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 get the gas out of the coalbeds in an economical manner. Recovesy of gas from coalbeds occurs in the presence of the low porosity and very low permeability inherent to these beds. The permeability is made up of natural fractures which are am-anged in a cleat system.
There are two cleat systems: the face cleat and the butt cleat. The face cleat is continuous throughout the reservoir and provides the largest permeability. Vertical wells drilled in the reservoir must be fractured to be in contact with the cleat system and maximize recovery. Coalbeds are much different than conventional natural gas reservoirs in that drilling and completion considerations are not the same due to the unique coalbed propetiies. Drilling vestical wells in coalbeds has become a very simple process in Alabarn%
Improvements in vertical well applications to coalbeds in the past few years have been centered on completion techniques. It has been shown that a vertical unli-actured well in a coalbed is difllcult to justify economicall~. However, with the help of hydraulic fracturing a vertical well can become economically feasible, Fig. 24. The use of a horizontal wellbore, however, allows control of the direction, so that the borehole can intersect the face cleat at right angles. The objective of this paper is to review the various horizontal drilling and completion techniques being used for the recovery of coalbed methane.