Temperature can be used as a tracer to detect leakage of fluids from a CO2 storage zone. Brine leakage from the injection zone to an above-zone interval will induce a temperature increase as a result of geothermal gradient. Leakage of CO2 can induce a temperature decrease owing to the Joule-Thompson effect associated with pressure drop toward shallow zones. A larger pressure drop at shallower depths is associated with more CO2 expansion upon leakage and could induce more cooling and, hence, a stronger temperature signal. We investigate the strength of the temperature signal as a function of depth for two scenarios in which either a well or a fault acts as leakage pathway. The hydraulic properties of the leakage pathway, modeled as a fractured medium or as a porous medium, also impact the results. Using dual-porosity and dual-permeability models for which CO2 relative permeability and average absolute permeability are modeled higher than in the simple porous medium case, we study the effect of fractures on the temperature signal. The leakage rate increases significantly for dual medium models. However, the temperature change in above-zone interval does not change much as it depends on the pressure gradient which is reduced compared to the single-porosity medium case.

1. Introduction

The value of CO2 geological storage sites may be limited when defective wells and faults create pathways allowing migration out of the CO2 injection zone (IZ). Various monitoring techniques are available to assure storage quality and to detect and characterize leakage pathways. Leakage of CO2, brine, or their mixture can induce pressure and temperature changes in an above-zone monitoring interval (AZMI) that is separated from the IZ by a sealing confining layer. Monitoring of the pressure signal in the AZMI has provided information about leakage pathways (Zeidouni and Pooladi-Darvish 2012a, b, Sun et al. 2013, Jung, Zhou, and Birkholzer 2013, Haghighat et al. 2013). More recent work by Zeidouni et al. (Zeidouni, Nicot, and Hovorka 2014) investigated the potential for leakage detection on the basis of the AZMI's temperature signal. Results showed that the thermal signal is controlled by several processes, including Joule-Thompson (JT) effect, heat of dissolution/vaporization of CO2/water, temperature discrepancy between the injected and native fluids, geothermal gradient, and heat exchange with the surrounding rock-fluid system.

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