Simulation of Physical-Chemical Processes During Carbon Dioxide Sequestration in Geological Structures

Abstract

Underground storage of carbon dioxide, CO2, is one way to reduce atmospheric releases of greenhouse gases. The sequestration of CO2 in a depleted gas reservoir is simulated, incorporating molecular diffusion between CO2 and the natural gas, dispersion, dissolution of CO2 in water, and chemical reactions of the CO2 with the aqueous phase and host rock. This study is applied to a depleted gas reservoir located in the North of Italy. Since the end of production the field has been used for gas storage, and in this work it has been considered for a pilot project of CO2 injection. The different physical and chemical phenomena acting in the reservoir are simulated to investigate their influence on the total storage capacity. The effect of injection rate, the point of injection and the purity of the injection fluid are analyzed. While molecular diffusion of CO2 in methane can be neglected, the dissolution of CO2 in water increases the maximum storage capacity. Both the pattern of reservoir heterogeneity and the degree of mechanical dispersion has a notable impact on the results. Particular attention should be given to the composition of the injection fluid, because the presence of impurities strongly affects the total stored mass. Low injection rates give a larger total stored mass, since gravitational forces act to transport the CO2 to the top of the aquifer, but only after long periods of injection. Near the gas/water contact, the residual gas saturation has a significant impact on storage.Precipitation reactions render the CO2 immobile in a solid phase, but over a very long timescale. Introduction A depleted gas reservoir represents an attractive target for carbon sequestration for several reasons. It contains a geological trap, and thanks to years of production, it has a transport and injection infrastructure. Although depleted, these reservoirs still contain natural gas: CO2 injection can be thought as a way to enhance gas production thanks to either reservoir repressurization or pressure maintenance. Many studies have already suggested that additional methane can be produced with CO2 injection[1]. Other work[2,3] has determined the optimal injection strategy, and has demonstrated that the best recoveries are obtained for the case of conventional depletion of the gas reservoir until abandonment with subsequent injection of CO2[2]. In order to evaluate the feasibility of CSEGR (Carbon Sequestration with Enhanced Gas Recovery) in geological formations it is essential to understand the physical and chemical phenomena acting in the reservoir. Pruess and co-workers[4,5] have performed extensive simulation studies of CO2 storage in saline aquifers with an emphasis on capturing details of geochemical reactions.Obi and Blunt6 used streamline-based simulation to capture the effects of aquifer heterogeneity on the flow, but used a very simple reaction model.Other authors have performed simulation studies of CO2 storage in gas fields, aquifers and oil fields[1,2,3,7,8,9,10,11,12,13], but have generally not considered the combined impacts of reservoir heterogeneity, dissolution and rate-limited reaction in detail. Once in the reservoir, CO2 will move due to the pressure gradient and gravitational effects[14]. CO2 is much denser than methane in both supercritical and gas conditions at all relevant pressures15, thus it will flow downwards through the gas reservoir, displacing the methane and repressurizing the reservoir. The displacement will be stable since CO2 is more viscous than CH4.

Laboratory Measurements of CO2-H2O Interfacial Tension at HP/HT Conditions: Implications for CO2 Sequestration in Deep Aquifers

“…Principle physical mechanisms by which CO 2 movement may be impeded or even halted in an aquifer include structural/stratigraphic; residual fluid; and hydrodynamic (Oloruntobi and LaForce, 2009;Larkin, 2010) trapping. The most dominant chemical trapping mechanisms include dissolution or solubility (Enick and Klara, 1990; Bahar and Liu, 2008;Taggart, 2009;Tabasinejad, et al, 2010Tabasinejad, et al, , 2011 and mineral or geochemical (Pruess and Garcia, 2002;Wellman, et al, 2003;Xu, et al, 2003;Calabrese, et al, 2005;Izgec, et al, 2005Izgec, et al, , 2006Zeidouni, et al, 2009). Though all of these mechanisms contribute to CO 2 sequestration, the stratigraphic/structural and residual fluid mechanisms have the largest and most immediate impact on trapping or retaining CO 2 in aquifers.…”

Streamline‐based simulation of carbon dioxide storage in a North Sea aquifer

“…They concluded that trapping of the CO 2 was a significant permanent storage mechanism. Other authors have considered the roles of relative permeability hysteresis [ Spiteri et al , 2005], dispersion [ Calabrese et al , 2005], and convective mixing [ Ennis‐King and Paterson , 2005] on storage.…”

Carbon dioxide sequestration in underground formations: review of experimental, modeling, and field studies

“…A considerable amount of research has been conducted in simulating and modeling CO 2 sequestration in the subsurface. Calabrese et al (2005) studied the physical and chemical processes during CO 2 sequestration in a depleted gas reservoir located in the north of Italy. They concluded that to maximize the volume of CO 2 injected, an optimum rate has to be defined.…”