NMR Response Mechanism And Evaluation Method For Shale Oil Reservoirs

Unconventional oil and gas resources such as shale oil and gas have become a hot spot in exploration and development and an important field of oil and gas exploration.Nuclear magnetic resonance technology plays an important role in oil and gas exploration and rock physics research by measuring hydrogen nuclear signals to obtain parameter information such as reservoir porosity and permeability.However,there are some problems in the evaluation of shale oil reservoirs by NMR technology,which are mainly manifested in the following aspects.Shale oil reservoirs develop micro-nano pore throats,and contain organic matter.Complex fluid types make NMR fluid evaluation difficult.There are differences in geological backgrounds such as shale oil deposition and structure in different basin blocks,and the NMR response of reservoirs is not consistent.The existing NMR fluid interpretation methods are not universal.Moreover,the measurement conditions and acquisition parameters of shale NMR logging and core experiments are inconsistent,and the influence of measurement parameters on NMR results is not clear.This paper combines theoretical analysis with physical experiments,analyzes the influence of measurement parameters on NMR results,studies the NMR response characteristics of fluids in different occurrence states of shale oil,summarizes the NMR relaxation characteristics of shale oil in different regions of China,and finally establishes shale oil reservoir fluid identification chart and reservoir oil saturation evaluation method.Firstly,the influence of instrument acquisition parameters and magnetic field environment on NMR measurement results is studied by numerical simulation and experimental measurement.The research shows that the magnetic field strength has an effect on the relaxation time,and the high and low frequency conversion relationship of relaxation time is established.The fluid interpretation chart obtained in the laboratory can be converted into a low frequency fluid identification chart of logging data.The research on the influence of waiting time and echo interval on NMR measurement of shale oil reservoir provides a reference for the setting of NMR acquisition parameters and the application and interpretation of measurement results.Through the theoretical analysis of relaxation time of different fluid components such as kerogen,oil and water,combined with the measurement results of multi-state core NMR experiments,the NMR response characteristics of different occurrence state fluids in shale oil reservoirs were obtained,and then the T2-T1 two-dimensional NMR fluid identification chart was constructed.Then,a series of nuclear magnetic resonance experiments were carried out on shale oil in the main blocks such as Lucaogou Formation in Junggar Basin,Fengcheng Formation in Junggar Basin,Qingshankou Formation in Songliao Basin and Yanchang Formation in Ordos Basin.One-dimensional and two-dimensional nuclear magnetic resonance response characteristics of shale oil reservoirs in different blocks were obtained,and the differences of shale oil reservoirs in different blocks were revealed.The oil content of shale oil core and the two-dimensional nuclear magnetic distribution of oil signal are further studied,and the oil-bearing characteristics of shale oil reservoirs in different regions are obtained.Finally,the T2-T1 two-dimensional NMR fluid identification chart is applied to the actual core and logging data,and the two-dimensional NMR core experiment and twodimensional NMR logging fluid evaluation method are established,which can accurately evaluate the fluid type and obtain the fluid saturation.The practical application shows that the fluid identification and saturation evaluation method of shale oil reservoir based on T2-T1 two-dimensional nuclear magnetic resonance has a good application effect,which gives full play to the advantages of nuclear magnetic resonance technology in shale oil reservoir evaluation,and is of great significance for shale oil exploration and development.At the same time,the applicability of different nuclear magnetic logging instruments for shale oil in each block was studied,and the applicability scheme of nuclear magnetic resonance logging instruments in shale oil reservoirs was obtained.