A Guideline to Optimize Drilling Fluids for Coalbed Methane Reservoirs
Authors: Barr Kristin Lee 
Published: April 2009

Abstract
Coalbed methane plays are unique when compared to conventional sedimentary reservoirs and also can vary greatly between the different locations and depths. Due to these characteristics, the drilling fluids must be tailored to each specific coal seam in a particular location in order to optimize the production capabilities and wellbore design. There are a series of ten analytical methods of analysis that will try to characterize critical aspects of the well program to design a fluid system for surface, intermediate hole and the drill-in fluid for the production zone. This systematic approach will initially target any previous drilling problems that have been observed in the area as well as the wellbore design to ensure that a cased well is drilled. It will then focus on designing a drill-in fluid to reduce formation damage and optimize productivity. A case study of a coalbed methane project from the Washakie Basin in Wyoming demonstrates the success of this method. The tailored coalbed methane fluid system reduced the previously experienced problems in the area such as mud rings and sloughing shales as well as reducing the overall drilling time. The fluid system was also designed to reduce the formation damage through a theoretical analysis of the chemistry and damage mechanisms. Productivity results have yet to be determined.

Introduction
Coal reservoirs possess unique properties that need to be preserved during the drilling process to ensure maximum productivity of the coals. Coal formations differ from traditional reservoirs such as carbonate or sandstone they also vary greatly between different coal seams and areas. As a result it is not appropriate to drill all coalbed methane wells with one specific fluid. The drilling mud should be tailored to each specific area relative to the composition of the coal, the produced water, whether it is a wet or dry reservoir, as well as the reactivity of the formations above and below the coal seam. This paper will outline a set of analytical procedures to optimize wellbore construction as well as preserve the coals productivity to create the appropriate fluid for each specific section of the well. The mud system should not only be designed to alleviate any previous drilling problems but it also should minimize formation damage. A case study will be discussed that will detail the coal analysis as well as compare a tailored fluid system to a traditional low solids non-dispersed (LSND) fluid system.

Analytical Methods of Analysis

Wellbore Construction and Drilling Fluid Design
Offset Study: This is an area study of the previous wells drilled in the vicinity. It would investigate all zones of the wellbore and highlight any problems associated with each section. This will aid in the design of all of the fluids including surface, intermediate as well as the drill-in fluid for the production zone.
Drill-in Fluid DesignReservoir Analysis: This is a detailed research study of the reservoir that will give insight into characteristics such as pressures, thickness of the coal seam, whether the reservoir is wet or dry, depth, and salinity of the produced water.Coal Rank: This analysis will quantify the moisture, ash, volatile matter and fixed carbon content.X-Ray Diffraction: This will determine the amount and type of clay minerals present in the crystalline inorganic portion of the coal or shale sample.