This study broke down the potential for CO2 recoverable potential reserves into three categories: Probable, Possible and Unfavorable. A comprehensive Excel spreadsheet was created summarizing the key reservoir characteristics and the authors’ assignment of reserves. (4)(5) (18)(19)(20)

Probable and Possible oil reserves were further broken down by recoverable oil reserves greater or less than 2.0 MMbo for the field analyzed. An Excel spreadsheet was created that summarizes the reservoir characteristics and reserve classification by each field.

Probable oil reserves are defined as those oil reserves associated with specific fields, which in the authors’ opinion, would most likely be recovered from reservoirs with the most favorable reservoir characteristics for CO2 flooding based upon empirical comparisons, other successful carbonate CO2 flood projects, or CO2 field pilots. Possible oil reserves are those associated with reservoirs, that in the authors’ opinion, have less favorable reservoir characteristics but CO2 flooding may be feasible at lower recovery factors. Unfavorable reserves are those associated with reservoirs that in the authors’ opinion have significant problems with successful CO2 application.

Within the 84 fields unitized, North Dakota contains Probable CO2 recoverable oil reserves of 181 MMbo. An estimated 153.8 MMbo of CO2 oil recoverable reserves is associated with 22 North Dakota fields with a recoverable CO2 flooding oil volume exceeding 2.0 MMbo. An estimated 17.3 MMbo of CO2 recoverable oil reserves is associated with 23 fields with recoverable CO2 flooding volume less than 2.0 MMbo. It is the authors’ opinion that fields with less than 2 MMbo recoverable CO2 reserves volumes are less likely to be developed than those with CO2 recoverable reserves exceeding 2 MMbo per field.

North Dakota contains Possible CO2 recoverable oil reserves of 105.9 MMbo. An estimated 103 MMbo is associated with 21 North Dakota fields with projected oil recovery of greater than 2.0 MMbo. Possible CO2 recovery of 2.9 MMbo is associated with five fields that would have possible CO2 recovery less than 2.0 MMbo.

Three of the 84 fields evaluated would have un-favorable recovery projection representing 11.7 MMbo.

Favorable Reservoir Characteristics for North Dakota CO2 Flooding Applications:

Reservoirs with the most likely characteristics for the probability of successful development include the Duperow, Madison, Spearfish Charles and Tyler Health formations. Madison reservoirs with the highest probability of development include the Glenburn, Sherwood, Mission Canyon, and Radcliffe members of the Madison formation.

Because North Dakota oil reservoirs are generally at a depth greater than 8,000 feet, achieving the minimum miscibility pressure (MMP) for CO2 injection is easily achievable. High bottom hole pressures are offset by higher hydrostatic pressure from injected CO2 and water at depth still allowing CO2 miscibility pressure to be reached. Published MMP for the Madison formation at Little Knife was 3200 psi. (4) Depth, API gravity and bottom hole temperatures are all favorable for CO2 flooding in North Dakota. In addition, porosity, permeability and oil saturations are all within range of other successful carbonate reservoirs such as those in West Texas and Canada. Presence of H2S in natural gas in sour crude is not a significant constraint. However, sour gas recycling costs will certainly be higher than sweet gas re-cycling costs.

Fields with the highest projected probable CO2 recoverable reserves assuming a recovery of 8% of the OOIP are:

1. Beaver Lodge Madison 17.6 MMbo
   
2. Tioga Madison 17.2 MMbo
   
3. Big Stick Madison 13.3 MMbo
   
4. Fryburg Heath 12.4 MMbo
   
5. Beaver Lodge Devonian 11.1 MMbo
   
6. Newberg Spearfish Charles 7.7 MMbo
   
7. Wiley Glenburn 7.6 MMbo
   
8. Blue Buttes Madison 7.4 MMbo
   
9. North Tioga Madison 7.2 MMbo
   
10. Charleston North Madison 6.4 MMbo
    

Fields with the highest Possible CO2 recoverable reserves assuming a recovery of 8% of the OOIP are:

1. Cedar Hills South RRB 28.8 MMbo
   
2. Cedar Hills North RRB 22.2 MMbo
   
3. Antelope Madison 8.0 MMbo
   
4. Cedar Creek Ordivian 7.7 MMbo
   
5. Medicine Pole Hills West RRB 3.4 MMbo
   
6. Medicine Pole Hills RRB 3.2 MMbo
   
7. Medicine Pole Hills South RRB 3.1 MMbo
   
8. Eland Lodgepole 2.8 MMbo
   
9. Lignite Madison (PA) 2.6 MMbo
   
10. Rough Rider East Madison 2.3 MMbo
    

Unfavorable Reservoir Characteristics for North Dakota CO2 Flooding Applications:

Several factors have contributed to CO2 flooding failures. Vertical reservoir containment is one of the key factors associated with failure. Reservoirs with high concentrations of vertical fractures should be avoided due to CO2 injection losses out of zone and or early CO2 breakthrough reducing sweep efficiency. An example of a highly vertically fractured reservoir would be the Fryburg or the Red Wing Madison reservoirs. Reservoirs with either very high or very low permeability are poor candidates.

High permeability reservoirs that are also thick will gravity segregate CO2 reducing sweep efficiency. Very low permeability reservoirs reduce both waterflood and therefore CO2 flood effectiveness due to low fluid injectivity. Thick reservoirs with no layered horizontal permeability barriers such as the Lodgepole reservoirs would require top down CO2 injection significantly adding to CO2 purchase costs. Lodgepole reservoirs also have a high recovery factor due to water imbibition further reducing the available residual oil target for CO2 flooding. Reservoirs with poor connectivity such as Horse Creek Field would also not be candidates for CO2 flooding due to poor sweep efficiency.

In general, fields or units with well spacing greater than 80 acres would be less likely CO2 flooding candidates due to sweep efficiency reduction. Fields with poor material balance characteristics during water flood indicating high water loss would also not be good CO2 flooding candidates. Some parts of the Cedar Creek Anticline would be questionable CO2 candidates due to the presence of thief zones above the Red River B formation that take up to 40% of the injected fluids through vertical fractures. Reservoirs demonstrating strong natural water influx, such as the Madison formation in Fryburg field and the Southern part of TR field, would also not be good CO2 candidates.

Generally speaking, the depth of most reservoirs in North Dakota exceeds 8,000 feet. This depth allows CO2 miscibility pressure to be easily reached but also is an economic constant for infill drilling development. Fields not currently already developed on 80 acre wells spacing will likely require significant additional capital investment for horizontal or vertical infill wells marginalizing the economic attractiveness of CO2 flooding development. Fields using water flood on 80-acre well spacing, however, will be good CO2 flood candidates.

Reservoirs with un-favorable characteristics for CO2 flooding include Interlake Silurian, Winnepegosis and highly fractured Madison reservoirs.