Abstract
The swelling effect of high-pressure carbon dioxide (CO2) in coal seam is obvious. In the restrained deep formation, it is converted to stress acting on the wellbore and the caprock. The action stress is concentrated near the wellbore and poses a threat to the cement-formation interface. Due to interface failure to micro-annulus, wellbore integrity will be lost and this will have an impact on carbon dioxide-enhanced coalbed methane recovery (CO2-ECBM) and storage. In this paper, the pseudo-steady pressure distribution and steady pressure distribution of CO2 injection process were established after considering the change in permeability of coal seam injected with high-pressure supercritical CO2, and the vertical stress distribution model was derived. A one-dimensional radial numerical simulation formed by iterative method was established. A model for calculating the failure length at the cement-formation interface is obtained, and the shear stress and debonding length at the interface at various injection rates and times are calculated. The results show that the shear stress on the cement-formation interface has the maximum magnitude on the height of the interface between coal seam and caprock. The shear stress generated by coal swelling may break the fragile cement-formation interface into a narrow debonding interface. The injection rate has an influence on the interface failure length. For the same total injection amount, low injection rate is beneficial to protect the cement-formation interface integrity. This study provides a reference for the design of maximum injection speed for CO2-ECBM and storage to avoid leakage.
Highlights
Fossil fuels such as coal, oil and natural gas provide above 80% primary energy of the world
With the same C O2 injection amount, the time is longer in the low injection rate, so the CO2 in the coal seam is more dispersed, and the radial pressure distribution is more balanced when the coal seam adsorbed CO2
With the injection of CO2, the swelling take place on coal seam and the vertical stress is formed along the interface between coal seam and caprock as the coal
Summary
Fossil fuels such as coal, oil and natural gas provide above 80% primary energy of the world. The high-pressure injection swelling effect near wellbore increases the stress on the cement-formation interface. The present study of geological integrity does not consider the cement-formation interface influenced by the effect of coal swelling which produce great stress under the consideration of high-pressure supercritical C O2. In this study, considering the pore pressure distribution after CO2 injection, combined with the swelling effect of supercritical CO2 in the coal seam, the stress distribution and the shear stress value at the cement-formation interface of the caprock are obtained. For a system containing CO2 and CH4, differential strain develops due to excess strain caused by dissolution of C O2 in coal compared to that of C H4, on unit concentration basis Whether it is the supercritical C O2 flooding or storage process, the pore pressure of coal seam reaches the pseudosteady state in a short period of time. When the shear stress equals to sc, the corresponding z is the cement-formation debonding failure length
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