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Abstract

Deep saline aquifers offer the potential for storing large volumes of CO₂.  However, with conventional CO₂ storage well design and operating practices, only a small fraction of this large CO₂ storage potential can be productively used.  Numerous alternative designs and practices, which utilize the reservoir’s internal architecture and draw on CO₂ storage engineering expertise, could enable CO₂ storage operators to more efficiently and fully utilize a saline formation’s storage capacity.  This would help reduce the areal extent of the CO₂ plume and its risk footprint.  It would also help reduce the number of CO₂ injection wells and patterns that would need to be developed.

This paper, entitled “CO₂ Storage Engineering: Real Solution to Real Problems”, discusses practical CO₂ storage options that have been proposed for significantly increasing CO₂ storage performance at three challenging saline reservoir settings.

Introduction

With large-scale CCS projects on the horizon, it is now timely to call on the emerging discipline of CO₂ Storage Engineering for concepts and designs that optimize the storage of CO₂.  The purpose of this paper - - “CO₂ Storage Engineering: Real Solutions to Real Problems” - - is to put into perspective the challenges of increasing CO₂ injection and usable CO₂ storage capacity while reducing the areal extent of the CO₂ plume.   The benefits from this include lower costs due to drilling fewer CO₂ injection wells and lower CO₂ storage risks due to a smaller area underlain by CO₂.  In addition, this paper sets forth a more rigorous reservoir characterization protocol essential for applying optimum CO₂ storage engineering practices.

The paper draws on three specific saline formation CO₂ storage sites for which detailed reservoir characterization, CO₂ plume modeling, and CO₂ storage design have been conducted, namely:

• The low permeability, moderately thick Oriskany saline formation in the Appalachian Basin;
• The moderate permeability, thick Paluxy saline formation in the Gulf Coast with discontinuous sand deposition; and
• The high permeability, thick Lower Tuscaloosa saline formation also in the Gulf Coast with continuous sand deposition.

For each saline formation, the paper discusses improvements in usable CO₂ storage capacity and reductions in well requirements from applying innovative CO₂ storage engineering practices.

Low Permeability Saline Formation

Background.   A sizeable portion of the CO₂ storage capacity provided by saline formations is in reservoirs with low permeability and only moderate thickness.  For example:

• In the western Pennsylvania portion of the Appalachian Basin, the Oriskany Formation, a major CO₂ storage option in this region, has low to moderate permeability.   
• In the deeper portion of the Illinois Basin, the Mt. Simon Formation, the dominant CO₂ storage option for the Midwest, also appears to have low to moderate permeability. 

Because of permeability-based restriction on CO₂ injection rates, alternatives to traditional vertical well drilling and completion designs will likely be essential for these reservoirs.  

Reservoir Setting.  The Oriskany is an important low permeability, moderately thick saline formation in the Appalachian Basin, with reservoir properties for the area we examined set forth in Table 1.  Our objective was to assess the CO₂ injectivity and storage capacity of this formation under alternative well drilling and completion designs.   Our first step was to construct a geological model and then place this model into a reservoir simulator.  Table 2 shows the model layers and reservoir properties for the Oriskany saline reservoir.