Effects of gas sorption-induced swelling/shrinkage on the cleat compressibility of coal under different bedding directions

Shoujian Peng,Jian Shen,Zhiming Fang,Jiang Xu,Geoff Wang

doi:10.1038/s41598-017-14678-1

Abstract

The cleat compressibility of coal is a key parameter that is extensively used in modeling the coal reservoir permeability for Coal Bed Methane (CBM) recovery. Cleat compressibility is often determined from the permeability measurement made at different confining pressures but with a constant pore pressure. Hence, this parameter ignores the sorption strain effects on the cleat compressibility. By using the transient pulse decay (TPD) technique, this study presents the results from a laboratory characterization program using coal core drilled from different bedding directions to estimate gas permeability and coal cleat compressibility under different pore pressures while maintaining effective stress constant. Cleat compressibility was determined from permeability and sorption strain measurements that are made at different pore pressures under an effective stress constant. Results show that the cleat compressibility of coal increases slightly with the increase of pore pressure. Moreover, the cleat compressibility of Sample P (representing the face cleats in coal) is larger than that of Sample C (representing the butt cleats in coal). This result suggests that cleat compressibility should not be regarded as constant in the modeling of the CBM recovery. Furthermore, the compressibility of face cleats is considerably sensitive to the sorption-induced swelling/shrinkage and offers significant effects on the coal permeability.

Highlights

The permeability of coal is one of the most critical parameters for the success of coal bed methane (CBM) recovery from coal reservoirs[1,2,3]
This study presents an alternative method to investigate the effects of sorption-induced swelling/shrinkage on coal permeability and cleat compressibility
The transient pulse decay (TPD) technique was employed for the laboratory characterization of the coal core to measure coal permeability with gas adsorption under a constant effective pressure that can be maintained by adjusting the confining pressure

Summary

Introduction

The permeability of coal is one of the most critical parameters for the success of coal bed methane (CBM) recovery from coal reservoirs[1,2,3]. The counteracting processes of matrix shrinkage and effective stress operate on cleat apertures during gas production These situations imply that coal reservoir permeability varies with time. Coal permeability is sensitive to reservoir stress conditions and the gas sorption-induced swelling/shrinkage behavior[5,6,7]. Connell et al.[15] presented the results of a laboratory characterization using coal core and estimated the properties that are required to apply the SD and Palmer–Mansoori models for coal permeability. Thereafter, these permeability measurements were used to estimate the cleat compressibility by fitting the Seidle model to the observations. A strong matrix shrinkage behavior may not lead to significant permeability rebound if the cleat compressibility is minimal

Methods

Results

Conclusion