Improving CO2-EOR In Shale Reservoirs using Dilute Concentrations of Wettability-Altering CO2-Soluble Nonionic Surfactants

摘要

The objective of this work is to determine if the efficacy of CO2-based enhanced oil recovery(EOR)techniques for unconventional liquid reservoirs(ULRs)can be enhanced by adding a dilute concentration of a non-ionic wettability-altering surfactant to the CO2.Addition of ionic and nonionic surfactants that significantly increase the water-wetness of rock surfaces has previously been shown to improve oil recovery during water-based hydraulic fracturing and water-based EOR in ULRs.In this study,nonionic surfactants are dissolved in CO2 in order to attain similar dramatic shifts in wettability,thereby providing another mechanism for the CO2-based recovery of oil from unconventional formations during EOR.Although most ionic surfactants are essentially CO2-insoluble,there are many oil-soluble and water-soluble nonionic surfactants that are CO2-soluble to ~0.1-1.0% at formation conditions.In fact,CO2-soluble and water-soluble nonionic surfactant solutions have been previously used to generate mobility control foams during EOR in conventional formations.In this study,however,the surfactant is primarily intended to alter wettability of unconventional rock toward water-wetness or oil-phobicity,and to determine,for the first time,if such a change is significant given the numerous other mechanisms by which CO2 recovers oil from shale(solvent strength,oil swelling,oil viscosity reduction,diffusion,extraction,etc.).The solubility of two nonionic,water-soluble,branched alkyl tail surfactants(Indorama SURFONIC~R N-100,an ethoxylated nonylphenyl alcohol with 10 ethylene oxide groups;and Indorama SURFONIC~R TDA-9,an ethoxylated branched tridecyl alcohol with 9 ethylene oxide groups)in CO2 has been measured between 25-75 oC.The surfactants exhibit a solubility of roughly 1 wt% at pressures of 2000-5000 psia,with lower pressures required for lower temperatures.SURFONIC~R TDA-9 is slightly more CO2-soluble than SURFONIC~R N-100.Although oil-soluble nonionic surfactants are also soluble in CO2,they may be less likely to induce the desired change in wettability and none have yet been assessed.Shale samples were first aged via immersion in dead Eagle Ford crude oil at high temperature to attain oil-wet characteristics.The contact angle of water drops on the shale exposed to air was then measured to verify oil-wetness.The shale was then immersed in either CO2 or CO2-surfactant solution for 16 h at 4000 psi and 80 °C.Contact angles were then measured to determine if any shifts in wettability occurred.No significant changes in wettability occurred for samples immersed in CO2,however a dramatic shift toward water-wet was detected for the sample immersed in the CO2-surfactant solution.CO2 huff'n puff experiments on small cylindrical samples of oil-saturated shale immersed in CO2-surfactant solution also resulted an ultimate recovery of about 75% after 8 cycles.The huff'n puff experiments using pure CO2 to compare with CO2-surfactant solution are ongoing.They will show whether the addition of surfactant resulted in faster recovery of oil and/or the recovery of more oil.