wettability is critical in assessment of fluid distribution, capillary pressure, relative permeability, and flow properties of fluids in reservoirs. Wettability of reservoirs can be affected by chemical composition of rock-fluid system, salinity, and reservoir temperature. Experimental assessment of wettability under reservoir conditions, while gaining control on the aforementioned parameters, may be tedious and challenging. Several published researches have used experimental studies to focus on determining the impact of individual factors on wettability of rock. However, studies on the combined effects of these factors are limited, although critical, for better understanding of wettability of hydrocarbon reservoirs. In this paper we introduce a workflow for assessment of wettability of rocks at reservoir condition using molecular dynamics (MD) simulations. The outcomes include (i) quantifying the wettability of pure minerals, (ii) quantifying the impact of reservoir temperature on wettability of pure mineral. The inputs to the simulation include molecules of pure minerals (quartz, calcite, albite) packed in a cubical simulation box. The molecules are condensed to form a flat surface. Subsequently, water and oil (hexane) molecules are placed on the surface of the mineral. We then perform simulations with constant number of particles, temperature and volume (NVT) on the system till equilibrium is reached. At equilibrium, the contact angle formed by the water droplet is measured. Contact angle is simulated for temperature conditions in the range of 300 to 380 K. The results showed that the contact angle between water-mineral for quartz, calcite, and albite at room temperature (300 K) ranges from 30º to 45º, indicating that the surface of these minerals is hydrophilic, with different degrees of hydrophilicity. This information is essential for reliable fluid flow simulations, which are often overlooked in conventional approaches. We also found that the temperature has a measurable impact on the contact angles formed by water droplet. We found that increase in temperature from 300 to 380 K decreases the contact angles by approximately 30% on quartz surfaces, 20% on albite surfaces, and 24% on calcite surfaces. The results for the hexane-mineral system show that the hexane behaved similarly in the three minerals surface. A thin film of hexane is formed at the surface corresponding to a contact angle of 0º. The method introduced in this paper has application for reliable evaluation of wettability at any reservoir of interest by knowing the molecular structure of clay and non-clay minerals as well as fluid content. Moreover, the challenges of wettability determination under high temperature and pressure conditions can also be efficiently addressed by using molecular dynamics simulations.
Biasing the Center of Charge in Molecular Dynamics Simulations with Empirical Valence Bond Models: Free Energetics of an Excess Proton in a Water Droplet
