Abstract

The early stages of a coalbed methane (CBM) project development often require more extensive use of currently available technologies than can be economically justified when approached from a conventional oil and gas drilling focus. As a result, key evaluation tools and technologies are either omitted or not considered before significant decisions are made regarding viability of a CBM play. Understanding that the various lifecycle phases will each affect different objectives and decision points is important. Following site acquisition and estimating basic drilling costs, at least five lifecycle phases can be identified:

  1. Regional Resource Reconnaissance,

  2. Local Asset Evaluation,

  3. Early Development,

  4. Mature Development; and

  5. Declining Production.

A systematic review of current and recently developed enabling technologies is presented in the context of their potential use and applicability. Environmental risk and other constraints that can impact development vary globally, as do economics and production forecasting. New and emerging chemical technologies, as well as hydraulic fracturing refinements, play key roles in various lifecycle phases and decision making to identify successful CBM development projects as early as possible. The paper presents strategies that can reduce development phase failure risk and help predict or rank production potential. Economic constraints usually become more restrictive as the lifecycle moves to Phase 4 and beyond, but key information needed to enter Phase 4 is often overlooked. Examples of this scenario are presented from a global perspective.

Globally, new and existing technologies combined with dynamic gas and electricity markets are changing the nature of CBM development opportunities. More accurate and timely go/no-go information needs to be used in the decision making process. Converting development opportunities to develop-ment successes involves integrating planning and evaluation methods, using targeted development technologies in the proper phase, and managing risk.

Introduction

Coal's unique gas-storage mechanism is weak Van der Waal's forces holding gas molecules on the coal-matrix surface. Concentration gradient causes gas to be released from the tens to hundreds of square meter surface area per gram of coal. Methane and other light gases diffuse (Ficks Law) from the coal matrix toward a lower concentration. Coal can store many times its equivalent volume in gas because the gas molecules are packed tightly onto the surfaces of the coal. Globally, coal is estimated to hold from 2.980 to 9.260 trillion cubic feet of methane.1 Gas adsorbed onto and within the coal macerals desorbs through a complex flow path of pores and cleats of varying sizes. The physics of migration is dominated by diffusion or diffusivity at various scales. Some coals are diffusion limited, while others are not. Water, and sometimes gas, exists at equilibrium gas saturation. Concentration gradients are most readily generated by removing this water or gas from the cleat system by reducing pressure.

Often, adsorption isotherms are used to help define a relationship between gas storage capacity and reservoir pressure; from this, a critical desorption pressure can be determined. Conventional porous-media fluid-flow concepts, such as Darcy's law, relative permeability, and permeability anisotropy quantify reservoir mechanics after gas is released from the coal matrix. Industry literature provides extensive discussion of all the above subjects. CBM technologies, when viewed from a development lifecycle perspective, illustrate departures from a conventional oil and gas approach and unique enabling aspects of a particular technology.

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