Study On NMR Experimental Of Shale Samples In The Lower Paleozoic Of Chongqing And Its Adjacent Area

In Chongqing and its adjacent areas, Lower Cambrian Qiongzhusi Formation (QZS),Upper Ordovician Wufeng Formation (WF) - Lower Silurian Longmaxi Formation(LMX) shale with favorable geological conditions of shale gas formation in the Lower Paleozoic is one of the main source rocks in the Sichuan Basin. The effective experimental methods to explore the shale nuclear magnetic resonance(NMR), can provide technical support for the exploration and development of shale gas. The physical properties such as porosity and permeability of the shale, supplemented by theoretical analysis, rock physics experiments and analysis, can better analyze and evaluate the development potential of the target shale reservoir in this area.Based on the principle of shale NMR, this paper chooses the representative samples of the study area, carries out experimental research. Firstly, we set up different experimental parameters to carry out shale NMR experiments, and analyse the influence of experimental parameters on the measurement results of NMR to study the method of selecting effective parameters of NMR experiment. Secondly, we conducte NMR experiments of shale in the study area to quantitatively calculate shale porosity,permeability and other physical parameters, combined with the shale core routine analysis of data, through numerical fitting to determine the nuclear permeability model undetermined coefficient C. Finally, we construct shale NMR T2 spectrum, classify shale in the study area, to quantitatively evaluate shale pore structure, pore size distribution and other characteristics, and establish NMR interpretation method of shale in study area.In this paper, the following parameters are determined by NMR experiments of shale samples: echo interval TE=0.11ms, echo number NECH=4096, sampling times NS=8,waiting time TW=6000ms, which is suitable for nuclear magnetic resonance test of shale in study area. Based on the distribution of the peak point T2 and the percentage of the pores in the shale sample pore size distribution, the shale of the study area can be divided into three patterns: the transition pore - micropore pattern, the transition pore -mesopore pattern, the mesopore - transition pore pattern. According to the difference of the T2 spectrum of the shale samples in different layers, the T2 spectrum distribution of each shale pattern is divided into three types: the single peak T2 spectrum, the double peak T2 spectrum and the three peak T2 spectrum. The results show that the LMX shale porosity and permeability of the study area is in the best conditions, the QZS shale is the second, the WF shale is poor. Through the experimental results and numerical fitting,we determine the shale NMR T2 cutoff value in the study area: T2cutoff = 2ms,the undetermined coefficient of the NMR permeability model is C=30.03, and the Coates model has better applicability in the study area. The NMR porosity and density,permeability and the peak point T2 value of shale samples have good correlation. The computational models can be established to quickly obtain physical parameters such as porosity and permeability of shale in the study area.