Evaluation of gas hydrate saturation by effective medium theory in shaly sands: a case study from the Qilian Mountain permafrost, China

Abstract

Gas hydrate saturation calculation is a complex problem in the evaluation of gas hydrate reservoirs in the Qilian Mountain permafrost. From observations of the actual rock samples and experimental results from scanning electron microscopy images, gas hydrate reservoirs contain some extent of laminated and dispersed clay that affects the resistivity response of formation rocks. In light of this situation, we need to consider the influence of clay content when we evaluate the saturation of gas hydrate reservoirs in the studied area. We establish two kinds of resistivity models according to the distribution of clay and obtain corresponding methods to evaluate the saturation of gas hydrates based on the effective medium theory (EMT). The results show that the clay content in the shaly sand reservoirs varies over a large range, from 3.0 to 45.0%, and the average value is 19.4% indicating a high clay content overall. The comparison between experimental resistivity and simulated results show that the models based on EMT can be used to characterize the resistivity of reservoirs with different clay distributions. In the DK-2 well, the mean value of gas hydrate saturation obtained by the EMT is 35.6%, whereas that estimated using the Archie equation is 9.5%. In the DK-3 well, similarly, the mean value of gas hydrate saturation obtained by the EMT is 34.4%, whereas that estimated using the Archie equation is 15.5%. The gas hydrate saturation obtained from the EMT is higher than that obtained from the Archie equation because the former takes into account not only the content but also the distribution of clay. The method based on EMT is more effective for identifying and estimating gas hydrate reservoirs in the Qilian Mountain permafrost.