Estimating CO2 Fluxes Along Leaky Wellbores
- Qing Tao (University of Texas at Austin) | Dean Checkai (University of Texas at Austin) | Nicolas Huerta (University of Texas at Austin) | Steven Bryant (University of Texas at Austin)
- Document ID
- Society of Petroleum Engineers
- SPE Journal
- Publication Date
- April 2014
- Document Type
- Journal Paper
- 227 - 238
- 2013. Society of Petroleum Engineers
- 1.14 Casing and Cementing, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 4.3.4 Scale
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Large-scale geological storage of carbon dioxide (CO2) is likelyto bring CO2 plumes into contact with a large number of existingwellbores. The flux of CO2 along a leaking wellbore requires a modelof fluid properties and of transport along the leakage pathway. Knowing therange of effective permeability of faulty cement is essential for estimatingthe risk of CO2 leakage. The central premise of this paper is thatthe leakage pathway in wells that exhibit sustained casing pressure (SCP) isanalogous to the rate-limiting part of the leakage pathway in any wellbore thatCO2 might encounter. Thus, field observations of SCP can be used toestimate transport properties of a CO2-leakage pathway. Uncertaintyin the estimate can be reduced by accounting for constraints fromwell-construction geometry and from physical considerations. We then describe asimple CO2 leakage model. The model accounts for variation inCO2 properties along the leakage path and allows the path toterminate in an unconfined (constantpressure) exit. The latter assumptionprovides a worst-case leakage flux.
By use of pathway permeabilities consistent with observations in SCP wells,we obtain a range of CO2 fluxes for the cases of buoyancy-driven(post-injection) and pressure-driven (during injection) leakage. Assuming thefrequency distribution is representative of SCP wells, we observe that inleakage pathways corresponding to the slow but nonnegligible buildup of casingpressure (several psi/D), the effective permeability of the leakage path is inthe range of microdarcies to hundreds of microdarcies, and the correspondingCO2 fluxes are comparable with naturally occurring background fluxesobserved at the ground surface. In pathways corresponding to intermediate andfast buildup rate of casing pressure (tens to hundreds of psi/D), the effectivepermeability is in the range of tenths to tens of millidarcies, and theCO2 fluxes are comparable with surface flux measurements at theIllinois basin and at the natural seep at Crystal Geyser (Utah). In pathwayscorresponding to very fast buildup rate (thousands of psi/D), the effectivepermeability is from tens to hundreds of millidarcies and the CO2fluxes are up to three orders of magnitude higher than those measured atCrystal Geyser.
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