Most carbon sequestration projects involve using the CO2 to pump into underground oil formations in order to get out more oil than they'd normally be able to extract. This has some obvious contradictions and should not be considered carbon sequestration, especially since much of that CO2 comes back up in the produced oil.
As this MIT report shows on pages 4-5, CO2 comes up with the oil, and is separated and recycled to continue the oil recovery process. The oil industry lives tries to recover as much CO2 as possible. Only about 1/3 to 1/2 of the CO2 pumped back into the oil formation is (inadvertently) sequestered (see page 6) -- and only because the oil industry's precious fluid is lost in the process.
There is also the reality that the oil produced in the process releases CO2 when burned.
In the Weyburn oil field in Saskatchewan, Canada – where CO2 from the Dakota Gasification Company's coal gasification plant in Beulah, ND is piped north to pump into the oil field, buying 25 more years of oil production – 2.8 times more CO2 would be released from all of the extra oil they expect to produce than the amount they "sequester" (ignoring reports of leakage).
In the Permian Basin (TX/NM), 47% of the amount of CO2 pumped into the ground is re-released by burning the extra oil produced (that would otherwise stay in the ground).
Burning coal, waste or biomass with gasification technology is not a climate solution, even if you expend enormous amounts of money and energy to capture much of the CO2 and pump it into the ground – known as carbon capture and sequestration (CCS). This awful, and taxpayer-subsidized, idea is often coupled with enhanced oil recovery (EOR), which uses that CO2 to get oil out of depleted oil fields. Doing this just means that oil that would otherwise be left in the ground will be burned, adding CO2 back to the atmosphere. That CO2 pumped into the ground could very well leak out of the fractured subsurface and get back into the atmosphere as well, negating the "solution" of trying to permanently store CO2 underground. The only way to sequester carbon is to leave it in the ground in the first place, and to end the use of combustion for energy.
Here are the calculations behind the numbers above:
Weyburn:
20,000,000 tons CO2 [1] expected to be injected to get 130,000,000 extra barrels of oil over 25 years. [2]
0.43 metric tons CO2/barrel of oil [3]
130,000,000 * 0.43 = 55,900,000 metric tons CO2 per extra oil to be liberated in Weyburn from EOR
55,900,000 / 20,000,000 = 2.80 times more CO2 released than sequestered
Permian:
1.6 BCFD (billion cubic feet per day) = 1,600,000,000 cu ft/day of CO2 pumped into the fields [4],[5
1 SCF CO2 = 0.0001 metric tons CO2 [6]
1,600,000,000 * 0.0001 = 160,000 metric tons CO2/day
180,000 extra barrels/day of oil per day produced [4]
170,000 extra barrels/day of oil per day produced according to another source [6]
175,000 average of these two estimates
0.43 metric tons CO2/barrel of oil [3]
175,000 * 0.43 = 75,250 metric tons CO2 per extra oil liberated daily in Permian Basin from EOR
75,250 / 160,000 = 0.47 (47%) of the amount of CO2 sequestered is re-released through burning the extra oil recovered
Sources:
[1] http://sequestration.mit.edu/tools/projects/weyburn.html and http://www.planetseed.com/relatedarticle/weyburn-oil-field-enhanced-oil-recovery
[2] http://www.netl.doe.gov/file%20library/research/oil-gas/CO2_EOR_Primer.pdf
[3] http://www.epa.gov/cleanenergy/energy-resources/refs.html
[4] http://www.nist.gov/pml/high_megawatt/upload/6_2-Hustad-Approved.pdf (p5)
[5] http://www.kindermorgan.com/business/co2/transport.cfm
[6] http://www.uigi.com/co2_quantity_convert.html
[7] http://enhancedenergy.com/eor/industry.html