Processing And Application Of Nuclear Magnetic Resonance Logging In Lacustrine Shale Formation

Affected by sedimentation,diagenesis and thermal evolution of source rocks,the reservoir characteristics of lacustrine shale formation,conventional formation and marine shale formation are obviously different.The lacustrine shale formation develops many silty laminae and layers,and has quite high sand content.The porosity and permeability of lacustrine shale formation are ultra-low.The lacustrine shale formation has relatively high clay minerals and ferromagnetic minerals content.The organic matter thermal maturity of lacustrine shale formation is generally low.The lacustrine shale formation has complex pore structures,such as nano organic matter pores,intergranular and intragranular pores of clay minerals and detrital particles.The fluid types of lacustrine shale formation are complex since formation water and different types of hydrocarbon components coexist.The reservoir characteristics above-mentioned bring great challenges to the comprehensive interpretation and evaluation of well logging.Nuclear magnetic resonance(NMR)logging only detects the hydrogen nuclei of fluid in the pore space and it is the most effective logging method that can simultaneously characterize the pore structure and fluid types of the formation.NMR logging plays an important role in formation evaluation in conventional reservoirs.However,the application of NMR logging faces huge challenges in the lacustrine shale formation.The key issues are:(1)The relaxation mechanism and NMR response characteristics of lacustrine shale are not clear;(2)The signal-to-noise ratio of NMR logging is low,making it difficult to detect the nanopores;(3)The relationship between NMR spectrum and pore space is not clear,and the existed pore diameter conversion model is not suitable in the lacustrine shale formation;(4)Conventional methods such as difference spectrum method and shift spectrum method cannot identify the complex relaxation components,making it difficult to identify fluid properties in the lacustrine shale formation;(5)Conventional one-dimensional and two-dimensional NMR techniques cannot effectively distinguish complex components with short relaxation time.In order to solve the problem that the NMR porosity becomes low and NMR spectrum shifts because of the echo spacing and the internal magnetic field gradient,a practical method to compensate for the effect of echo spacing on the NMR porosity of lacustrine shale is proposed.Based on the NMR experiments with different echo spacing,the relationship between the NMR porosity and echo spacing can be established to recover the signal amplitude.A time-domain correction method to correct the influences of the internal gradients on the NMR T2 spectrum is proposed.Based on the T2-G two-dimensional NMR experiments,the internal magnetic field gradient estimation function can be established to correct the T2 spectrum.Based on these two correction methods,the NMR logging of lacustrine shale formation can be processed precisely.In order to solve the problem that the relationships between the transverse relaxation time and pore radiu are not clear and exact,the theory of NMR,mercury injection capillary pressure and nitrogen adsorption are detailedly discussed,the conversion models of transverse relaxation time and pore radiu for different pore types are established.In order to solve the problem that the T2 spectrums of lacustrine shale are cross-overlapped,a gaussian decomposition method is proposed to decompose T2 spectrum into multiple individual T2 spectrums.The NMR response characteristics and petrophysical significance of decomposed individual T2 spectrums can be determined,then full-scale pore radiu characterization of lacustrine shale formation can be realized by conversing different pore types with different conversion model.In order to solve the problem that the fluid types of lacustrine shale formation are diverse and the differences between NMR response characteristics of different fluid types are small,the NMR responses can be considered to be the linear superposition of multiple relaxation components,the one-dimensional or two-dimensional NMR data of core sample or formation can be considered to be the observation signal,the relaxation component with specific NMR response characteristics can be considered to be the source,the NMR response characteristics of each relaxation component can be considered to be the source signal,and the content of proportion of each relaxation component can be considered to be the signal amplitude.Then a fluid identification method can be established based on NMR blind source separation.Lacustrine shale formations are characterized by diverse pore types,complex relaxation components and cross-overlapped NMR spectrums.Conventional one-dimensional and two-dimensional NMR techniques are limited in the formation evaluation in the lacustrine shale formation.A tri-window pulse sequence to collect the full decaying information of porous media is proposed.In the first window,the inversion-recovery pulse sequence is applied for T1 encoding.In the second window,D and T2 are encoded by an adjustable continuous pulse field gradient and echo spacing.In the last window,CPMG with the shortest TE is used to acquire diffusion-free relaxation information.A joint inversion algorithm named‘composite-data-processing’ is also proposed to obtain the 3D correlation map.Our results suggest that the novel pulse sequence and inversion algorithm of 3D NMR can be effectively applied to the exploration of lacustrine shale resources.