Depleted Oil and Gas Reservoirs

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Depleted oil and gas reservoirs are one of the most promising methods of sequestration. Although they do not have the storage capacity of aquifers, they do have a history of success. Data is available for these fields, and often the volume capacity is known.Ê In addition, the reservoirs have proven to be able to securely sequester gas for geological periods of timeÑmillions of years [1] . In total, it is expected that 107 x 10⁹ tons C could be stored in depleted fields [2] .

 

There are two methods for sequestering CO₂ in depleted reservoirs.Ê The first, pressure-depleted fields, is advantageous over water drives. Pressure-depleted fields offer pressures lower than standard geological pressure, while the pressure is higher in water drive fields.

 

 

 

Problems with Sequestration in Depleted Fields

 

There are several problems associated with sequestering CO₂ in depleted oil and gas fields. Capturing, processing, and transporting CO₂ is costly.Ê Technology to monitor and verify sequestration progress is underdeveloped. Perhaps the worst problem is the possibility of leakage.Ê The wells may not have been properly sealed when abandoned. Also, the depletion of oil affects the pressure underneath the caprock, possibly compromising its integrity [3] .

 

Further research and development are required before beginning to sequester in depleleted oil and gas fields.

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Enhanced Oil Recovery (EOR)

 

source EOR Cross Section

Enhanced oil recovery is by far the most technologically mature method of sequestration. Developed by a Chevron affiliate in the seventies, carbon dioxide aided recovery of oil has been time proven [4] .Ê Carbon dioxide is miscible with oil, and, once dissolved, causes the oil to swell, thereby becoming less viscous and more mobile. Through EOR, an additional 5 to 20% of oil is recovered [5] .Ê

 

The purchase of pure CO₂ is the single largest cost involved with EOR.Ê Approximately 80% of the carbon dioxide used comes from naturally occurring sources.Ê The largest of these is McElmo Dome in Colorado, which supplies 40,000 tons CO₂/day to the Permian basin oil reservoirs [6] .Ê The other 20% uses anthropogenic CO₂ that would otherwise have been let out into the atmosphere [7] .

 

The CO₂ pumped into the reservoir generally stays in the reservoir as long as it is operational. When the field is decommissioned, the ideal situation would be to move the CO₂ into near by depleted fields. However, operators currently do not have incentive to do this, considering cost is high [8] . It is expected that there is a total worldwide sequestration capacity of 120 Gt CO₂ in EOR.

 

Injection Method

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EOR requires that CO₂ dissolve in the oil, causing it to swell.Ê This method is called solubility trapping. The effectiveness of solubility trapping depends upon many factors: the sweep efficiency of CO₂ injection, the formation of fingers, and formation heterogeneity [9] .

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

EOR Activity in the United States

 

Currently, the United States accounts for 95% of EOR activity worldwide.Ê Within the U.S., EOR is expected to account for 10% of total oil production by the end of the decade. During 1998 alone, a total of 43 million tons CO₂ were injected at over 65 EOR sites [10] .

 

Most EOR sites are in southwestern United States. Two primary examples of EOR projects currently underway are the Rangely Weber field in northwestern Colorado and the Wasson-Denver unit in the Permian basin.

 

Rangely Weber project. Operational since 1986, this is the third largest CO₂ flood in the Rocky Mountains.Ê The project contains floods extending over 61 km², with 341 production wells and 208 injection wells.Ê CO₂ is purchased from the Exxon-Mobil LaBarge natural gas processing facility in Wyoming, and piped to the Rangely field.Ê In the end, an estimated 25 million tons of CO₂ will have been sequestered [11] .

 

Wasson Denver EOR unit.Ê One of the worldÕs largest EOR facilities, the Wasson Denver EOR unit covers 115 km² in Yoakum and Gaines counties, western Texas. CO₂ is transported via 900 km of pipe from McElmo Dome. At the fieldÕs maximum productivity in 1996, 23,000 t CO₂/day were being injected. Ultimately, .09 Gt CO₂ is expected to be sequestered.

 

Depleted Gas Field Sequestration

source Production Field

 

Enhanced Gas Recovery (EGR) is not as attractive an option as EOR, but can nevertheless be used as a possible CO₂ sequestration method. Quite simply, EGR is not as effective as EOR because there is less natural gas that needs to be recovered using alternative methods.Ê Conventional methods are effective in recovering approximately 90% of the gas.Ê There is also concern about contamination of natural gas reservoirs with CO₂. However, preliminary testing indicates that CO₂ may still be effective with minimal contamination.Ê Further tests are necessary to confirm this [12] .

 

Still, there is the possibility of storing CO₂ in depleted gas fields. It can be assumed that space taken up by natural gas can all be used for CO₂ storage.Ê The quantity of CO₂ that can be injected will vary with depth, pressure, and temperature.Ê Overall, it is estimated that depleted natural gas reservoirs will be able to hold 25 years worth of carbon dioxide emissions [13] .