Adsorption/desorption characteristics for methane, nitrogen and carbon dioxide of coal samples from Southeast Qinshui Basin, China

This paper presents an experimental and modelling study of the adsorption/desorption of pure gases CH4, CO and N and their binary and ternary mixtures on coal samples obtained from southeast Qinshui Basin, China. Results show that the adsorbed amounts of N, CH4 and CO have approximate ratios of 1.0:1.3:2.4, respectively. No significant hysteresis from adsorption to desorption is observed for pure N and CH4 whereas significant hysteresis is measured for CO in CO -CH4 and CO-CH4-N mixtures and CH4 in the N -CH4 mixture. The experimental observations are modelled using three different models, namely the extended Langmuir (EL), the Langmuir-based ideal adsorbed solution (L-IAS) and the Dubinlin- Radushkevich-based ideal adsorbed solution (D-R-IAS). The models predict well the experimental observations for desorption tests. But the measurements for the low adsorbate capacity in binary and ternary mixtures are overestimated by the prediction models. It is found that the EL model predicts the CO -CH4 desorption test better while the D-R-IAS model is the best model for the CO-CH4- N adsorption

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

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

Characteristics of Elastoplastic Consolidation by Compaction and Its Effects on Coal Permeability

This paper presents a combined experimental–analytical investigation of coal strain development under fluctuating applied hydrostatic stress. The laboratory setup mimics the isotropic volumetric compaction of coal under burial–uplift cycles in the absence of tectonic stress. Special emphasis is placed on the corresponding permeability evolution of the coal strata. Our results show that the stress–strain path is exponential, approaching a linear relation in the logarithmic stress–strain space with the monotonic increase in stress. A similar behavior is found for the strain–permeability path in the logarithmic strain–permeability space. The permeability recovery undergoes hysteresis with respect to the stress in a stress loading–unloading cycle, but the hysteresis is not manifest with respect to the strain. A theoretical geomechanical consolidation analysis was performed using an elastoplastic modelling framework. The analysis suggests that plastic strain is the cause of the hysteresis of the strain recovery in a stress loading–unloading cycle. The closed hysteresis loops manifested in stress loading–unloading–reloading cycles are promoted by the plastic strain during stress unloading and the difference in evolution rates of the elastic core between loading and unloading. The results of this study are helpful for understanding the mechanism of permeability evolution and optimizing water and coal seam gas production

A Novel Equivalent Numerical Simulation Method for Non-Darcy Seepage Flow in Low-Permeability Reservoirs

The low permeability and submicron throats in most shale or tight sandstone reservoirs have a significant impact on microscale flow. The flow characteristics can be described with difficultly by the conventional Darcy flow in low-permeability reservoirs. In particular, the thickness of the boundary layer is an important factor affecting the formation permeability, and the relative permeability curve obtained under conventional conditions cannot accurately express the seepage characteristics of porous media. In this work, the apparent permeability and relative permeability were calculated by using non-Darcy-flow mathematical modeling. The results revealed that the newly calculated oil–water relative permeability was slightly higher than that calculated by the Darcy seepage model. The results of the non-Darcy flow based on the conceptual model showed that the area swept by water in non-Darcy was smaller than that in Darcy seepage. The fingering phenomenon and the high bottom hole pressure in the non-Darcy seepage model resulted from the larger amount of injected water. There was a large pressure difference between the injection and production wells where the permeability changed greatly. A small pressure difference between wells resulted in lower variation of permeability. Consequently, the non-Darcy simulation results were consistent with actual production data