Numerical study on complex resistivity measurement of porous media containing gas hydrate

摘要

Natural gas hydrate is potentially a new energy source. Most of the naturally occurring gas hydrate are present in the marine sediments. As the stability of gas hydrate in porous sediments is largely influenced by pressure, temperature and properties of the pore water, it is rather difficult to estimate its concentration. Electrical complex resistivity measurements have been used for estimating the gas hydrate saturation with the aid of Archie's method. However, the various distribution forms of gas hydrate in porous media and existence of interfaces between the hydrate and sediment particles made the imaginary part of conductivity highly sophisticated. Traditional saturation evaluation methods used for oil and water cannot be used for gas hydrate any more. To elucidate the mechanism of hydrate saturation evaluation by complex resistivity measurement that implemented in the experimental work and to investigate the connections between gas hydrate distribution and electrode responses, simulations on the resistivity measuring equipment and porous media containing gas hydrate were carried out. Numerical models with different distribution forms (such as layer, block and vein) of gas hydrate in a reactor were constructed. The EM field inside the reactor and full current responses of electrodes were calculated by finite element method (FEM). Then the apparent conductivity σ_sand apparent permittivity ε_swere obtained for the inversion of hydrate saturation. The simulation results have shown that: firstly, there are obvious correlations between the gas hydrate distribution forms and electrode responses, from which the distribution forms can be deduced qualitatively; secondly, the current responses to the media containing gas hydrates are very similar to those containing oil at high frequency; thirdly, the imaginary part of current response for the case with gas hydrate in the form of layer, block or vein increase abnormally at extremely low frequency, and it depends on the shape and size of gas hydrate. It is postulated that the current relaxation exists at the interface between the gas hydrate and sediment particles, and it implies that induced polarization occurs in these models.