Insights into Salinity Variations for Waterfloods,Frac-Fluids and Drilling Mud in Clay-Hosted Pores using Molecular Simulations

摘要

Clays,with their charged surfaces,are characterized by strong interactions with dissolved ions in brines,injected water,frac-fluids,and drilling mud-filtrate.While we do have strong evidence and science to address clay swelling,little to no progress has been made on fluid distributions and their impact on transport through clay-pores when exposed to fluids of varying salinity.In this work,we use a molecular dynamics approach to characterize fluid distributions when oil and water are present in clay-hosted pores and to finally provide guidelines for optimal salinities in our drilling mud,frac-fluid,and injected water.This work considers the distribution of oil and water at varying salinities in illite slit-pore systems because illite is the most common clay mineral.We use a wide range of concentrations(0-37000ppm NaCl)in these pores and quantify the surface electric potentials,electric double layers(EDL),mean square displacement(MSD),and ion hydration shells to model the interaction between clays,water,oil,and salts.The results indicate that at high salinity(above 16000ppm)Na+ and Cl-ions primarily adsorb on the clay surface due to the charge screening effect.The unabsorbed Na+ and Cl-ions appear as aggregates or within their hydration shell,which impedes oil flow.At low salinity(below 8100ppm),water bridges across the pore due to a strong surface potential that is not neutralized by the low salt concentration.These water bridges severely hamper the flow of any free water and oil.At intermediate values of salinity(8500-12400ppm),ionic aggregates are largely absent and water molecules merely adsorb on the surface of the pore.At these intermediate values of salinity,the mobility of the oil phase is the highest and constitutes the best range of salinity for the pore surface chemistry that we consider.This paper extends our understanding of the effect of salinity variations on fluid distribution and its impact on oil transport in clay pores.Prior work has largely focused on the impact of swelling but little has been done to understand the microscale phenomena such as water bridging and ion aggregation on fluid flow and storage.Our work provides optimal values of salinity that enhance oil mobility and therefore,provides insights into the design of low salinity waterfloods,frac-fluid,and drilling mud compositions for formations where clay may be present.