Abstract

Just over ten years ago, coal bed methane production from Wyoming's Powder River Basin was virtually non-existent. Today, total gas production from Powder River Basin coals is almost 1 Bcf per day from nearly 10,000 wells. This tremendous resource is unique compared to other commercial coalbed methane plays with gas content an order of magnitude lower, and reservoir permeability values several orders of magnitude higher than other producing coal bed plays.

This paper documents the results of an ongoing evaluation of the reservoir and production characteristics of the Wyodak coals located in the fairway of the Powder River Basin activity. Most of the drilling and production to date has focused on the Wyodak and equivalent coal horizons. A reservoir simulation model has been constructed covering 136 sections and including over 1300 wells. A detailed geological description and a large data base of core data and test results were used in the construction of the model. Historical gas production, water production, and reservoir pressure data were successfully matched from 1993 to the present during the calibration of the simulation model. The calibrated model was then used to evaluate optimum development strategies for this shallow, high permeability coal resource.

This paper discusses the production characteristics of the Wyodak coal, the impact of well spacing and well timing on the recovery factor, and the influence of outside factors such as recharge from the regional aquifer. Our results contradict conclusions reported in prior studies concerning the influx of water from adjacent sand horizons. In addition, the effects of multiple well interference and depletion of undrilled portions of the coal by existing wells are documented and discussed.

Introduction

The first significant production from shallow coals in the Powder River Basin occurred in 1993. A total of 63 wells were drilled in the Marquiss area located in T48N R72W south of Gillette¹. The amount of water withdrawal from this area was substantial, and a significant region of pressure drawdown occurred in the Wyodak coals. As a result of this pressure drawdown, gas production began and continued on a steady incline. Additional development of the coal resource followed rapidly, and over the next ten years the exploitation of the Wyodak coal seam would emerge as one of the most active gas plays in the world.

The subbituminous Wyodak coals of the Powder River Basin are dramatically different than other commercially developed coals. Gas contents are typically an order of magnitude (or more) lower than one would encounter, moisture content is high, ash content is low, and the coal permeability is several orders of magnitude higher than other commercial coal plays². A 2001 study by the Potential Gas Agency estimates the total recoverable gas resource associated with coals in the Powder River Basin at 24 Tcf³.

The Wyodak interval of the Fort Union Formation contains the Anderson and Canyon coal seams. Prior to 2000, nearly all of the coalbed methane activity in the Powder River Basin focused on the Wyodak “fairway” located south of the city of Gillette. More recently, development activity has included the Big George coals to the west, and the exploitation of multiple coal seams to the north (Figure 1). The Wyodak coals have been developed at depths ranging from 250 feet to 1000 feet, while the stratigraphically higher Big George coals are found at depths of 900 feet or greater due to their location further west in the basin (Figure 2).