Coalbed Methane Simulator Development for Improved Recovery of Coalbed Methane and CO2 Sequestration

Author:

Syahrial Ego1

Affiliation:

1. Lemigas

Abstract

Proposal Sequestration of CO2 in deep unmined coal seams is currently under development for improved recovery of coalbed methane (ICBM) as well as permanent storage of CO2. Recent studies have shown that CO2 displaces methane by adsorbing more readily onto the coal matrix compared to other greenhouse gases, and could therefore contribute towards reducing global warming. In order to carry out a more accurate assessment of the potential of ICBM and CO2 sequestration, field based numerical simulations are required. Existing simulators for primary CBM (coalbed methane) recovery cannot be applied since the process of CO2 injection in partially desorbed coalbeds is highly complex and not fully understood. The principal challenges encountered in numerical modelling of ICBM/CO2 sequestration processes which need to be solved include:two-phase flow,multiple gas components,impact of coal matrix swelling and shrinkage on permeability, andmixed gas sorption. This paper describes the development of a compositional simulator for improved recovery of coalbed methane and CO2 sequestration. The new features that describe the complex process of CO2 injection are implemented here. The numerical results for enhanced recovery indicate that matrix swelling associated with CO2 injection could results in more than an order of magnitude reduction in formation permeability around the injection well, hence prompt decline in well injectivity. The model prediction of the decline in well injectivity is consistent with the reported field observations in San Juan Basin USA. Also, a parametric study is conducted using this simulator to investigate the effects of coal properties on the enhancement of methane production efficiency based on published data. Introduction Sequestration of CO2 in deep unmined coal seams is currently under development for improved recovery of coalbed methane (ICBM) as well as permanent storage of CO2. Recent studies have shown that CO2 displaces methane by adsorbing more readily onto the coal matrix compared to other greenhouse gases, and could therefore contribute towards reducing global warming. In order to carry out a more accurate assessment of the potential of ICBM and CO2 sequestration, field based numerical simulations are required. Existing simulators for primary CBM (coalbed methane) recovery cannot be applied since the process of CO2 injection in partially desorbed coalbeds is highly complex and not fully understood. The principal challenges encountered in numerical modelling of ICBM/CO2 sequestration processes which need to be solved include:two-phase flow,multiple gas components,impact of coal matrix swelling and shrinkage on permeability, andmixed gas sorption. Coalbeds are heterogeneous and are usually characterised by two distict porosity systems - micropores and macropores.The macropores are know as the cleat that can be subdivided into the face cleat, which is continuous throughout the reservoir, and the butt cleat, which is discontinuous and terminates at intersections with the face cleat. Permeability ofcoal is regarded as the most important parameter controlling coalbed methane production rate. Due to its dual-porosity structure, where the permeability is predominantly provided by the cleat network which make up the secondary porosity system, the permeability of coal is highly stress-dependent.The face and butt cleats in coal seams are usually sub-vertically oriented. Thus changes in the cleat permeability can be considered to be primarily controlled by the prevailing effective horizontal stresses that act across the cleats. Coal matrix has been shown to shrink on desorption of gas and to expand again on readsorption. During primary methane production, two distict phenomena are known to be associated with reservoir pressure depletion, with opposing effects on coal permeability.[1] The first is an increase in the effective horizontal stress under uniaxial strain conditions in the reservoir. The second is gas desorption from the coal matrix, resulting in coal matrix shrinkage and thus a reduction in the horizontal stress.

Publisher

SPE

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