Investigation On The Mechanisms Of Enhanced Oil Recovery Of Tight Sandstones Under A DC Voltage Treatment

Tight sandstones charactered with low porosity,low permeability,and widely developed pore structure are difficult to be developed and exhibit a low oil recovery because of the difficulty in replenishing energy by water injection.DC electric field has the advantages of high transmission,high penetration,high efficiency and easy operation,which can effectively improve the connectivity of reservoir space,thereby achieving the purpose of enhancing oil recovery.This technology has great application potential.Especially for tight sandstone reservoirs,the electroosmotic flow may have a stronger excitation effect in a finer throat.Conducting the research on the mechanism of DC electric field to enhance the oil recovery of tight sandstones can lay a solid theoretical foundation for the application of related electric field technology in unconventional energy.Up to now,the understanding of the mechanism of DC electric field to enhance reservoir recovery is relatively lack and unclear.This paper firstly verified that the DC electric field can improve the oil displacement efficiency of tight sandstones through the indoor electric displacement experiment.Then,combined with basic theories such as electrodynamics,electric double layer theory,surface chemistry,and surface physics,to investigate the pore structure properties,surface chemical properties and surface physical properties of tight reservoirs.The main understandings of this paper are as follows:The recovery factors of tight sandstones under the action of DC electric field were characterized.The influenced characteristics of different electric field strengths,different solution concentrations and different displacement methods on the recovery of tight sandstones were analyzed based on the behaviors of indoor electrokinetic oil displacement experiments.There is an optimal electric field strength value,which induces the tight sandstone recovery reach a maximum value.At lower concentrations,simultaneous flooding has a higher degree of recovery than that of sequential flooding.At higher concentrations,the opposite results will be shown.The mechanism of the change in the pore structure of tight sandstone reservoirs under the action of DC electric field was clarified.The changed characteristics in the pore structure of tight sandstones under different electric field strengths were studied by nuclear magnetic resonance(NMR)method,focusing on revealing the change mechanism of electrokinetic effect on pore structure.Through high pressure mercury injection(HPMI),low nitrogen adsorption(LNA)and NMR methods,the characteristics of pore changes in different regions under the action of DC electric field were analyzed.Electrophoresis is responsible for the difference of pore structure in the anode and cathode regions.The change mechanism in the surface chemical properties of tight sandstone reservoirs under the action of DC electric field was revealed.The changed characteristics of surface wettability under the action of DC electric field were characterized.Fourier transform infrared spectroscopy(FTIR)and X-ray photoelectron spectroscopy(XPS)were combined to reveal the alteration mechanism of different functional groups.The results showed that these functional groups can be transformed into each other.The relationship between oxygen-containing functional groups and the wettability of tight sandstones was established.Moreover,the influence of different types of oxygen-containing functional groups on wettability was determined.By introducing the concept of electronegativity,the inherent electronegativity properties of different kinds of oxygen-containing functional groups were clarified.The difference in electronegativity of oxygen-containing function groups leads to different abilities to form hydrogen bonds and to control wettability.The change mechanism in the physical properties of tight sandstone reservoir surface under the action of DC electric field was disclosed.The characteristics of roughness variation in different regions of the reservoir under the action of DC electric field were obtained by atomic force microscope(AFM).Combined with the change in wetting state,it is elucidated that the increase in roughness and the change from CassieBaxter to Wenzel state are responsible for the increase in surface hydrophilicity.Through nanoindentation marking,rapid characterization of the topographic features of the same location region was achieved using AFM and cold field-Scanning electron microscope(CF-SEM).Based on this method,the variation characteristics of the intraparticle and inter-particle roughness in the same position region before and after the action of the DC electric field were respectively characterized.After that,the variation mechanism of the roughness was revealed by analyzing the pore-related parameters(number,depth and scale).