円盤状堆積層試料を用いた圧密・浸透連成試験によるメタンハイドレート堆積層における絶対浸透率の定式化―メタンハイドレート貯留層の浸透率評価に関する研究（第５報）―

Abstract

Methane hydrate (MH) is one of the potential natural gas resources in near future, because it exists in marine sediments or in permafrost regions worldwide. To evaluate the productivity of methane gas from MH reservoirs, it is necessary to develop a gas production simulator for doing case studies.In this study, for the purpose of numerical simulation on MH dissociation process, we carried out an experimental study for estimating permeability of a MH reservoir with compaction of the sediments due to decrease of effective stress in pore space. In actual hydrate fields, it is supposed that vertical consolidation of sediments occurs whereas gas and water flow in a horizontal direction. To reproduce such a situation, we attempted to measure the permeability of artificial MH sediment under the horizontal radial flow condition by using a specially designed apparatus.First, we investigated the relationship between absolute permeability and MH saturation in this radial flow system. When MH saturation increased from 0 to 0.30, permeability decreased from 12.0μm2 to 1.19μm2 that was one 12th of the value of sand matrix without any MH formation. The same tendency of permeability changes was obtained in liner flow system as previously reported, so the measuring technique of permeability by using radial flow system has been established.Second, by using the similar apparatus, compaction-permeability tests were conducted to formulate absolute permeability as a function of porosity and MH saturation. From experimental results, it was found that permeability decrease due to consolidation was remarkable in the cases of small sand grain diameter and large initial porosity. We determined the parameters of permeability reduction factor for porosity through a numerical analysis, and derived the final form of absolute permeability equation for MH reservoirs.