Study Of The Microscopic Remaining Oil State And Availability Of Extra-Low/Ultra-Low Permeability Sandstone Reservoirs

Due to long-term exploitation,most oil fields have entered a high water cut period,and the recovery rate has dropped significantly.However,there are still abundant remaining oil resources in ultra-low permeability and ultra-low permeability sandstone reservoirs,and ultra-low permeability and ultra-low permeability reservoirs It will also become a hotspot for future development.The reservoir characteristics of ultra-low permeability and ultra-low permeability reservoirs are complex,which affects the development of remaining oil.Therefore,only the characteristics of the pore structure of ultra-low permeability and ultra-low permeability sandstone reservoirs,the occurrence state of microcosmic remaining oil and the law of recoverability can only be studied.In order to provide scientific basic data and guidance for enhancing oil recovery.In this paper,taking the Chang 6 reservoir in Wangyao District of Ansai Oilfield as an example,aiming at the particularity of ultra-low permeability and ultra-low permeability sandstone reservoirs,microscopic CT scanning is used to reconstruct 3D digital cores,nuclear magnetic resonance,and 2D visualized frozen slices for fluorescence quantification The microscopic pore structure characteristics and remaining oil occurrence state of ultra-low permeability and ultra-low permeability sandstone reservoirs are studied by combining the method with microscopic displacement experiments,and the producing law of remaining oil is revealed.Firstly,the samples in this study area were classified by the gas-measured pore seepage parameters,and the samples with porosity>11%and permeability>1 m D were taken as Class A pore structure samples,and the samples with porosity<11%and permeability<1 m D were taken as Class B pore structure samples.Using CT scanning technology and digital core technology,3D pore structure images were obtained,and after image processing such as artifact removal,noise removal,threshold segmentation and construction of digital core,different digital core data bodies were obtained,and after establishing the pore throat model,the pore throat parameters（average pore radius,average throat radius,coordination number and tortuosity）were evaluated,and the average pore radius of class A samples was 20.26μm,the average roar radius 1.41μm,the average pore radius of Class B samples was 11.86μm,and the average roar radius was 0.54μm.Then,using NMR technique,the NMR T2 spectral patterns were obtained to classify the different pore sizes of the reservoir,with T2<1 ms as micropore,1 ms≤T2<10 ms as small pore,10 ms≤T2<100 ms as medium pore,and T2≥100 ms as large pore,quantifying The pore structure distribution pattern of the cores in the target block was quantified.Secondly,the core replacement experiments were carried out to quantify the oil utilization law of each type of remaining oil before and after water drive in different types of samples by using the freeze filming method,and to study the real distribution state of the remaining oil in the pore throat in two-dimensional state.The oil remaining in cluster form mainly changed into intergranular adsorption,pore surface film and corner form.After the CT scan of the cores of the replacement experiment,the remaining oil in different types of samples before and after the water drive was characterized,and the remaining oil changes were visualized and quantified to investigate the remaining oil in the three-dimensional state.The remaining oil in Class B samples is basically in the form of clusters,mesh and porous,and the remaining oil in isolated form is in a great proportion.The core replacement experiment+NMR coupling method was used to quantitatively characterize the proportion of remaining oil in different pore sizes in the core,and reveal the residual oil utilization pattern in each pore under saturated oil and water drive stage.Under saturated oil conditions,the medium pores of 10～100 ms are the main oil enrichment locations in Class A samples,while the micropores and small pores of 1～10 ms and less than 1 ms in Class B samples are their main distribution ranges and the main remaining oil storage locations.After the water drive,regardless of the type,the remaining oil is concentrated in the micropores less than 1ms,and the mesopores are the most used,followed by the small pores.Finally,digital cores were constructed using the images after CT scanning,and the core pore distribution map,ball-and-stick model and pore throat parameters were analyzed to obtain pore size classification and calculation of movable fluid saturation using nuclear magnetic resonance T₂mapping.After the replacement experiments,the changes of different residual oil in two-dimensional state were quantified by using freeze-production,the changes of residual oil distribution in three-dimensional state were analyzed by using three-dimensional CT in situ characterization of microscopic residual oil,and the degree of mobilization of different pore diameters were quantified by using NMR technology,and the above methods were combined to better study the characteristics of microscopic pore structure,the state of microscopic residual oil distribution and the degree of mobilization.