Experimental Study And Application Of NMR Response Mechanism In Tight Oil And Gas Reservoirs

In recent years,with the rapid increase in energy demand at domestic and global levels,the exploration and development of complex,nonconventional oil and gas resources,such as tight oil and gas,and shale oil and gas,have become increasingly important.As important means of exploration,nuclear magnetic resonance(NMR)core analysis and logging technology plays an irreplaceable role in oil and gas exploration and development.However,the application of NMR logging faces huge challenges due to the poor petrophysical property and complex pore structure of tight reservoirs,source rock and reservoir association in tight reservoirs,weak hydrocarbon signals and low signal to noise ratio of NMR logging.In order to address these issues,this paper carried out research on the response characteristics and application of NMR in tight hydrocarbon reservoirs,which is crucial to the exploration and development of tight reservoirs.Impulse sequence is the basis for NMR data acquisition and application.In this paper,the commonly used one-dimensional,two-dimensional and NMR pulse sequences were compared and analyzed,and the pulse sequence design and data acquisition methods suitable for tight reservoirs were improved.It was proposed that the CPMG pulse sequence for non-uniform sampling at varying echo train spacing for a single measurement can select proper echo train spacing and sampling density according to the characteristics of relaxation components,reduce the number of radio-frequency pulses and energy loss,and improve data acquisition efficiency.By combining high-precision quantification of the long and short relaxation components of T₂ spectrum with the advantage of the medium and long relaxation components of two-dimensional D-T₂ spectrum in accurate classification of pore fluid types,a testing method integrating D-T₂ with T₂ spectra was designed to analyze water wetted cores containing oils of below-medium consistency,achieving good effects.Considering that it is difficult to determine the data acquisition parameters of improves CPMG pulse sequences in T₂-G measurement,a t0 adaptive method was designed,which is able to reduce the operational difficulty and improve the measuring efficiency of T₂-G measurement.The NMR signals of tight reservoirs are characterized by fast attenuation,small amplitude and low signal to noise ratio,which increases data processing difficulty.The methods for noise reduction and inversion of NMR data were introduced and in terms of noise treatment,a time-domain and frequency-domain dual filtering method was used to reduce the noise,achieving certain effects.For inversion,the effects of inversion when regularization parameters were selected for norm smoothing,slope smoothing and curvature smoothing were compared.The results show that better effects can be achieved through curvature smoothing at low signal to noise ratio.Excessively large internal magnetic field gradient and echo train spacing result in excessively low NMR porosity.Therefore,with reference to the idea of water-spectrum construction,T₂ spectra,from which diffusion and relaxation components were removed,were reconstructed,so as to eliminate the impact of internal magnetic field gradient.The application of NMR technology in computation of the organic porosity of tight reservoirs was investigated through scanning electron microscope(SEM)and spectroscopic analyses based on the characteristics of the NMR T₂ spectra of rocks of varying saturation.Based on centrifugal and NMR experiment data analysis,the modes of occurrence and laws of seepage of pore fluids were studied and a model for computing the permeability of tight sands based on NMR double cut-off values was proposed.For pore structure evaluation,the methods for computation of pore diameter distribution based on one-dimensional T₂ spectra distribution,heterogeneity evaluation based on two-dimensional SLICE imaging and visual evaluation based on three-dimensional SPRITE imaging technology were studied.