4. What We Have Learnt from CO2 Sequestration in Saline Aquifers
CO2 emission reduction becomes one of the main concerns of the world; hence deep saline aquifers can be used as CO2 storage  to decrease CO2 emission. Aquifers are the favorable storage places comparing to depleted reservoirs and coal seams, mainly because of safety issues   . Poorly-sealed abounded wells are reducing the safety of storage capability of depleted reservoirs, while water-bearing permeable rock nature of saline aquifers is increasing the safety factor  .
To perform a successful CO2 sequestration project in saline aquifers, monitoring and verification of storing process is a must. Underground distribution of CO2 needs to be monitored during the life of project. It is essential to identify when the minimum volume and saturation of CO2 has been reached in the storage reservoir.
Lithology, geomechanics and geology of the storage reservoir is main elements which drive CO2 detection capability within the reservoir. Having said that, it must also be noted that all of these reservoir characteristics have some level of uncertainties. There is no doubt about that these uncertainties may have an impact on CO2 distribution detection in the reservoir. Therefore, these uncertainty elements also need to be taken into account for the CO2 sequestration and CO2 distribution detection planning stages  .
CO2 detectability also relies on the phase in which CO2 is injected in the storage reservoir. CO2 is customarily injected as a liquid form to be able to transform to a supercritical phase. Supercritical state has both a liquid and a gas characteristic; hence supercritical fluid expands like a gas, while keeping its density as a liquid level. CO2 density is adequate to fill the pores at depths lower than 2600 ft.
State of CO2, and the environment into which it is sequestered, have a significant bearing on the detectability of CO2. Typically, CO2 is injected as a liquid which transforms to a supercritical fluid  . In this state, it has properties of both a liquid and a gas so that it expands like a gas, but with a density of a liquid. At depths below 800 m, CO2 density is high enough to allow efficient filling of pore space. There is also a reduction in the buoyancy difference between the CO2 and other pore fluids.
Miscibility is another important characteristic which varies with CO2 sequestration conditions. As it is a known fact that CO2 and natural gas are miscible, therefore CO2 is able to relocate water in the pores. However, since our interest here is CO2 sequestration in saline aquifers, multiphase relationship of CO2 and the aquifer dictates the pore space volume which can be filled by CO2   .
5. CO2 Sequestration Capacity of Shale
Coming from the extremely tight and low permeability nature of shale reservoirs, to produce from or to inject any fluid into shale reservoirs is tough. However, technological improvements in hydraulic fracturing technique made these reservoirs producible. Hydraulic fracturing basically opens artificial flow channels for reservoir fluid to flow with cracking the formation via pumping of highly pressurized fluid into the formation. Once these shale reservoirs depleted, existing fractures in the reservoir could be storage for CO2 sequestration. Another advantage of shale reservoirs is that they do not require capital expense cost such as new well drilling which is the case for saline aquifers  .
Viability of CO2 storage in shale reservoirs can be predicted by reservoir models, since it is critical to identify CO2 storage capacity of the reservoir. There are two major criteria while performing feasibility study for CO2 sequestration in shale formations. The first criterion is the diffusivity which gives an idea about formation deliverability. The first criterion in this regard is gas adsorption/desorption characteristics of shale formations. It is an ongoing study and many researchers claim that CO2 may adsorb the shale formation more favorable than methane.
Storage capacity of shale formations for CO2 storage is promising. It is also claimed that CO2 injection time in shale reservoirs would be much faster than the methane production from these reservoirs. In addition, it is estimated that around 14.5 billion tons of CO2 can be sequestrated into shale formations in the next two decades. The estimated capacity is almost half of the U.S. CO2 emissions from power plants within the next 20 years, which is equal to 20% of total expected CO2 emission of the U.S.
Based on numerical investigation, it concluded that the maximum CO2 storage capacity for eastern U.S  shale gas is 1.12 million metric tons per square kilometer, and the sorbed CO2 storage capacity is estimated to be 0.72 million metric tons per square kilometer. The total CO2 storage capacity of organic-rich shales at supercritical conditions as a function of pore pressure was measured by  by considering the pore compressibility and sorption effects. The results state that kerogen, the organic part of the shale, acts as a molecular sieve and accounts for the gas sorption on shales. The sorption capacity of shales is affected by its TOC content, clay minerals and micropore structure. There is a large body of literature investigating the sorption capacity  .
6. CO2 Trapping Mechanisms
Trapping mechanisms are important in CO2 sequestration feasibility studies. Four trapping mechanisms are present to our knowledge which are given in Figure 3, and can be named as: mineral trapping, solubility trapping, residual CO2 trapping, and structural/stratigraphic trapping. Trapping time is considered up to ten thousand years same as the nuclear storage projects. To give an idea about 10,000 years consideration time, it has been 11,000 years up to today since the last ice age  .
Figure 3. Schematic representation of the security of CO2 trapping mechanisms over time.
The most common type of trapping mechanisms is geological structures for CO2 storage in depleted reservoirs. Deep saline aquifers also have structural trapping mechanism coming from their depositional environment. When upward migration of CO2 is blocked by impermeable rock layer, CO2 is trapped due to buoyancy effect.
Moving from geological structures for trapping, solubility type of trapping takes place when CO2 dissolved in formation water  . Reservoir pore pressure and temperature, along with the formation water salinity are the key elements for solubility trapping. CO2 solubility rises with increasing pressure, on the other hand decreases with increasing temperature and salinity. With this mechanism CO2 is trapped as a liquid phase sinking under the gas phase due to gravitational forces. Solubility type of trapping is secure and favorable for CO2 storage  .
All in all, CO2 sequestration feasibility studies are not depending on one unique trapping mechanism. It is concluded that different geophysical characterizations will be necessary throughout the life of the project. For instance, earlier and later stages of the sequestration project might be sensitive to different characteristics, hence it is suggested that time lapse surveys must be performed through the life of the project to confirm sequestrated CO2 is trapped securely   .
7. Active Carbon Capture and Sequestration (CCS) Projects Worldwide
Carbon Capture and Sequestration projects are mainly performed in the U.S. and Europe, pie chart distribution of the world CCS projects is given in Figure 4. Canada, Australia and New Zealand, and China are also other major players of CCS projects  –  .
Figure 4. Countries with active CCS program.
Unconventional shale reservoirs are main hydrocarbon resources of today’s world, they are developed for oil and gas production. However, depleted unconventional shale reservoirs can be good candidates for CO2 sequestration and storage operations. Extremely low permeability nature of shale reservoir might seem like their lack, but on the other hand decent amount of CO2 can absorb on shale fracture surfaces. Existing natural and hydraulic fracture networks in shale reservoirs made these reservoirs attractive to permanent CO2 storage projects. That being said, when it comes to reservoir modeling for CO2 storage, many characteristic factors need to be taken into account such as buoyancy, heterogeneity of shale reservoirs, and the existence of formation water, because they will directly affect the storage capacity of the particular reservoir.
In conclusion, our extended literature review shows that shale reservoirs are good candidates for CO2 storage with the capacity of 5 to 10 kg/t per formation. These results verify the feasibility of CO2 sequestration in shale reservoirs. It can be stated that the long term feasibility of CO2 storage in shale reservoirs needs to be extended. However, the information available to our knowledge manifests that shale reservoirs can be good storage candidates for permanent CO2 storage, therefore studies need to be focused on to make these projects practical. It is concluded that unconventional shale reservoirs have many favorable characteristics for CO2 storage, and it is expected that these reservoirs will become very attractive for CO2 sequestration projects all around the world in the very near future.
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- Mohammad O. Eshkalak
- Petroleum and Geosystems Engineering Department, The University of Texas at Austin, Austin, TX, USA
- Email: [email protected]
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