The gas saturation in flushed zone of gas layers is low and the observed gas signal is weak because the mud filtrate invasion reduces the native gas saturation.A(T2,T1/T2) two-dimensional nuclear magnetic resonance(NMR) method of typing gas layers was developed.The performance of(T2,T1/T2) method for typing gas at different kinds of reservoirs,signal-to-noise ratios and gas saturations was studied in details through numerical simulation.The results show that(T2,T1/T2) method is especially useful for identifying gas formations.It is suitable for typing lower gas saturation formation where drilling mud filtrate flushes strongly.
Porosity is a basic parameter for evaluating reservoir,and NMR logging is an effective method to obtain the porosity. However,we have often found that there exist significant differences between NMR po-rosities and formation core porosities in the complex reservoir. In this paper,we list the factors which affect the NMR porosity response in the complex reservoir,such as longitudinal relaxation time (T1),transverse relaxation time (T2),hydrogen index (HI) and borehole environment. We show how these factors affect the NMR porosity response and suggest methods to correct them. This should improve the accuracy of NMR logging porosity in complex reservoirs for the terrestrial formation.
Special requirements for design of tools used for wireline NMR logging and NMR logging while drilling and for interpretation model are demanded due to the dispersion properties of NMR relaxation for crude oil. NMR longitudinal relaxation time (T_1) and transverse relaxation time (T_2) of the dead oil samples with different viscosities were measured by NMR spectrometers with a Larmor frequency of 2 MHz and 23 MHz at five different temperatures respectively. The results showed that T_1 was obviously dependent on the Larmor frequency of NMR spectrometer. The degree of T_1 dispersion became stronger with the increasing crude oil viscosity, Larmor frequency and the viscosity/temperature ratio. T_2 was independent of NMR spectrometer measuring frequency. It is suggested that the resonance frequency should be selected lower than 2 MHz when measuring T_1 in logging while-drilling and that T_1 dispersion should be corrected when Larmor frequency is higher than 2 MHz.
