Published by the DTI Oil & Gas Directorate for the reservoir engineering and IOR community in the UK.
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CO₂ Sequestration: Modelling Enhanced Coal Bed Methane Production and Trapping in Sedimentary Basins

David Hicks of CMG Ltd (david.hicks@cmgl.ca) discusses advanced numerical modelling capability related to using CO₂ to enhance the production of methane from coal beds and to trapping CO₂ in sedimentary basins. (http://www.cmgl.ca)

Global warming, and the contribution of CO₂ emissions to this effect, has been a topic of much discussion and research. The past decade has seen a dramatic increase in public awareness of this issue and a demand to do something about it. This culminated in the Kyoto Protocol, signed by many countries in 1999. The Kyoto Protocol brought forward a framework of recognition and commitment to this problem, and provided the impetus to actively seek ways of mitigating green house gas production. The CO₂ reductions agreed by the EU countries, and the possibility of CO₂ trading and tax credits, has provided financial viability and impetus to processes which can reduce CO₂ emissions to the atmosphere.

The Computer Modelling Group (CMG) has been providing simulation tools for the oil and gas industry since it was formed in 1978, primarily in the area of advanced recovery techniques. Recently, in conjunction with partners in the Far East and North America, CMG has developed additions to its GEM (EoS) simulator to provide modeling capabilities for all aspects of CO₂ disposal. Known as GEM-GHG (Green House Gas), special consideration has been given to the controlling processes governing two distinct areas: Enhanced Coal Bed Methane (ECBM); and trapping in sedimentary basins, such as depleted oil and gas reservoirs and aquifers.

The injection of CO₂ into deep unmineable coalbeds is both an attractive way of enhancing methane production as well as trapping the injected CO₂. As many centres of energy production are already situated near coal seams, we can create a virtuous circle. This both benefits the operator and the environment through the re-cycling of flue gas emissions into the coal seams, trapping the produced CO2 and increasing the amounts of gas produced to run the plant. CO₂ preferentially absorbs onto the coal matrix with CO₂ molecules displacing CH⁴ molecules from sites on the coal surface. Thus the CO₂ remains trapped in the coal seam while liberating extra CH⁴ molecules. Carbon tax credits coupled with higher natural gas prices make this an exciting commercial opportunity.

For strictly sequestration purposes CO₂ can also be injected into deep aquifers, where CO₂ has a high density and high solubility in the aqueous phase at the high pressures that exist in these structures. There are two ways in which CO₂ can be trapped in aquifers: (1) structural (or hydrodynamic) trapping and (2) mineral trapping. The first process consists of trapping CO₂ into a flow system with low flow velocity over geological periods of time. The second process converts CO₂ to carbonate minerals and renders it immobile. The latter is very desirable as CO₂ is sequestered in a form that is harmless to the environment. Government restrictions and tax credits will also hopefully make this an economically viable consideration.

Geomechanics also plays an important role in the long-term entrapment of CO₂. It is desirable to inject CO2 into aquifers with impermeable cap rock and sealing geological faults. However, during injection, cap rock may become permeable and sealing faults may become conductive because of geomechanical deformations. When CO₂ is injected into the reservoir or aquifer, the pressure of the reservoir may increase to values that are much greater than the initial pressure. Because of this increase in pressure, the structure of the reservoir also changes. Cap rocks or boundary rocks may dilate, resulting in porosity and permeability changes. If the pressure further increases, cracks may occur, resulting in the escape of gas from the reservoir, invalidating the purpose of this CO₂ store.

In order to achieve true, long term, CO₂ sequestration the controlling factors must be able to be modeled, and understood, over extensive periods of time. Government bodies must be convinced of the effectiveness of each project, and the economic and environmental benefits must be proven. Simulation is a powerful way of achieving this, and allows all parties to understand the benefits and risks associated with any specific venture. CMG's GEM simulator provides the tool to do this, and CMG invites input into its research program from any European based organizations.

Gas saturation (click image for larger view)	CO2 mole fraction in aqueous phase (click image for larger view)
Aqueous phase density (kg/m3) (click image for larger view)	CO3-- molality (click image for larger view)
Calcite precipitation (mol/m3) (click image for larger view)	Dolomite precipitation (mol/m3) (click image for larger view)

Figures show profiles after 100 years, 21,500sm³/day CO₂ injected for first 2 years. Gas phase collapses as it is absorbed by aqueous phase. Heavy CO₂ saturated water descends through lighter formation water and carbonate minerals gradually precipitate driven by ionic equilibrium.

Reservoir properties for CO₂ injection into aquifer

CO₂ sequestration for cross-sectional run after 100 years

Moles of CO₂ sequestered under various forms for cross-sectional run

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