Reactive Transport Modeling of Geologic CO{sub 2} Sequestration in Saline Aquifers: The Influence of Intra-Aquifer Shales and the Relative Effectiveness of Structural, Solubility, and Mineral Trapping During Prograde and Retrograde Sequestration
In this study, we address a series of fundamental questions regarding the processes and effectiveness of geologic CO{sub 2} sequestration in saline aquifers. We begin with the broadest: what is the ultimate fate of CO{sub 2} injected into these environments? Once injected, it is immediately subject to two sets of competing processes: migration processes and sequestration processes. In terms of migration, the CO{sub 2} moves by volumetric displacement of formation waters, with which it is largely immiscible; by gravity segregation, which causes the immiscible CO{sub 2} plume to rise owing to its relatively low density; and by viscous fingering, owing to its relatively low viscosity. In terms of sequestration, some fraction of the rising plume will dissolve into formation waters (solubility trapping); some fraction may react with formation minerals to precipitate carbonates (mineral trapping); and the remaining portion eventually reaches the cap rock, where it migrates up-dip, potentially accumulating in local topographic highs (structural trapping). Although this concept of competing migration/sequestration processes is intuitively obvious, identifying those sub-processes that dominate the competition is by no means straightforward. Hence, at present there are large uncertainties associated with the ultimate fate of injected CO{sub 2} (Figure 1). Principal among these: can a typical shale cap rock provide a secure seal? Because gravity segregation will always keep the immiscible CO{sub 2} plume moving towards the surface, caprock integrity is the single most important variable influencing isolation security. An extremely thick shale cap rock exists at Sleipner (several 100 m); here, however, we examine the performance of a 25-m-thick cap, which is more representative of the general case. Although the cap rock represents the final barrier to vertical CO{sub 2} migration, what is the effect of intra-aquifer permeability structure? Because this structure directs the path of all CO{sub 2} migration processes within the target formation, it will effectively determine the spatial extent of plume-aquifer interaction, and thereby exert a controlling influence on all sequestration processes. Here, we consider three common settings: a homogeneous saline aquifer, one with inter-bedded laterally continuous shales (continuum representation of microfractured shales), and one with inter-bedded laterally discontinuous shales (discrete representation of lateral facies changes). For each configuration, we examine the unique character of immiscible CO{sub 2} migration paths, describe the dependent location, timing, and extent of associated solubility and mineral trapping, and detail the relative partitioning of injected CO{sub 2} among the immiscible plume, formation waters, and carbonate precipitates. While intra-aquifer permeability structure establishes the spatial framework of plume-aquifer interaction, the effectiveness of solubility and mineral trapping within this setting is largely determined by compositional characteristics of the aquifer and (if present) its inter-bedded shales. Here, we focus on Sleipner, where the saline aquifer consists of unconsolidated impure quartz sand saturated with a seawater-like aqueous phase, and there is strong evidence of thin interbedded shales. Based on our modeling results for this environment, we infer the effect of varying fluid composition from dilute to saline to brine, and the effect of varying sand and shale mineralogy within relevant limits. In addition, we describe those compositional characteristics required to maximize solubility and mineral trapping for a given permeability configuration. We also address the fundamental yet infrequently posed question: what happens when CO{sub 2} injection is terminated? Hydrologic and geochemical evolution may be very different during the relatively brief ''prograde'' (active-injection) and subsequent long-term ''retrograde'' (postinjection) regimes of geologic sequestration. Most importantly, are prograde trapping mechanisms enhanced or reversed during the retrograde phase (which spans geologic time scales)? We will demonstrate that there are indeed significant differences between prograde and retrograde sequestration.
- Research Organization:
- Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
- Sponsoring Organization:
- US Department of Energy (US)
- DOE Contract Number:
- W-7405-ENG-48
- OSTI ID:
- 15002519
- Report Number(s):
- UCRL-JC-146932; TRN: US200416%%83
- Resource Relation:
- Conference: First National Conference on Carbon Sequestration, Washington, DC (US), 05/14/2001--05/17/2001; Other Information: PBD: 24 Apr 2001
- Country of Publication:
- United States
- Language:
- English
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