Water Material Balance

Introduction

Based on the definition of compressiblity, pure water compressibility can be defined as:

With no free gas in pores (cleats) etc, water production in an undersaturatd coal is controlled by water and cleat compressibility which implies

However, because cleat compressibility is significantly larger than water compressibility, one could argue the following:

Substituting engineering terms

Assuming we are starting at some Pressure, Pr, greater than desorption pressure (Pd), then we can use the following as a water material balance for coal seam gas during undersaturated conditions.