Page 86 - Principles of Applied Reservoir Simulation 2E

Part I: Reservoir Engineering Primer 71

successful commercial microbial EOR projects. The projects reflected a diversity
of locations, lithologies, depths, porosities, permeabilities, and temperatures.
Two of the projects were in the U.S., two in China, and one in Argentina, and
included sandstone, fractured dolomite, siltstone/sandstone, and fractured
sandstone reservoirs. Reservoir depths ranged from 4450 to 6900 feet, tempera-
tures from 110° to 180° F, porosity from 0.079 to 0.232, and effective permeabil-
ity from 1.7 to 300 md. Evidence from laboratory research and case/field studies
shows that microbial EOR processes can result in the incremental recovery of
oil and also reduce water production from high permeability zones. However,
more research needs to be done to maximize the potential for microbial EOR.
Some effort in this direction has been conducted. A microbial transport simulator
was developed under the auspices of the U.S. Department of Energy as a
modification to the black oil simulator BOAST.

8.3 Nonconventional Fossil Fuels

Clean energy refers to energy that is generated with little environmental
pollution. Natural gas is a source of clean energy. Oil and gas fields are
considered conventional sources of natural gas. In the following, we discuss two
nonconventional sources of natural gas: coalbed methane, and gas hydrates.

(oalbed Methane
Coalbeds are an abundant source of methane [Selley, 1998; Rogers, 1994].
The presence of methane gas in coal has been well known to coal miners as a
safety hazard, but is now being viewed as a source of natural gas. The gas is
bound in the micropore structure of the coalbed. It is able to diffuse into the
natural fracture network when a pressure gradient exists between the matrix and
the fracture network. The fracture network in coalbeds consists of microfractures.
The microfractures allow Darcy flow and are called "cleats."
Gas recovery from coalbeds depends on three processes [Kuuskraa and
Brandenburg, 1989]. Coalbed methane exists as a monomolecular layer on the
internal surface of the coal matrix. Its composition is predominately methane,
but can also include other constituents, such as ethane, carbon dioxide, nitrogen