A Review of Horizontal Drilling and Completion Techniques for Recovery of Coalbed Methane
Samuel O. Osisanya, The University of Oklahoma, SPE and Robert F. Schaffitzel, Texaco Exploration and Production Inc., SPE
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In the last few years the oil and gas indusby has been turned around with respect to gas production and availability. Clean burning and abundant natural gas resources have caused high
demand, and because of the tax incentives, coalbed methane is a popular gas source. Coalbed reservoirs are much different
from conventional natural gas reservoirs because drilling and completion considerations are not the same due to the rock
Horizontal wellbores are considered to be vesy
effective in reservoirs that are relatively thin, naturally
fi-actured, and anisotropic with regard to permeability.
Coalbed reservoirs have all of these features. The concern
today is how to get the gas out of the coalbeds in an economic manner.
Coalbed cleat systems are made up of natural
fractures, hence vertical wells drilled in the reservoir must be Horizontal well drilling and completion is an
alternative technique to overcome low production as well as
reservoir heterogeneities in coalbed reservoirs. Some authors have shown that a properly drilled and completed horizontal
well can increase production by about seventy-five percent
over that of a fractured vertical well.
This paper discusses coalbed properties essential to the
application of horizontal drilling and completion.
techniques used to drill and complete horizontal wells in
coalbed reservoirs are discussed and compared with each
other to distinguish their application.
US Bureau of Mines has demonstrated that methane
drainage by horizontal and directional boreholes is a safe and effective method of removing methane in advance of mining
and of controlling methane emissions during mining’-2.
Horizontal holes were found to have the advantages of
relatively low drilling costs and the ability to intersect the coalbed cleat or tlacturc system, thus increasing permeability to gas flow. Hydraulically stimulated vertical holes on the
other hand had the disadvantages of requiring large numbers
of surface sites, higher costs, and production and maintenance problems. The concept of directionally drilled degasification holes was originally considered by the Bureau of Mines as a
means of combining the best elements of the surface vertical borehole and underground horizontal drilling techniques.
In the past few years the oil and gas industry has been
turned around to be called the gas and oil industry. The clean burning, easily accessible, and overly abundant natural gas
resource has created a high demand. For many years, coal
mines were degasified and most operators thought little of the value of the gas. But recent demand and tax incentives have
made coalbed methane gas a popular item. It is believed there is a large amount of gas stored in coalbeds throughout the United States with current estimates of 400 trillion cubic feet of gas being stored in coalbeds (Fig. 1, Table lY.
Today’s concern is how to...