NMR Relaxation Mechanism And Logs Interpretation Method Of Tight Sandstone

Tight sandstone is an important unconventional reservoir.Because of its low porosity and permeability,conventional logging response is insensitive to fluids,and resistivity logging response has few differences.It is difficult to identify fluids in tight sandstone reservoir.Nuclear magnetic resonance?NMR?logging is sensitive to fluids with hydrogen which in pore of saturated rocks.This technology can provide porosity information without regarding to the matrix of rock.It has important advantages in the evaluation of porosity,permeability,oil content and pore structure of low-porosity and low-permeability reservoirs.Therefore,this thesis attempts to use NMR technology to carry out tight sandstone logs interpretation methods.In this thesis,the Chang 8 reservoirs in Ordos Basin are taken as the object of research.Firstly,we verified the geological characteristics of the reservoirs with the help of petrophysical experiments.Secondly,we improved the processing parameters for NMR logging data according to the study with core-NMR experiments,so that the processing results of low signal-to-noise ratio data have been optimized.In addition,there are many kinds of NMR logging data,such as MRIL-P,MRIL-C,CMR and MRT.In this thesis,we carried out the study of adaptability and deepened application of the NMR data through comparing a variety of software.Considering with wettability,we took the research of the relaxation mechanism of NMR in porous media of tight sandstone by constructing theoretical model and digital core model.And we studied the relaxation and response characteristics of NMR in different wetting conditions with numerical simulation.Their effects on the methods of NMR fluid typing have been also clarified.Based on these studies,we have found out new methods of NMR fluid typing in view of the wettability characteristics of the formation.We have built cross-plot of T₂ geometric mean from T₂ differential spectrum and porosity difference inverted from the echo-strain difference.The plate has good application effect in this area and effectively improves the accuracy of fluid typing.Because 1D NMR has some limitations in fluid typing,we also carried out the study of 2D NMR logging in this thesis.2D NMR can measure more NMR information such as lateral relaxation time,longitudinal relaxation time,diffusion coefficient,internal gradient field,etc.Analyzing the?T₂,D?maps from different detection depths,we can get the information of the change in fluids on mud invasion profile and calculate the volume of fluids quantitatively.2D NMR has a good effect in fluid typing in complex reservoirs.The research results of this thesis have been applied to Chang 8 reservoirs in Ordos Basin.The effects of application is good.