The production of methane from wet coalbeds is often associated with the production of significant amounts of water. While producing water is necessary in order to desorb the methane from the coal itself, the damage from the drilling fluids used is difficult to assess because the gas production follows weeks to months after the well is drilled. Commonly asked questions include: What are the important parameters for drilling an organic reservoir rock that is both the source and the trap for the methane? Has the drilling fluid affected the gas production? Are the cleats plugged? Does the "filtercake" have an impact on the flow of water and gas? Are stimulation techniques compatible with the drilling fluids used?
This paper describes the development of a unique drilling fluid to drill coalbed methane wells, with a special emphasis on horizontal applications. The fluid design incorporates products to match the delicate surface chemistry on the coal, a matting system to provide both borehole stability and minimize fluid losses to the cleats, and a BREAKER method of removing the matting system once drilling is completed.
Field results from three horizontal wells will be discussed, two of which were drilled with the new drilling fluid system. The wells have demonstrated exceptional stability in coal for lengths to 1000 m, controlled drilling rates and ease of running slotted liners. Methods for, and results of, placing the BREAKER in the horizontal wells are covered in depth.