Abstract

This paper presents a scoping study to investigate the economic viability of nitrogen injection to improve the recovery of coalbed gas. The sensitivity of the analysis to key reservoir parameters is also presented.

The results of the study suggest that nitrogen injection may be more attractive than conventional pressure depletion for lower permeability formations where the loss in permeability with pressure decline is significant. Nitrogen injection economics are shown to be very sensitive to the costs associated with injection and separation and are adversely affected by reservoir heterogeneity.

Introduction

Conventional pressure depletion methods for the recovery of coalbed methane are inefficient with ultimate recoveries generally not exceeding 50% of the initially sorbed gas volume. This has led to an increasing interest in enhanced recovery processes using nitrogen and carbon dioxide injection. Puri and Yee report modelling studies which suggest that both the rate of methane recovery and ultimate recovery are significantly increased by the injection of nitrogen. However, the improved methane recovery is accompanied by increased costs associated with the supply of nitrogen and the need to separate the produced gas stream as a result of early nitrogen breakthrough at production wells.

Although Puri and Yee suggest that the economic benefits of nitrogen injection more than offset the nitrogen injection and separation costs, they provide no details of their economic evaluation and no indication of the sensitivity of project economics to the high levels of uncertainty in estimating important reservoir parameters such as permeability, reservoir heterogeneity, gas-water relative permeability and the stress dependence of cleat permeability and porosity. Their study assumed a homogeneous seam, core measured relative permeabilities and constant rock properties. These assumptions largely determine the production characteristics of the seam - methane and water production rates and nitrogen breakthrough - and therefore determine the economic viability of the process. There exists a clear need for a parametric study to investigate the effect of reservoir properties on the economic viability of nitrogen injection which utilises more realistic predictions of potential field performance.

The purpose of the present paper is to present the results of a preliminary economic evaluation of the nitrogen injection process for potential application to the Fruitland Formation in the San Juan basin. This particular formation was chosen both because it is the focus of considerable development activity and because it has been the subject of numerous recent studies to evaluate reservoir properties. The sensitivity of nitrogen injection economics to key reservoir parameters is demonstrated on the basis of reservoir simulations using a three-dimensional, two-phase, dual porosity, fully compositional numerical code which models multicomponent sorption in a thermodynamically consistent manner.

Reservoir Description

The reservoir data used in this study is similar to that for the Cedar Hill field in the Fruitland formation of the San Juan basin. The data used is summarised in Table 1.

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