Aquifers

 

What is an aquifer?

source Sleipner Schematic

 

 

Aquifers are looked to as one of the most promising places for sequestration of carbon dioxide due to the tremendous amount of carbon dioxide they can potentially store [1] .  An aquifer is a porous rock bed below the surface of the earth or the sea in which gases can be trapped.   If the rock bed has a low permeability or obstructions to the free flow of fluids, pressure can build up, causing the injection rate and the total capacity for carbon dioxide to decrease [2] . Aquifers, therefore, must have a high permeability value, meaning liquids (usually water) can flow through them with ease [3] . There are aquifers that satisfy these criteria on every continent, and estimates for their total capacity vary from 500 gigatons [4] to thousands of gigatons [5] .

To be suitable for sequestration, and aquifer must satisfy the following criteria:

1) the highest point must be at least 800m deep (carbon is in a supercritical state)

2) must be sealed (have a "regional aquitard")

3) adequate porosity. "near-well permeability should be high to allow good injection capability, but the regional permeability values should be low so that the residence time of the carbon dioxide is high [6] "

 

In addition, the aquifer should be close to the source of the carbon dioxide producer to cut down on transportation costs [7] .

 

 

How is CO₂ injected into the aquifer?

source

 

There are two methods that are generally accepted in the mainstream as the best way to inject carbon dioxide into the aquifer. 

 

Hydrodynamic trapping. Carbon dioxide is trapped either as a gas or in a supercritical state. Below 800m, CO₂ has reaches its supercritical state.  Once inside the aquifer, the carbon dioxide flows with the same rate as the formation water in the system.  Generally, this rate may be between 1-10 cm/year.  At this rate, the carbon dioxide will be within tens of kilometers in millions of year [8] . This may be the most important method in the short term [9] .

 

Mineral Trapping.  Carbon dioxide reacts with minerals and organic matter within the aquifers to form carbonate minerals that become permanently fixed solids.  Carbon dioxide will remain immobile forever, making mineral trapping the only method that makes carbon dioxide completely harmless [10] .  When carbon dioxide dissolves in water the following reaction occurs:

 

H₂O + CO₂ = HCO₃^- + H⁺

 

Small amounts of the bicarbonate ion (HCO₃^-) and the proton (H⁺) are produced, regardless of the pressure of CO₂. The proton acts, however, to lower the pH of the formation water, and this creates the opportunity for "attack" on silicate minerals.  The result is the release of calcium (Ca), magnesium (Mg), and iron (Fe) free ions:

 

H₂O + CaAl₂Si₂O₈ + 2H⁺ = Ca²⁺ + Al₂Si₂O₅.(OH)₄ [11]

 

Calcium free radicals rapidly react with bicarbonate ions, forming calcite (CaCO₃), which is generally agreed to be the best form for storing carbon dioxide.

 

Ca²⁺ +HCO₃^- = CaCO₃ + H⁺

 

The sum of these reactions is

 

CaAl₂Si₂O₈ + 2H₂O + CO₂ = Al₂Si₂O₅.(OH)₄ + CaCO₃

 

Similar reactions occur for the formation of calcium-magnesium carbonate and iron carbonate.

A problem with this method of sequestration is that only minimal amounts of bicarbonate ions are formed.  To overcome this problem, it is optimal to use mineral trapping where there are large proton sinks.  This means that sandstone aquifers are preferable to carbonate aquifers [12] .

 

Geological conditions of an aquifer

 

No two aquifers are alike.  Pressure and temperature varies among and even within the rock bed.  Generally, temperature increases by about 30°C for every kilometer below the surface of the Earth.  Pressure is approximately equivalent to hydrostatic pressure, although when the pore space is not connected to the ground surface the pressure can be higher.  Pressure higher than hydrostatic pressure is called overpressure, and lower is called underpressure [13] .

 

Above a temperature of 31.1°C and pressure of 72.8 bars, CO₂ exists its most dense form, a supercritical liquid.  This is desirable, because CO₂ in its most dense phase takes up the least amount of room [14] . 

 

How much CO₂ can be stored?

 

Theoretically, aquifers alone should be able to accommodate more than all the CO₂ emitted worldwide over the next hundred years [15] . However, very little data is known for the storage capacities of specific aquifers. It is important to determine the volume capacity of an aquifer before beginning a sequestration project.  Overfilling an aquifer can lead to several undesirable effects.  Overpressure can occur.  CO₂ may escape and interfere with other subsurface activities, such as coal mining or oil exploitation.  Groundwater may be polluted.

 

The storage capacity for an aquifer can be calculated by estimating the total volume of the rock formation, and dividing it by the proportion of the rock that can be used for storage. However, this is a very rough estimate of total volume, and it can't take into account the distribution, thickness, and porosity of the individual formation. The migration path of stored CO₂ can also affect the total possible volume because of variations in the number of pockets from aquifer to aquifer.  Unfortunately, there is almost no current data to refer to for this information [16] .

 

What is in the future for aquifers?

 

Injected as a gas, carbon dioxide is predicted to remain sequestered in aquifers for approximately a thousand years. As a mineral, the carbonate minerals should be immobile forever.  However, there are several other factors to consider.  There is some concern about the environmental impact of sequestering CO₂ in aquifers.  If the aquifer is not completely sealed, leakage may occur, resulting in escape of CO₂ to the atmosphere or contamination of groundwater [17] .  Also, rock beds constantly evolving.  The effect this may have on sequestered CO₂ is not fully understood, but it is possible for CO₂ to escape should it migrate to a lower pressure area [18] .

 

Aquifer disposal is feasible and probably the best option for landlocked areas, but it will be costly.  The high price is a result of expenses related to carbon dioxide capture, purification, and compression, as well as facilities for the site.

 

 

The Sleipner project

 

The Sleipner project has been sequestering carbon dioxide in the Utsira formation since 1996.  The Utsira formation is a large sand formation approximately 800 m below the Norwegian sector of the North Sea. The project began as a way to avoid paying the Norwegian tax on CO₂ emission-- $50 per ton CO₂ [19] .  Between 1996-1999, over two million tons of CO₂ were stored in this formation [20] . Twenty thousand tons are being added each week [21] .  A project called the Saline Aquifer Carbon Dioxide Storage (SACS) project is monitoring the site, specifically 1) creating a new seismic survey of the formation to infer how the carbon dioxide bubble is forming and 2) measuring the distribution to predict the density of the CO₂ for thousands of years to come [22] . First results of this study click link (http://www.ieagreen.org.uk/thermie.doc).

 

 

source Sleipner Platforms

 

 

 

Alberta Basin

An extensive proof of concept study was conducted in Alberta, Canada as a joint project between Alberta Energy, Environment Canada, Natural Resources Canada, TransAlta Utilities, Edmonton Power, and the Alberta Research Council.

 

 

note: for more info go to http://www.ieagreen.org.uk/sacs2.htm#co2facility

-           (www.fe.doe.gov/coal_power/sequestration_geologic.shtml 2)