According to IPCC the maximum amount of carbon that might be sequestered by global afforestation and reforestation activities for the period 1995-2050 was estimated at 60-87 Gt C (70% in tropical forests, 25 % in temperate forests, and 5% in boreal forests). The issue most important is to decrease the rate of deforestation: there are 13 billion meters squared of tropical regions that are deforested every year. There is potential for these regions to reduce rates of deforestation by 50% by 2050, which would be a huge contribution to mitigate the global warming.
CO2 can remain trapped underground by means of different mechanisms that include:
Despite CCS (Carbon Capture and Sequestration) in geological formations is presented as a new way to combat the climate change by means of storing CO2, it must take into account the need of carring out the processes of CCS in a environmentanlly friendly form.
ABANDONED OIL AND GAS FIELDS
Oil and gas abandoned field are suitable for CO2 storage because:
The capacity of the reservoir will be limited by the need to avoid exceeding pressures that can damage the caprock. Shallow reservoirs usually have a low CO2 storage capacity since CO2 may be in the gas phase. Abandoned oil and gas fields are easier to assess than are saline formations because the geological structure are usually well characterized from existing wells.
ENHANCED OIL RECOVERY (EOR)
Opportunities for enhanced oil recovery (EOR) have increased the interest in CO2 storage. Although not being designed for CO2 capture, EOR projects have demonstrated an associated storage of CO2. EOR uses a miscible flooding that contains CO2, with an incremental oil recovery of 7-23% of the original oil in place. These mechanisms can use oil swelling and viscosity reduction for injection of immiscible fluids or completely miscible displacement in high-pressure applications in which more than 50% of the injected CO2 returns with the produced oil and is usually separated and re-injected into the reservoir (this minimizes the operating costs).
The remaining CO2 is trapped in the reservoir by irreducible saturation and dissolution in reservoir oil that it is not produced and in pore space that is not connected to the flow path for the producing wells.
The depth of the reservoir normally is more than 600 m.
Injection of immiscible fluids must often be enough for heavy- to-medium-gravity oils. The more desirable miscible flooding is applicable to light, low-viscosity oils. For miscible floods, the reservoir pressure must be higher than the minimum miscibility pressure needed for achieving miscibility between reservoir oil and CO2, depending on oil composition and gravity, reservoir temperature and purity of CO2.
To achieve effective removal of the oil, thin (less than 20 meters), high angle, homogeneous reservoirs with a low vertical permeability are required. For horizontal reservoirs, the absence of natural water flow, major gas cap and major natural fractures are preferred. The density difference between the lighter CO2 and the reservoir oil and water leads to movement of the CO2 along the top layerof the reservoir.
Consequently, reservoir heterogeneity may have a positive effect, slowing down the rise of CO2 to the top of the reservoir and forcing it to spread laterally, giving more complete invasion of the formation and better storage potential. It is shown that the use of CO2 enhanced oil recovery for CO2 storage can be a lower cost solution than saline formations and depleted oil and gas fields.
However, there are studies that prove that the injection of 1 ton of CO2 in a EOR project would allow the extraction of 0,6 tons of petrol that generate 3m2 tons of CO2. For this reason, coolmyplanet refuses EOR projects.
ENHANCED GAS RECOVERY (EGR)
Although up to 95% of original gas in place can be produced, CO2 could be injected into depleted gas reservoirs to enhance gas recovery by repressurizing the reservoir. The natural gas which is in the field is mixed with the CO2 that is injected and degrades gas production. This is one of the reason because this technique was believed to receive less attention. However, this mix have physical propierties for reservoir repressurization. These propierties include density: CO2 has higher density than CH4 and also a lower mobility. Due to these two propierties CO2 can migrate down and this will stabilize the displacement between CO2 injected and methane (the original gas in the field)
Wells are located at the upper layers of the reservoir to make easier the CO2 injection. Heterogeneity causes an increase in CO2 but the re-pressurization can happen before that CO2 advance that avoids major problems. Costs associated with CO2 capture must be reduced since it is the most costly part of the cycle of Carbon Capture and Sequestration. It is proved that CO2 injection is a good way to capture CO2 while enhancing methane recovery. To conclude, the process of EGR with CO2 injection is economically and technically possible and even favourable
Unlike EOR projects, EGR projects work with natural gas which is more environmentally friendy than petrol.
Saline formations are other kind of geological reservoirs. Their theoretical storage capacity range from 1000 to 104 GtCO2. They are deep sedimentary rocks saturated with brines containing high concentrations of dissolved salts. These formations contain enormous quantities of water, but they are not suitable for agriculture or human consumption. Saline brines can be used by the chemical industry and formation waters of varying salinity are used in health spas and for producing low-enthalpy geothermal energy.
Geothermal energy is likely to increase. For this reason, potential geothermal areas may not be suitable for CO2 storage. Areas with a great geothermal energy potential are generally less suitable for CO2 geological storage because of the high degree of faulting and fracturing and the sharp increase of temperature with depth. In very arid regions, deep saline formations may be considered for water desalinization.
The CO2 is injected into poorly cemented sands about 1000 m below the sea floor. The sandstone contains secondary thin shale or clay layers, which cause the internal movement of injected CO2. The upper layer is a seal which is an extensive thick shale or clay layer. The saline formation into which CO2 is injected has a very large storage capacity. Reservoir studies have shown that the CO2-saturated brine will become denser and sink,eliminating the potential for long-term leakage.
Its theoretical storage capacity varies from 3 to 200GtCO2. Coal contains fractures that give permeability to the system. Between cleats, solid coal has micropores into which gas molecules from the cleats can diffuse and be adsorbed. Coal can adsorb many gases and may contain up to 25 normal m3 methane per tonne of coal at coal seam pressures. It has a higher affinity to adsorb gaseous CO2 than CH4. The volumetric ratio of adsorbable CO2:CH4 ranges from as low as one for mature coals such as anthracite, to immature coals such as lignite. Gaseous CO2 injected through wells will flow, will diffuse into the coal matrix and will be adsorbed onto the coal micropore surfaces, freeing up gases with lower affinity to coal such as the methane. The process of CO2 trapping in coals seams for temperatures and pressures above the critical point is not well understood.
The transition temperature depends on:
Some studies suggest that injected CO2 may react with coal, further highlighting the difficulty in injecting CO2 into low-permeability coal. If CO2 is injected into coal seams, it can displace methane, thereby enhancing CBM recovery. Carbon dioxide ECBM has the potential to increase the amount of produced methane to nearly 90% of the gas, compared to conventional recovery of only 50% by reservoir-pressure depletion alone
Favourable areas for CO2 ECBM include:
If the coal is never mined or depressurized, it is likely CO2 will be stored for long time, but, as with any geological storage option, disturbance of the formation could void any storage.