SPE Journal
Volume 17, Number 2, June 2012, pp. 502-522

Summary

Cold mixed CO₂/water injection into hot-water reservoirs can be used for simultaneous geothermal-energy (heat) production and subsurface CO₂ storage. This paper studies this process in a 2D geothermal homogeneous reservoir, a layered reservoir, and a heterogeneous reservoir represented by a stochastic-random field. We give a set of simulations for a variety of CO₂/water-injection ratios. In this process, often regions of two-phase flow are connected to regions of single-phase flow. Different systems of equations apply for single-phase and two-phase regions. We develop a solution approach, called the nonisothermal-negative-saturation (NegSat) solution approach, to solve efficiently nonisothermal compositional flow problems (e.g., CO₂/water injection into geothermal reservoirs) that involve phase appearance, phase disappearance, and phase transitions. The advantage of this solution approach is that it circumvents using different equations for single-phase and two-phase regions and the ensuing unstable switching procedure. In the NegSat approach, a single-phase multicomponent fluid is replaced by an equivalent fictitious two-phase fluid with specific properties. The equivalent properties are such that the extended saturation of a fictitious gas is negative in the single-phase aqueous region.

We discuss the salient features of the simulations in detail. When two phases are present at the injection side, heterogeneity and layering lead to more CO₂ storage compared with the homogeneous case because of capillary trapping. In addition, layering avoids movement of the CO₂ to the upper part of the reservoir and thus reduces the risk of leakage. Our results also show that heterogeneity and layering change the character of the solution in terms of useful-energy production and CO₂ storage. The simulations can be used to construct a plot of the recovered useful energy vs. maximally stored CO₂. Increasing the amount of CO₂ in the injection mixture leads to bifurcation points at which the character of the solution in terms of energy production and CO₂ storage changes. For overall injected-CO₂ mole fractions less than 0.04, the result with gravity is the same as the result without gravity. For larger overall injected-CO₂ mole fractions, however, the plot without gravity differs from the plot with gravity because of early breakthrough of a supercritical-CO₂ tongue near the caprock. The plot of the useful energy (exergy) vs. the CO₂-storage capacity in the presence of gravity shows a Z-shape. The top horizontal part represents a branch of high exergy recovery and a relatively lower storage capacity, whereas the bottom part represents a branch of lower exergy recovery and a higher storage capacity.

© 2012. Society of Petroleum Engineers

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History

• Original manuscript received: 5 February 2011
• Meeting paper published: 24 May 2011
• Revised manuscript received: 21 September 2011
• Manuscript approved: 22 September 2011
• Published online: 16 April 2012
• Version of record: 11 June 2012