A Review of Horizontal Drilling and Completion Techniques for Recovery of Coalbed Methane
Authors: Osisanya Samuel O. , Schaffitzel Robert F. 
Published: December 2007

In the last few years the oil and gas industry 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 properties. Horizontal wellbores are considered to be very effective in reservoirs that are relatively thin, naturally fractured, 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 fractured. 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. Several techniques used to drill and complete horizontal wells in coalbed reservoirs are discussed and compared with each other to distinguish their application.

Introduction

The 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. Horizontal holes were found to have the advantages of relatively low drilling costs and the ability to intersect the coalbed cleat or fracture 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 1).

Today's concern is how to get the gas out of the coalbeds in an economical manner. Recovery 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 arranged 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 properties.