Experimental Study On Spontaneous Imbibition Law Of Tight Reservoirs Based On Nuclear Magnetic Resonance Technology

Tight reservoirs are important reserve resources.Horizontal drilling and hydraulic fracturing has been extensively used for the exploitation of these unconventional resources,and fracturing fluids absorbed into formations by spontaneous imbibition is an important mechanism of oil production.But tight reservoirs are in poor physical properties,which lead to unclear about the imbibition law of fluid in reservoir.Meanwhile,the normalized model of imbibition has limitations.Therefore,the studies about pore structure of tight reservoirs,law of fluid spontaneous imbibition and optimization of normalized model are vital to improve the recovery efficiency of tight oil and gas resources.Firstly in this paper,X-ray diffraction experiment and gas measurement are used to measure mineral composition,porosity and permeability parameters of tight sandstone reservoirs;SEM and nuclear magnetic resonance(NMR)technologies are used to characterize pore types and pore dameter distribution of tight reservoirs quantitatively;and then physical characteristics of tight reservoirs can be analyzed comprehensively.Secondly,means of using improved imbibition bottle and NMR technology are used to study characteristics and laws of spontaneous imbibition under different conditions,variables in the experimental process mainly include: comparison of oil-water and gas-water imbibition processes;boundary conditions of cores;types of surfactants added in imbibition solution;types of cores.Dimensionless time model and normalized recovery rate model are used to analyze and dispose the results of the imbibition experiments.The results show that the tight sandstone features a multiscale pore structure,which is dominated by micropores and small mesopores.As the imbibition process begins,white oil is preferentially displaced from these relatively small pores by water and a large amount of oil production comes from the micropores.For the gas-water system,however,water could not enter mesopores readily if only driven by capillary pressure owing to the snap-off effect of NWP.Boundary conditions are shown to have a significant impact on imbibition rate and ultimate recovery.Both are higher as the areas available for water imbibition increase.As for gas-water system,boundary conditions exert little effects on final recovery but remarkable impacts on the imbibition rates.Deionized water with low concentrations of surfactants altered the wettability of the samples,from weakly water-wet to a strongly water-wet on the rock surfaces,while lowering interfacial tension(IFT)at the oil-water interface.Therefore,a higher oil recovery could be obtained to some extent,but enough IFT is still needed to ensure a large capillary force.The traditional scaling equations can be used to scale the imbibition data for both oil-water and gas-water systems to some extent,and predict the recovery by imbibition if the wettability of rock medium remains unchanged.As for the normalized recovery rate model,parameter is optimized to be suitable for different conditions in oil-water and gas-water systems in tight reservoirs which can be used as reference in the processing of imbibition data and recovery prediction of tight reservoirs in the future.This research demonstrates the imbibition characteristics of tight sandstone,several relevant affecting factors and parameters in the normalized recovery model of tight reservoir are optimized,providing crucial theory foundations for the development.