A Numerical Simulation Study Of Formation Permeability As A Function Of Methane Hydrate Concentration

Methane hydrate is one of the most important potential energy in the world. Recently, the technology of hydrate exploration and recognition is becoming a hot issue in the geophysical field. In this paper, we modeled the permeability of methane hydrate bearing sediments as a function of methane hydrate concentration by using either the Schlumberger's T₂ relaxation time formula (SDR model) or a direct calculation based on Darcy's law assuming Poiseuille flow. On this basis, we tried to establish the new methods for hydrate recognition and reserve estimates.First, we studied the nuclear magnetic resonance (NMR) and the SDR model, introduce the basic principle, the measuring and calculation of parameter T₂ andφNMR,and the process of NMR experiments. By reviewing reported experimental results and analyzing the hydrates formation in the pore medium, we established the numerical simulation method based on SDR model. In the simulation test, we varied the proportion of irreducible and movable water as well as the total porosity associated with the T₂ distribution and studied if the normalized permeability as a function of methane hydrate saturation is dependent on these variations. The results suggest the behavior of the normalized permeability is determined by the component of fine sand and coarse sand, regardless of the pore size variation of the movable water.Second, using a tube-sphere model, we built up the methane hydrate saturation by randomly placing methane hydrate crystals in the pore space and computed the permeability using a direct calculation based on Darcy's law assuming Poiseuille flow. In this process, we found the effect of hydrate formation and pore connectivity on the changes of permeability. Through the nucleation rate, we also analyzed the way of each pore body and pore throat impact on the permeability, and confirmed that pore throat play a key role in fluid flow capacity.Earlier experimental measurements reported in the literature show that the rate of permeability declining is not fixed and there is a methane hydrate saturation range where the permeability remains relatively constant. We found that, when the SDR model is used, the simulation results show a curve of normalized permeability versus methane hydrate saturation quite close to that predicted by the Masuda model with N=10, however, different from Minagawa's result. When the permeability was directly calculated based on Darcy's law assuming Poiseuille flow, the simulation results show a much higher normalized permeability, and only show a trend consistent with the experimental results, i.e., with a permeability plateau, when the methane hydrate crystals are preferentially placed in the tubes, and the higher the preferential probability, the larger the range where the permeability has a plateau.