Shale Oil Mobility Study With Nuclear Magnetic Resonance

Unconventional oil and gas exploration and development has made a breakthrough,as unconventional oil and gas,shale oil has become an important force in the global energy structure.The study of shale oil mobility is an important content of shale oil reserve evaluation and production prediction.Due to its poor physical properties,low pore and permeability parameters and low fluid saturation,conventional experimental methods are not suitable for the experimental study of shale oil mobility.In this paper,based on NMR deconvolution analysis method,the mobility of shale oil in original core under different conditions was studied,the main factors affecting the mobility of shale oil were identified,and the methods to improve the mobility of shale oil were found,so as to provide technical support for the effective exploitation of shale oil reservoir.The optimization study of measurement parameters of NMR T₂spectrum of shale oil reservoir rock was carried out,and the influences of phase cycle,waiting time,repeated scanning times,half-echo time and echo number on measurement results were analyzed,and the optimization parameters of NMR T₂spectrum of shale oil reservoir rock were determined:The Tau value is 70μs,the number of echoes is 1500,the phase cycle is 8,the number of repeated scans is 3500,and the waiting time is 100 ms.Based on NMR relaxation theory,the method of characterizing pore radius with NMR T₂spectrum of shale oil reservoir rock was studied,and the power exponential model of NMR T₂value and pore radiuslg r（28）alg T₂（10）b was deduced.The model shows a strong positive correlation between T₂value and pore radius,and the correlation coefficientR²（28）0.9935,which provides an efficient and accurate method for the characterization of pore structure of subsequent shale oil reservoirs.Based on imbibition experiments,the feasibility of shale oil was evaluated.The results showed that the imbibition recovery of formation water,fracturing fluid and surface activator were 9.7%,12.3%and 15.7%respectively under room temperature and room pressure,and the imbibition recovery of surfactant was the highest.As the temperature of formation water imbibition increases from 50.1℃to 90.4℃,the recovery of shale oil changes from 23.5%to25.8%,an increase of 2.3%.Therefore,imbibition temperature has limited influence on the recovery rate of shale oil.As the pressure of formation water imbibition increases from5.12MPa to 20.46MPa,the recovery of shale oil changes from 13.1%to 28.4%,increasing by15.3%.Therefore,imbibition pressure has a significant impact on the recovery of shale oil.The results of shale oil production in different pores show that the shale oil production is the most in the pores with the size of 150nm,and the pores with the radius of 100nm-200nm and above 1200nm are the main contribution pores to oil production.Based on the CO₂huff and puff experiment,the feasibility evaluation of shale oil was carried out.The results show that with the increase of CO₂huff and puff temperature from30.1℃to 90.7℃,the recovery of shale oil changed from 27.2%to 40.1%,increasing by12.9%.Therefore,CO₂huff and puff temperature has a significant impact on the recovery of shale oil.With the increase of CO₂huff and puff pressure from 5.26MPa to 20.60MPa,shale oil recovery changed from 23.6%to 72.2%,increasing by 48.6%.Therefore,CO₂huff and puff pressure has a significant impact on shale oil recovery.The production results of shale oil in different pores show that the larger the pore radius is,the higher the production degree of shale oil by CO₂huff and puff is.However,the main pores of oil production are not macropores,but medium pores.Compared with the imbibition experiment,both the temperature and pressure of CO₂huff and puff have influence on the recovery of shale oil,and the influence of CO₂huff and puff pressure is more obvious.Therefore,high-pressure CO₂huff and puff is the most effective way to improve the recovery of shale oil.The research results of this paper have important application value for improving the recovery of shale reservoirs.