Research On Response Of Nuclear Magnetic Resonance Logging While Drilling

The porosity measured by NMR LWD tool is almost not affected by rock matrix and the tool can provide real-time information of the formation,so NMR LWD plays an irreplaceable role in formation evaluation.However,NMR LWD tool works under complex motion,and there are many factors affecting the logging responses.Due to the radial vibration,the magnetic field applied to the sample in the sensitive area is changing all the time,so it is necessary to study the nuclear magnetic resonance response in a non-uniform field.With the knowledge of coherence pathways,the formula for calculating the echo amplitude in the non-uniform field is derived.Since the pathways and calculation amount increase sharply as the echo numbers increase,the formula needs to be optimized.With the absence of relaxation and diffusion,the magnetization vector in the non-uniform field will also decay.The echo decay in a non-uniform field is affected not only by T₂ but also by T₁,which is an attenuation factor consisting of 1/T₁ and 1/T₂.By establishing a radial vibration model of NMR LWD,the finite element method is used to divide the samples into small voxel,??₀/?₁ can be calculated by assuming that B₀ in every voxel is a fixed value.Then the total echo signal in each vibration state is obtained.The effects of vibration frequency,vibration amplitude and field gradient on echo signals are discussed separately,and a correction method is also proposed.Finally,with the echo amplitude derived from coherence pathway,the response characteristics of the NMR LWD at the formation interface are also studied when the tool is in radial vibration.Based on the interface model,the effects of antenna length,well direction and formation thickness on the response characteristics are discussed by changing different formation parameters.With the three parameters mentioned above bigger and bigger,the porosity is more and more inaccurate.