Why Low Permeability and How Does It Affect Coalbed Methane Supply in Coal Seams in China
Authors: Meng Y.F. , Luo P.Y. , Jin L. 
Published: December 2007

This paper analyses the main effects of permeability of coal seams on the coalbed methane exploration in the light of mechanisms of methane adsorption and desorption and the micropore structures. It points out that the obstacle to a successful exploitation of coalbed methane is the low permeability of the coal seams. Further, the stress constraint of the seal structure with gas saturated fractures is the primary cause of the low permeability of the coal seams in China. The critical depth that is affected by this cause, based on the existing coalbed exploration technology, is calculated in this paper.

Introduction - Characteristics of Porous Structures of Coal Seams

Coal rocks are organic rocks with cleats and pores being very mature. Most coal matrix contains very fine and interconnected micropores with pore diameters in the order of less than 100 angstroms. The porosity of the micropores is about 3 to 5%, and the permeability is in the range of microDarcy. Coal seams have very mature cleats that can be classified in two types. One type is the primary cleats, the other type is the secondary cleats. The primary cleats are formed as a result of volumetric shrinkage of carbogel during the carbonization process under temperature and pressure. Therefore, the distribution density of the primary cleats is related to the rank of the coal as shown in Fig. 1(c). The primary cleat systems usually consist of two orthogonal cleats. The dominant cleats are referred to as the face cleats.

Compared to the other cleats, called butt cleats, the face cleats have better interconnection such that the cleat porosity is 2% to 3%, and the permeability is in the milli-Darcy range. Conceptually, we can represent the face cleats by a dual-porosity system as shown in Fig. 1(b).

The secondary cleat system is formed by the tectonic setting and in-place stress changes. The density of distribution and the geometric configuration of the secondary cleats depend on the structural movement and the direction of the primary stress. Sometimes the coal matrix may contain middle to large size pores that may also contribute to the permeability. These pores are classified into five types according to their diameters. Each type of pores has a different effect on the diffusion and osmosis of methane as listed in Table 1. Furthermore, based on the proportion of pore types the structure of coal seams is divided into three categories: open, transition, and closure (Fig. 2).

The open coal seam contains many large and mid pores. These pores provide a good osmotic path for the methane desorbed from micropores and transition pores and can be drained easily. Due to the lack of large and mid pores in closure seams, it is difficult for the methane to be drained since it must pass through a long and narrow path in order to diffuse and migrate into the fractures. Methane from transition seams is less accessible than that from the closure seams but not as easy to drain as from the open seams. Measured porosity distribution results shown in Fig. 3 indicate that the cleat system is the main contribution to the permeability. The micropores are rarely permeable.

With large amount of micropores and cleats, coal rocks have huge specific surface areas in their seams. The internal surface area of one gram of coal may reach 100 to 400 m2. Methane in coals is either absorbed on the coal surfaces, or as free methane molecules in fractures and large pore spaces (some are dissolved in ground water stored in the coal structures). About 80% to 90% of the methane is absorbed onto the huge internal surfaces of the coal. The adsorption and desorption of methane molecules equilibrate under certain pressure and temperature.

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