Low-salinity water and surfactants for hydraulic fracturing and EOR of shales

Abstract

There are significant recoverable oil and gas resources stored in shale reservoirs throughout the world. Horizontal well with multistage hydraulic fracturing is an enabling technology for economic production of these shale resources. Recovery factors of liquid rich shale reservoirs is typically less than 10%, hence, there is an incentive for implementing enhanced oil recovery (EOR) technologies to improve oil production from these very tight reservoirs. Brine, chemical (surfactant), gas, or combination of these fluids imbibition or injection in huff-n-puff scenario could improve oil recovery of liquid rich shale reservoirs. In this study, low-salinity water and low-salinity water + surfactant imbibition are assessed as both hydraulic fracturing and EOR fluids in liquid rich shale reservoirs. Osmosis, capillary pressure, wettability alteration, and other effects are among the mechanisms by which these fluids can improve oil recovery from shale reservoirs. These favorable effects were investigated through imbibition experiments. Experiments show: (a) low-salinity water huff-n-puff in tight formations can improve oil production from tight formations. This is because wettability alteration, capillary, and osmotic forces; (b) surfactant + low-salinity-water huff-n-puff can further improve oil production due to additional IFT decrease and wettability alteration by surfactants; hence, improve oil production from stimulated ultra-tight formations. Based on experimental observations and literature reviewed, low-salinity water huff-n-puff (until economical), followed by low-salinity water + surfactant huff-n-puff EOR process is recommended to optimize production from liquid rich shale reservoirs, while minimizing cost. These fluids can be used to reduce matrix-fracture skin damage. The effectiveness of these hydraulic fracturing and EOR fluids in shales is highly dependent on the existence and extent of natural fractures or laminations within the stimulated reservoir volume; this is because osmosis and diffusion are very slow processes.