Distribution Characteristics Of Microscopic Remaining Oil In Low Permeability Reservoirs

The pore structures are complex and heterogeneity is strong in the low-permeability reservoirs.So the oil recovery rate is low,and there is still more remaining oil retained in the reservoir in the actual development process.Therefore,carrying out microscopic remaining oil distribution studies on low-permeability reservoirs,revealing the distribution pattern and distribution law of remaining oil in the microscopic pore-throat network,can provide a geological basis for later measures to enhance the oil recovery.Taking the beach-bar sandstone reservoirs of Fan 147-151 Block in Zhenglizhuang Oilfield as an example,through the analysis of the basic characteristics of the reservoirs,three types of reservoirs were divided.The experimental schemes were designed based on this.And we carried out core water flooding simulation Experiments,nuclear magnetic resonance testing and oil-bearing thin slice analysis to reveal the microscopic remaining oil distribution.The reservoirs in the study area are mainly fine-grained lithic feldspar sandstones,and the storage spaces are mainly composed of primary intergranular pores and secondary intergranular dissolution pores.Most of them are mixed pores,with poor pore throat separation.The radius is small and the microscopic heterogeneity is strong.The oil-producing layers can be divided into three types of reservoirs:type I conventional reservoirs,type II medium-low porosity and low permeability reservoirs,type III low-porosity low to ultra-low permeability reservoirs.And types II and III are the main reservoirs.The results of core water driving simulation experiments showed that the initial oil saturation and final oil displacement efficiency of type II reservoir samples are higher than those of type III reservoir samples.The same type of reservoir,the displacement effect of the fractured rock sample is better than that of the un-fractured rock sample,reflecting that the micro-fractures produced by the fracturing have a positive impact on the water driving efficiency.To the same type of reservoir,fracturing after a rock sample is reformed with micro-fractures.It is easy to increase the heterogeneity of the rock sample,which causes its water driving efficiency to decrease instead.According to T₂ relaxation time,the pores are divided into three intervals:micro pores,small pores and medium-large pores.The middle-large pore interval has the best displacement effect in Type II and Type III reservoirs,followed by the small pore interval.Type II fractured rock samples have better displacement effects in each interval than un-fractured rock samples,while type III fractured rock samples have worse displacement effects in each interval than un-fractured rock samples.Through the analysis of oil fluorescence technology,there are 6 remaining oil types:pore-throat filling type,dispersed droplets distribute type,pore wall membrane type,throat stranded type,crack retention type and corner remaining oil type.Among them,the pore-throat filling type has the most remaining oil content,which is mainly distributed in the small pore throats with poor connectivity.And the content of dispersed droplets distribute type remaining oil is least.Compared with type II rock samples,the remaining oil in the micro-pores of type III rock samples increased in the proportion of pore-throat filling type remaining oil,while the remaining oil ratio of other types decreased.Compared with the un-fractured rock sample,the remaining oil content in the pores near the fracture in the fractured rock sample is significantly reduced,but the remaining oil content in the area farther from the fracture is increased.Combining the research results of micro-remaining oil distribution with the analysis of the effect of macro-development measures,suggestions for tapping the potential of remaining oil are put forward.