A laboratory investigation of permeability variation associated with fines migration during water flow in coal

Abstract

Permeability is a key parameter in the production of coal seam gas (CSG). Laboratory experiments have been carried out extensively to study the coal permeability. Typical approach includes injection of fluids (e.g. water and gas) through coal samples and permeability can be determined from measured pressure drop across the sample and flow rate. Some of these studies have demonstrated coal permeability variation during water/gas flow. These permeability variations have been attributed to physical blockage of flow paths by fines migration and/or coal creep. Investigation of these studies has indicated some limitations:

(i) Effluent water was not characterized which could include characterization of produced fines, dissolved elements in the effluent water. (ii) Coal characterization (coal rank and mineralogy etc.) was not conducted so that a correlation between the permeability variation and coal and fines characteristics could not be investigated. This is an important missing information which can help understand the root causes for the permeability variation. (iii) The effect of coal creep was not separated from that of fines migration on the permeability variation. So, it was difficult to analyze and conclusively demonstrate the impact on coal permeability of each mechanism.

The purpose of this thesis is to investigate to what degree fines migration is responsible for coal permeability variation during water flow. A systematic physical and analytical modeling approach is proposed for this purpose. The approach includes designed water flow tests, characterization of coal samples, analysis of effluent water and analytical modeling of permeability damage.

The results demonstrate that permeability is affected by fines migration during water flow in coal. A good correlation between the permeability variation and fines production is observed. The fines produced from coal are found to be mainly coal and clay. Analytical calculations of the interaction energy between the observed fines and coal surface suggest that clay fines are easier to be mobilized than coal fines. Bituminous coal fines appear to mobilize more easily than anthracite coal fines. The analytical tool predicts reasonably the observed permeability variation.