Abstract

One of the risks associated with CO2 geological sequestration is CO2 migrating upwards from storage reservoir along vertical leakagepaths, which may be the abandoned or improperly cemented wellbores, or thereactivated faults. CO2 migrating upwards is a process generallycontrolled by formation pressure gradient and CO2 gravity, affectedby the changes of CO2 properties such as density, viscosity, enthalpy and phase state along the paths, and companied with heat transferbetween escaping CO2 and the leakage paths and heat conduction information.

Considering absorbing or releasing latent heat in phase changes, this paperdeveloped models for CO2 flow and heat transfer during the leakageprocess. Then a procedure was developed to couple these models. Our studieshave shown that CO2 leakage tends to approach a steady state withinseveral years, thus it is reasonable to deal with this problem using the steadystate model generated in this study for the CO2 migrating upwards ina thousand-year long process. Sensitivity analyses of the factors affecting CO2 leakage rate, the pressure and temperature profiles along thepaths were studied. These factors are the permeability and cross-section areaof the leakage path, the pressure and temperature in the storage reservoir, andthe surface temperature. In terms of the temperature profile, CO2cooling effect caused by forming the super heated gas is dominant. It is alsodiscovered that, during the CO2 migrating upwards process, CO2 cooling effect along the paths is essentially described by thesaturation line in the CO2 P-T phase diagram.

A vertical fracture pattern with a radius of 1m and a permeability of 100mD wasassumed in case studies. Within reasonable ranges of formation pressure andtemperature gradients the altered temperature profile for the interval of 300m~ 700m, where CO2 phase is close to the saturation line in P-T phasediagram, provided a clue to detect CO2 leakage. A simple linearrelationship between the CO2 leakage rate and the maximumtemperature difference between the original and the altered temperatureprofiles was obtained through regression analysis, so that the CO2leakage rate could be estimated with the temperature profile logged along thewellbore in the field practice.

Introduction

Disposal of CO2 into geologic formations, including depleted oil andgas reservoirs, coal beds, and saline aquifers, has been investigated as apromising means to reduce atmospheric emissions of the greenhouse gas. The Weyburn CO2 Inject Project has been implemented to enhance the oilrecovery through CO2 injection while most of the CO2injected reaches a long-term geological storage within a Mississippianreservoir in southeastern Saskatchewan, Canada. However, under certainunfavorable conditions stored CO2 can leak back into the atmospherethrough vertical leakage paths, such as the improperly abandoned/cementedwellbores and the reactivated conductive faults, thus reduces the utility ofthis technology [8]. CO2 leakage also represents a potential vitalhazard to uppergeosphere and biosphere ecological systems.

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