Cleats & Joints
In normal coal, gas desorbs from the internal coal surfaces to diffuse through the coal matrix
and micropores to cleats and then enters the laminar flow regime. Cataclastic coals, which result
from the formation of sheared coals through a brittle deformation mechanism, have interconnected
pores and continuous cleats. They are divided into blocks of sizes intermediate between the cleat
and microfractures size. It is clear that, in as much as the dimensions of these blocks are smaller,
the mean diffusion distances are shorter, implying that the gas quickly reaches fractures and cleats
for laminar flow. There are three different stages in the transport of gases through sheared coals.
The first stage involves diffusion from and through the micropores to microfractures. Secondly, the
flow of gas proceeds through microfractures to cleats or fractures, and the last stage is gas movement
through cleats and fractures to the open surface. Ductile deformed coals are called mylonitic coal
and are located in special structural positions such as thrust ramps and small-scale folds. In this
kind of coal all fractures are tightly compressed, which means less ability to conduct the gas as
well as a vast specific surface area, which is the specification of a good gas reservoir. This type
of coal always exhibits low connectivity and results in the appearance of high pressure gas pockets,
which represent one of the most well defined outburst prone places in coalmines.
Larger fractures are called joints and are found to extend over the whole or part of the coal seam
and are much less frequent than cleat. These are of the exogenic origin and are related to tectonic
movement. The frequency of joints increases rapidly when approaching shear structures and faults, large
joints running over several times the thickness of the seam and at low angle to the coal are of endogenetic
origin. Joints can cut across the lithological boundaries in the seam, but are in general limited to the
seam thickness. Increase in their frequency is an indication of an approaching geological structure.
Cleats & Joints pg 1 |
Cleats & Joints pg 2
Cleats & Joints pg 3
