An Evaluation Model Of Adsorbed And Free Shale Oil And Its Geological Application

The amount of free oil is the potential recoverable maximum in tight nanoporous shale reservoirs using the existing technologies.A variety of geochemical parameters related to the retention,migration and expulsion of generated oil,have been developed to estimate the amount of free oil in shale.However,there is still no a theoretical model.Therefore,based on the occurrence states?i.e.adsorbed and free states?of hydrocarbons accumulated in pores with various sizes and shapes,an unsaturated adsorption mathematical model under the conditions of hydrocarbon vapor adsorption?0<P/P₀<1?was established.Subsequently,this model was deduced to evaluate monocomponent hydrocarbon or mixed oil under formation conditions?P/P₀?1??i.e.,a saturated adsorption mathematical model?.Finally,the saturated adsorption mathematical model was applied to estimate the amount of oil confined in nanopous shale that are mainly in adsorbed and free states,and to reveal the characteristics of oil accumulation mechanism.The main results were as follows:?1?For 11 monominerals?i.e.pyrites,calcareous minerals?dolomite,calcite?,felsic minerals?quartz,orthoclase and albite?,clay minerals?kaolinite,ripidolite,illite,mixed-layers illite/smectite and Ca-montmorillonite??and 10 shale samples from 7 wells in Dongying Sag,Hydrocarbon vapor adsorption,low temperature nitrogen adsorption/desorption and low pressure CO₂ adsorption tests were carried out.The adsorption properties of n-decane,methylcyclohexane and toluene on monominerals and shale samples were revealed,and the control mechanism of pore microstructure of monominerals and shale samples on hydrocarbon adsorption was investigated.The results shows that clay minerals have the largest adsorption capacities for hydrocarbons,followed by pyrite,calcareous minerals and felsic minerals;the clay minerals adsorption properties for hydrocarbons are ripidolite,illite,mixed-layers illite/smectite and Ca-montmorillonite,from low to high in sequence from low to high in sequence;the adsorption properties for hydrocarbons on monominerals and shale samples are positively correlated with their total pore volume and BET specific surface area,and negatively correlated with their average pore diameter.?2?Considering the differences in hydrocarbons states?i.e.adsorbed and free states?in pores with various sizes and shapes,an unsaturated adsorption mathematical model was established to evaluate the adsorbed,condensed and total amounts of monocomponent hydrocarbon under the conditions?0<P/P₀<1?of Hydrocarbon Vapor Adsorption tests.Subsequently,this model was deduced to evaluate the adsorbed and free amounts of monocomponent hydrocarbon or mixed oil?shale oil?and the weight ratio of adsorption phase under formation conditions?P/P₀?1??i.e.a saturated adsorption mathematical model?.?3?Based on the saturated adsorption mathematical model of shale oil and its related data,the content of absorbed and free shale oil in Shahejie Formation of Dongying sag were calculated.With the correction of the light hydrocarbon loss of chloroform bitumen"A",the evaluation results shows that the amounts of free oil and adsorbed oil in shale samples range from 0.134?6.391 mg/g?mean 2.630 mg/g?and 0.089?2.262 mg/g?mean 0.884 mg/g?,respectively;the amount of free oil is about 2.9 times upon that of adsorbed oil;the favorable section with high free oil for the well LY1 in the Dongying sag is buried at the depth of 3630?3670 m belonging to the Lower-third Member of the Paleogene Shahejie Formation?Es₃^x?.When the volume of oil-bearing pores is constant,the free?adsorbed?oil amount gradually increases?decreases? with increasing mean pore diameter,and the free amount is largest?smallest? in slit-shaped?spherical? pores,corresponding to the smallest?largest? adsorbed amount in slit-shaped?spherical? pores.