What is Carbon Dioxide Sequestration?

Carbon dioxide (CO₂) sequestration refers to the process by which CO₂ is removed or captured from the atmosphere to lower the effect of global warming. Sequestration is a natural phenomenon occurring on its own. However, the rate of CO₂ generation has surged sharply, and the natural sequestration process is unable to cope up with the situation.

Different Ways of Carbon dioxide (CO₂) sequestration in an oil and gas field

Theoretically, if carbon dioxide or any other gas is injected in close vicinity of an oil and gas well, the injected gas would expand and push the fluids in one direction. This particular principle is used in one of the many types of Enhanced Oil Recovery (EOR).

The oil and gas wells lack natural pressure to bring the hydrocarbons from the deep reservoirs to the surface. Some may have the required pressure, but due to regular production over the years, the pressure depletes. There comes a point in an oil and gas well when the well starts pumping more water than the hydrocarbon and thus becomes uneconomical.

Most of the oil and gas fields are abandoned because the cost of pumping hydrocarbon becomes uneconomical. The abandoned fields still have 30-40% of hydrocarbon left in their reservoir. Companies around the world are working on developing ways to produce that 30-40% of the oil economically.

Carbon dioxide sequestration is receiving considerable attention since most of the countries are working on cutting their carbon emissions and slow the rate of global warming. The CO₂ produced from the industrial processes or the atmosphere could be collected and sent through pipelines or gas tankers to the fields where they can be injected into the depleted reservoirs.

Flow chart for CO₂ sequestration in Oil & Gas field (Image Source: © Kalkine Group 2020)

The CO₂ sequestration is done in two ways in an oil and gas field:

• Abandoned natural gas fields could be used to fill in the void with captured carbon dioxide from the industrial processes. The reservoir will have storage and seal already in place to accommodate the gas.
• CO₂ could be used in enhanced oil recovery (EOR) process to extract the residual oil from the reservoir. It involves injecting the gas into the reservoir using one of the drilled wells.

A miscible fluid is formed when injected CO₂ mixes with reservoir oil under proper conditions. When miscibility occurs, the forces of capillary pressure which formerly held the oil, disappear. The oil is then free to be carried to the producing well. The concept of miscible displacement of oil by CO₂ has been known for many years and has been tried in a few full scale-field projects.

Some of the benefits and drawbacks of CO₂ sequestration for producing oil in the purview of the oil and gas industry are discussed further.

Criteria for application of CO₂ injection

• A large gas cap is an unfavorable factor. If the reservoir pressure is considerably lower than the miscibility pressure, a large volume of CO₂ is needed to obtain miscibility.
• A fractured reservoir is considered unfavorable because it provides a conduit from injection to producing well.
• A good and reliable source of CO₂ at a reasonable cost is a prerequisite.
• Relative thin permeable zones (15-25ft) in the reservoir are technically advantageous because they diminish the tendency of gravity override, but the thicker zones have an oil volume advantage.
• The depth of the reservoir plays a vital role as the miscibility pressure is around 1200 psi. The required depth to achieve the pressure is equal to or greater than 2,500 ft.
• Temperature is also an essential factor.
• The lower limit of oil gravity is in the range of 25-30? API depends whether the oil is aromatic or asphaltic etc.
• Pure CO₂ is preferred for injection, but contamination by methane increases the miscibility pressure. However, 5-10% of methane is tolerable.
• Hydrogen Sulphide (H₂S) lowers the miscibility pressure but causes corrosion and adversely affects the environment.

Benefits of using CO₂

• The most significant advantage of using CO₂ compared to other gases is that it can extract heavier components up to C-30. The solubility of CO₂ in oil causes it to swell.
• Swelling of oil by CO₂ is to a greater extent than caused by methane.
• The colorless gas lowers the oil viscosity and increases oil density.
• It is water-soluble and reduces its density.
• It achieves miscibility at relatively lower pressures range of 100-300 bar.
• CO₂ decreases the difference between oil and water density and eliminates the chances of gravity segregation.
• The greenhouse gas decreases the surface tension of oil and water, which leads to more effective displacement.

Disadvantages of using CO2 in EOR operation

• The main challenge in successful CO₂ injection operation is the mobility of the gas. The relatively lower density and viscosity of CO₂ compared to that of reservoir fluids can cause gravity tonguing and viscous fingering.
• The adverse effect of CO₂ is more damaging than those with water flooding.
• A very high volume of CO₂ is required to achieve the desired result. For instance, it may require 5-10 million cubic feet of CO₂ gas to produce just one barrel of stock tank oil.
• Supplying CO₂ for long distances will require pipelines. Maintenance and installation of such long-distance pipelines are capital extensive.
• CO₂ can form highly corrosive acid upon contact with water known as carbonic acid. Special metal coatings and alloys may be required in pipelines and equipment which are handling the odorless gas_.