Cleat (Formation) Compressibility

Introduction

Cleat or void volume compressibility of a coal is analogous to pore compressibility of of conventional reservoirs. This value can also be called formation compressibility.

According to Seidle (2011), coal formation compressibility is very difficult to measure (controls and predictive capability are not well understood), and values within a factor of two are considered as good agreement.

Various reported values of coal compressiblity (from industry) are shown in the table below (modified from Seidle ). For comparison, pore volume compressibilities of conventional rocks range from 10^-6 to 10^-4, making cleat compressibility up to 1,000 times larger.

The large formation compressibilities have a significant impact on water saturation, relative permeability and material balance calculations and therefore predicted gas production rates.

References:

• John Seidle, Fundamentals of Coalbed Methane Reservoir Engineering, 2011 PennWell Corporation