Evaluating the CBM reservoirs using NMR logging data

Abstract

Abstract This paper discussed the application of nuclear magnetic resonance (NMR) logging in evaluating the pore structure of coal seams, physical properties, aquosity and sealing of the coal seam roof and floor. The study results show that T2 relaxation time spectrum could reflect the pore structure characteristics of the coal. The NMR transverse relaxation time (T2) of coal is 0.5~2.5, 20~50, and more than 100 ms, three spectral peaks represent meso pores, large pores and fractures. The larger the spectral peaks are where more pores and fractures develop. The T2 relaxation time spectrum of coal reservoirs in the research area is generally characterized by double-peak structure characteristics, which shows that the coal reservoir is microporous with meso pores. There are obvious gaps and poor continuities between the two peaks, and the break point is approximately 10 ms between the two peaks point. This indicates that the bound water and movable fluid in the coal reservoir cannot be connected effectively. NMR can reflect the level of porosity and permeability of the coal reservoir. Greater development of fracture and better permeability in the coal reservoir cause greater right peak of T2 relaxation time spectrum of the NMR. The effective porosity of coal reservoirs in the study area is generally less than 3%, which reflects the matrix pores of the coal seam. The fractures of large pore have little contribution to the porosity. The left peak of the T2 spectrum of a coal seam and its roof and floor are higher, which indicates that the meso pores and tiny pores develop, and there is less movable water. The low left peak and high right peak of T2 spectrum show that the meso and macro pores develop, and there is more movable water. In a well area with good sealing capability, the right peak of the NMR logging T2 spectrum of the coal seam roof and floor is low, the T2 spectrum peak amplitude is low, and there are low porosity and poor permeability. NMR logging technology can be used to evaluate the CBM (coalbed gas reservoir) effectively, and then correctly evaluate and effectively develop the CBM reservoir.