Page 245 - Fundamentals of Enhanced Oil and Gas Recovery

CHAPTER EIGHT

Enhanced Gas Recovery Techniques

From Coalbed Methane Reservoirs

1 1 2
Alireza Keshavarz , Hamed Akhondzadeh , Mohammad Sayyafzadeh
1
and Masoumeh Zargar
1
School of Engineering, Edith Cowan University, Perth, WA, Australia
2
Australian School of Petroleum, The University of Adelaide, Adelaide, SA, Australia

8.1 INTRODUCTION

Over the past recent years, coalbed methane (CBM) has been among the fastest
growing unconventional reservoirs in the world. Although coal mining is well known
and has been regarded as one of the most reliable fuel supplies for a long time, gas
production from coal seams had not previously drawn oil and gas industries’ attention.
This is mainly attributed to the fact that the gas content in these reservoirs was not
considered to be substantial before the occurrence of some major explosions in min-
ing sites. Subsequent to perceiving the deep coal layers as potential unconventional
gas resources, the production was not yet encouraging due to the unique mechanism
of this reservoir type, rendering the gas companies disinterested in exploring CBM
fields. However, owing to economic obstacles in heavy oil production as well as
depletion of conventional reservoirs, in the past two decades, gas production feasibility
from CBM reservoirs has been investigated extensively, and CBM has proven to be a
promising unconventional gas resource, and such reservoirs are being extracted world-
wide for the time being. The world cumulative CBM in place has been estimated to
be over 8000 Tcf, with North America being the richest region, globally [1].
CBM reservoirs are, in essence, naturally fractured reservoirs, and the fluid bulk flow
occurs inside the fractures (cleats) toward the wellbore. The fracture system in CBM
reservoirs consists of two distinct sets of fracture, namely face cleats and butt cleats,
which are normally perpendicular to the reservoir bedding layers. Face cleats are the
well-developed cracks inside the reservoir that are fairly parallel and play the most sig-
nificant role in transferring the fluids toward the production well. Butt cleats, on the
other hand, comprise a set of less-developed parallel fractures that expectedly end at
their interconnection with face cleats, almost vertically. Normally, the natural cleats in
CBM reservoirs are initially filled with mobile water and contain negligible sorbed gas.

Fundamentals of Enhanced Oil and Gas Recovery from Conventional and Unconventional Reservoirs. © 2018 Elsevier Inc.
DOI: https://doi.org/10.1016/B978-0-12-813027-8.00008-4 All rights reserved. 233