Application Of Low Field Nuclear Magnetic Resonance In Characterizing The Physical Properties Of Coal Reservoir And The Occurrence And Migration Regularity Of Fluid In Coal

In this paper,low field nuclear magnetic resonance(LFNMR)was used to probe the fluid(water and methane),and we focused on the relationship of physical properties of coal sample and NMR parameters.Meanwhile,the occurrence and migration regularity of fluid in coal reservoir is discussed.The total amplitude of T2 spectrum is proportional to the mass of water in the sample.The NMR porosity of coal sample is determined by comparing the total amplitude of 100%saturated coal sample to that of the calibration sample with known volume.Compared with the porosities by weighing,NMR porosities are generally lower.By analyzing the quality changes of saturated sample in natural evaporation process and the corresponding amplitude changes of T2 spectrums,we think overlong echo spacing lead to the lower NMR porosity.After corrected by the relationship of the quality change and the amplitude change,the NMR porosities were in good agreement with the porosities by weighing.Comparison of mercury injection curve and the calibrated T2 spectrum shows that the results of two methods are equivalent.From the perspective of coal reservoir and coalbed methane production,the water in coal is divided into adsorbed water,capillary water and free water by the evaporation rate.The water within saturated coal sample evaporates with two stages(the initial fast evaporating stage and the followed slowly evaporating stage).The evaporating ratio of water in the initial stage to square root of time is proportional to the permeability of the sample.The two most commonly used expressions for permeability in NMR logging is Coates model and SDR model.We tried calculated the NMR permeability by these two models,but results is not satisfactory,especially for the sample with air permeability lower than 0.01mD.After taking out the sample with extra low permeability(<0.01 mD),the permeability derived from the modified SDR model correlated well to the air permeability.In this paper,we developed a non-magnetic sample can and the NMR characteristics of CH4 and CH4 in coal powder and fragment was explored.The NMR amplitude and relaxation time of pure CH4 both increase linearly with the increase of gas pressure.Because of the contact of CH4 and sample can inwall,the T2 spectrum of CH4 shifts to the fast relaxation time.In T2 spectrum of CH4 within coal,the amplitude less than 1ms contributed from the adsorbed methane on coal.We tested the T2 spectrums of CH4 within coal in different gas pressure,and a NMR adsorption isotherm was obtained.Compared with volumetric method,Langmuir volume derived from NMR adsorption isotherm is lower with a relative error of 32%.At last,T2 spectrums of coaly debris with different grain diameter were compared,indicating that the use of LFNMR in well log site is feasible.The obvious difference of T2 spectrums appeared in the region>100ms,which mainly because the water on debris surface was not wiped adequately.In most cases,the relative errors of T2 spectrums amplitude of coaly debris with different grain diameter in the two region,0.1?1ms and 10?100ms,are less than 10%?The difference shifts to the fast relaxation time for the sample rich in paramagnetic minerals,which must be considered when the physical property of debris is determined by LFNMR.