NMR Response Characteristics And Influencing Factors Of Tight Sandstone In Ordos Basin

Tight sandstone reservoirs are widely distributed and rich in resources in Ordos Basin,and they are the focus of exploration.Nuclear magnetic resonance(NMR)technology is widely applied in reservoir evaluation with its unique advantages.In this paper,56 pieces of tight oil cores and 17 pieces of tight gas cores are used to carry out the experiment of movable fluid by using NMR,scanning electron microscopy images,the casting thin sections,X-ray diffraction and high-speed centrifugal technology,etc.Firstly,the experimental parameters are optimized.NMR test parameters are selected: scanning times Ns=64,waiting time Tw=5s,echo times Ne=6000,echo apacing Te=0.27 ms.According to the analysis of variation laws of water saturation and T2 spectrum shapes under different centrifugal force and centrifugal time,it is confirmed that 3h is the best centrifugal time,and 2.07MPa?2.78 MPa are the best centrifugal force respectively of tight oil and gas cores,and the corresponding lower limit of effective throat radius are 0.070?m and 0.052?m.The movable fluid exists in the pore of the throat radius greater than the value,which can provide reference for the evaluation of the tight reservoir in the future.The movable fluid is located in pores with a throat radius greater than the value.The conclusion can provide references for the future evaluation of tight reservoirs.Secondly,the T2 spectrum characteristics of the experimental cores are analyzed.The T2 spectrum distribution under the condition of saturated water mainly show bimodal left high and right low pattern,a few show unimodal or three peaks,and the range of effective signal are in 0.1~1000ms.The T2 spectrum distribution after centrifugation mainly show unimodal pattern with a range of 0.1~100ms.The movable peak disappears basically,and immovable peak decreases in a small range.The worse core physical properties,the smaller the T2 spectrum interval before and after centrifugation,and the less movable water content.Finally,the T2 cutoff value?the variation law of movable fluid parameters and their influencing factors are studied.The distribution is scattered and the value is low.The average T2 cutoff values of the tight oil and gas cores are 6.509 ms and 3.767 ms,respectively.The average values of the movable fluid saturation are 31.711% and 28.304%.The average values of movable fluid porosity are 2.149% and 2.230%.Most reservoirs belong to medium or even poor reservoirs.There are also various influencing factors,mainly including elementary substance and microscopic pore structure factors such as reservoir properties,pore types and the presence of clay minerals.