Storage of Carbon Dioxide in Geologic Formations
Authors: Orr Franklin M. 
Published: June 2014

Distinguished Author Series articles are general, descriptive representations that summarize the state of the art in an area of technology by describing recent developments for readers who are not specialists in the topics discussed. Written by individuals recognized to be experts in the area, these articles provide key references to more definitive work and present specific details only to illustrate the technology. Purpose: to inform the general readership of recent advances in various areas of petroleum engineering.

Abstract
Concerns about rising concentrations of carbon dioxide (CO2) in the atmosphere have led to considering the possible large-scale storage of anthropogenic CO2 in the subsurface. This paper reviews options for capturing and storing CO2 in geologic formations (i.e., oil and gas reservoirs, aquifers, and coalbeds). Advantages and limitations of each of these approaches are discussed.

Introduction
Measurements of the concentrations of CO2 and other greenhouse gases in the atmosphere show clearly the effect of the industrial revolution. Fig. 1 shows,1 for example, that the concentrations of the most important greenhouse gases [CO2, methane (CH4), and nitrous oxide (N2O)] all have risen substantially from their preindustrial-age levels, with the rise starting in the late 1800s. The concentration of CO2, which is emitted to the atmosphere as a result of burning of fossil fuels, clearing of forests, and manufacture of cement, has increased by approximately a third from the preindustrial-age level of approximately 280 to 370 ppm today. That level is higher than the concentrations observed over the last 400,000 years.1
Emissions of CO2 are approximately 24&#x00D7;109 tonnes/yr (24 Gt/yr), or approximately 6.5 Gt/yr of carbon. While amounts of CO2 emitted annually resulting from human activities are modest compared to the large natural cycles that exchange CO2 between the atmosphere, oceans, and terrestrial biosphere, there is no doubt that current emissions exceed the capacity of the natural systems to absorb them. The accumulation of CO2 in the atmosphere is the result. Those accumulations are reflected by an accompanying reduction in the pH of the upper ocean of approximately 0.1. Thus, there is considerable evidence that human activities are interacting with the geochemistry of the planet on a global scale.
Greenhouse gases in the atmosphere also influence climate.1,2 Without water vapor and other greenhouse gases in the atmosphere, the average temperature of the planet would be below the freezing temperature of water, and the planet would be a very different place. Rising concentrations of CO2, and other greenhouse materials such as CH4, N2O, and black soot, raise the issue of interactions with global climate. These materials capture additional radiation in the atmosphere (although sulfate aerosols and clouds reflect sunlight and reduce the effects of this capture). The average temperature of the planet will adjust, eventually, to balance this change in radiative forcing. But planetary energy transport is a complex coupled nonlinear system involving different time scales and thermal masses for circulations in the ocean and atmosphere. Large-scale computations are used to estimate the influence of changing greenhouse-gas concentrations on climate. Estimates of the magnitude and timing of the response differ with the formulations of the models (which differ in the ways they represent physical mechanisms that act at subgrid scales and in the estimates of future emissions), but all the models show increasing average planetary temperatures over the course of this century. The estimated magnitudes of the temperature changes are a significant fraction of the difference in average planetary temperature between glacial and interglacial periods (5 to 7&#x00B0;C), but that warming would be in addition to the current temperature, which is near the maximum observed for the last 400,000 years.
CO2 has the greatest effect of the greenhouse gases, but taken together, CH4, chlorofluorocarbons, N2O, ozone, and black carbon currently have comparable effects on radiative forcing. Estimates of economic growth, energy use associated with economic activity, and CO2 emissions associated with energy suggest that the concentration of CO2 in the atmosphere will continue to grow during this century unless significant steps are taken to reduce releases of CO2 to the atmosphere.3 Wigley et al.3 investigated CO2 emission rates required to stabilize atmospheric CO2 concentrations. They found that substantial reductions in expected emissions would be needed by midcentury to stabilize concentrations at much higher levels than exist now. For example, stabilization at 550 ppm, approximately double the preindustrial-age level, would require steady reductions in the amount of CO2 emitted per unit of energy generated and addition of new systems for supplying energy that have essentially no CO2 emissions. The new systems will be needed to accommodate the growth in energy demand that will accompany population growth and the economic development of nations such as India, China, and Brazil. The estimates of Wigley et al.3 suggest that by the middle of this century, energy supplies that emit no CO2 that are approximately equal to current energy supplies will need to be added if demand is to be satisfied and atmospheric concentrations are to be stabilized. Because 85&#x0025; of the primary power is supplied by fossil fuels now, creating this new supply will be a very big challenge.
Hoffert et al.4 argued that meeting the challenge of stabilizing CO2 concentrations will require more-efficient use of energy and development of a variety of new technologies. They concluded that one technology that could contribute to meeting that challenge is the capture and storage of CO2 that would otherwise be emitted to the atmosphere. This paper reviews geologic options for CO2 sequestration and current activities in the development of that technology.
At least three options exist for geologic storage of CO25&#x2013;8: oil and gas reservoirs, deep saline aquifers, and unmineable coalbeds. Table 1 summarizes estimates of the capacity of the three storage options made by Parson and Keith5 and by the Intl. Energy Agency (IEA) Greenhouse R&#x0026;D Programme.7 These IEA estimates were those expected at storage costs of U.S. &#x0024;20/tonne or less (these costs do not include the cost of separating the CO2 from combustion gases, compression, or transportation to the storage site). While these estimates inevitably involve significant uncertainties, as the differences between estimates illustrate, the amounts shown are large enough compared with current and estimated future emission rates to suggest that there is potential for storing sufficient quantities of CO2 to have an effect on overall emissions if remaining research questions can be resolved and if costs can be reduced sufficiently.