Geochemical Response And Alteration Mechanism Of Oil Components During Water Flooding Process

Water flooding technology is a common method to increase oil production,recovery rate and recovery factor in the process of oilfield development in China.It is a process of oil flowing and being exploited by injecting water to maintain or restore reservoir pressure.During water flooding process,the injected water,crude oil,reservoir and the substance brought by water will interact,which will cause the change of crude oil composition.In order to reveal the geochemical response and alteration mechanism of crude oil components in water flooding development process,12 wellhead crude oils in Qudi oilfield in different water flooding development periods were analyzed by internal standard method.The dynamic changes of saturated hydrocarbon and aromatic hydrocarbon contents and their parameters were studied,and the alteration mechanism was analyzed.During the process of water flooding,the relative content of saturate hydrocarbons in crude oil increases,the relative content of aromatic hydrocarbons decreases,and the ratio of saturated hydrocarbons to aromatics increases,which also proves the existence of water washing in this process.ΣnCl4-ΣnC15+、ΣnC21-/ΣnC22+and(nC21+nC22)/(nC28+nC29)reduce continually.The absolute concentrations of ΣnC14-,ΣnC21-and nC21+nC22 reduce continually,while the absolute concentrations of ΣnC15+,ΣnC22+ and nC28+nC29 Keep relatively unchanged or slightly increased.The change of parameters is the result of the simultaneous action of water washing and biodegradation.Pr/nC17 and Ph/nC18 shows an initial increase and subsequent decrease,or the opposite,indicating an effect of a strata chromatography adsorption,along with the water washing effect or biodegradation.During the process of water flooding,the parameters C15-RD/C15-D and C15-D/C16-HD of dicyclic sesquiterpanes increased slightly.The parameters ETR,TTs/C30H and TS/C30H increased slightly.The absolute concentration of compounds in the parameters increased slightly,and the increase of molecular compounds was higher than that of denominator compounds.Water washing has little effect on tricyclic terpenes and pentacyclic triterpanes.Therefore,the increase of these parameters proves the existence of biodegradation.Other parameters of tricyclic terpenes and pentacyclic triterpanes are basically stable or slightly reduced.RS/C30H and C27-DS/C27-RS of steranes increased gradually,while the other steranes parameters were basically stable.Concentrations of RS,C27-DS and C27-RS increased as a whole.The increase range of RS was slightly larger than C30H,and that of C27-DS was slightly larger than that of C27-RS.The parametric changes of steranes are also mainly caused by biodegradation.During the process of water flooding,MNR,DNR and TNR1 of alkyl naphthalenes first increase and then decrease.The absolute concentration of alkyl naphthalene compounds is complex and lack of regularity.Alkyl naphthalene compounds are biodegraded and water washed at the same time,but their effects are opposite.The parameters of Alkyl phenanthrenes and trifluorene compounds are basically stable,and the concentration of the compounds is basically stable or slightly reduced.C2620S/C2820S,C20/(C20+C2820S),(C20+C21)/(C20+C21+C26+C27+C28),(C20+C21)/(C26+C27+C28)of three aromatic steranes parameters decreased slightly;The absolute concentration of the compounds in the parameters increased as a whole,while the concentration of three aromatic steranes parameters with smaller molecular weight increased relatively little.During the process of water flooding,the carbon isotope values of crude oil are basically stable at-26‰ while the carbon isotope values of saturated hydrocarbons,aromatic hydrocarbons and asphaltenes are slightly increased,and the carbon isotope values of non-hydrocarbons are basically stable as a whole.The dynamic changes of saturated hydrocarbons and aromatic hydrocarbons reflect that the secondary effects of crude oil in the process of waterflooding mainly include water washing,oxidation and biodegradation and fluid-rock(geochromatography)effect molecular weight,structure,position and number of methyl groups and polarity of compounds all affect the effect of these secondary actions.