Abstract

In this study, pore structures and water permeation property of a shale rock are analyzed by means of X-ray CT. The target samples are produced Kushiro district in Hokkaido, Japan. Underground of this area is consisted by Cretaceous formation. Firstly, one dimensional permeation tests were performed by using core sample retrieved from Cretaceous formation, and intrinsic permeability was evaluated. Secondly, the internal structure of rock samples was observed by X-ray CT scanner, and porosity distributions were also evaluated by comparing CT image data between the dry and the water-saturated conditions. It was found that Cretaceous formation has relatively low permeability as k=10−19 ~ 10−18 m2. It was also found that the porosity of each sample was approximately 7%~13%, however, porosity distribution was not uniform, and was strongly influenced by density distribution in samples.

1. Introduction

Natural gas/oil has been developed all over the world, and most of gas/oil has been supplied from so called conventional natural gas/oil deposits for long time. However, as technology and geological knowledge is advancing, many unconventional natural gas/oil deposits have been discovered, and total amount of supplied energy is increasing [1]. The representative of unconventional natural energy deposits is shale rock layers [1]. In Japan also, there are several promising shale rock layers as gas/oil reservoirs [2], and certain amount of gas/oil deposits are inspected.

In this study, pore structures and water permeation property of a shale rock are analyzed by means of X-ray CT. The target samples are produced Kushiro district in Hokkaido, Japan. Underground of this area is consisted by Cretaceous formation. This is mainly formed by sandy shale and it is spreading under Kushiro coal seam. It is estimated that huge amount of methane gas exists in the Cretaceous formation [3], however, the origin of the methane and the storage process in the formation including its total amount are still not clear. The purpose of this study is to obtain fundamental characteristics or properties of the shale rocks located at Kushiro, such as nominal porosity, permeability, porosity and density distributions. Here, nominal porosity is evaluated through water absorption tests. One dimensional permeation tests are performed, and intrinsic permeability of the shale samples are evaluated. In order to inspect the internal structure of the shale, X-ray CT method is applied. From the CT image data, the internal structure of rock samples is discussed, and porosity distributions are also evaluated by comparing CT image data between the dry and the water-saturated conditions.

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