Westport Oil and Gas Co., L.P. Evaluation of Oil Reservoir Characteristics To Assess North Dakota Carbon Dioxide Miscible Flooding Potential Ralph Nelms Westport Oil and Gas Co. Randy Burke North Dakota Geological Survey North Dakota Geological Survey Geologic Investigations Number 21

North Dakota CO2 Mini-Test Pilot 1981 Gulf Oil Company. Little Knife North Field . Five Acre 5 Spot. Mission Canyon Formation. Recovered 13% of OOIP. MMP 3,100 psi. Gulf Projected Recovery of 8% of OOIP on 160-acre Well Spacing.

Previous North Dakota CO2 Studies 1988 and 1990 Basin Electric Cooperative. North Dakota and Montana 26 Fields. 4,467 MMbo In Place. 858 MMbo Produced as of 1988. 1,038 MMbo Primary + Sec. EUR (~24% OOIP) 3,549 MMbo Remaining In Place 232 MMbo Potential CO2 Flooding Recoverable. (5.2% of OOIP)

Dakota Gasification Co. CO2 Pipeline Location CANADA M.T. N.D. CO 2 PIPELINE

CO2 Screening Methodology Empirical reservoir characteristics. Analog dimensionless models (Shell Morgan Kinder). Preliminary reservoir simulation studies (IFLO). PVT tests, core analysis and core floods. Advanced reservoir simulation studies (ECLIPSE,GEM, VIP) Preliminary economic analysis. Field pilot. Large scale simulation and project economics. Full scale development.

Favorable Reservoir Characteristics for Empirical Screening Reservoirs with good waterflood response are best Candidate for CO2. 20% OOIP< Recovery Factor <50% OOIP. Depth >2500 ft to reach MMP. Oil Gravity >27 Degrees API. Oil Viscosity <10 cp. Porosity >12%. Permeability >10 md.

Quick Rules of Thumb Recovery factor using miscible CO2 is 8% -11% OOIP. Immiscible CO2 50% of miscible. MMP equals initial bubble point pressure. CO2 requirement is 7-8 Mcf/bo plus 3-5 Mcf/bo recycle. Water injection required to fill gas voidage and increase reservoir pressure to original BHP. WAG is alternative but 10 Mcf/bo still required. Top down CO2 injection alternative is effective but requires more capital investment for higher CO2 volume.

Unfavorable Reservoir Characteristics for Empirical Screening High concentrations of vertical fractures. Very high, or very low, permeability. (Vertical segregation or fracture channeling) Thick reservoirs with no layered horizontal permeability barriers. Reservoirs with poor connectivity. Well spacing >80 acres. Poor material balance during water flood. (High water loss out of zone, water influx or high water cut during primary production)

U.S. Active CO2 Floods Texas 48 (26 Succ, 8 TETT, 13 Prom, 1 Fail) Wyoming 5 (5 Success) Oklahoma 4 (2 Success, 1 TETT, 1 Promising.) Utah 2 (1 Success, 1 Promising) Mississippi 2 (1 Success, 1 Promising) Michigan 2 (1 Success, 1 TETT) New Mexico 2 (1 Success, 1 Promising) Colorado 1 (1 Success)__________________ Total 66 (38 Success, 10 TETT, 17 Prom.) * First CO2 Flood SACROC Jan 1972

Summary of US CO2 Active Floods (CEED 2002) Total Active CO2 Floods = 66 Total Successful = 38 Total Successful Carbonates = 29 Total Successful Sandstones = 9 Of 29 Successful Carbonate Units, 18 are in San Andres Dolomite.

Reservoir Characteristics of 29 Successful Carbonate CO2 Floods 2002 US Average North Dakota Madison Porosity 11% (7%-13.5%) 10.9% Permeability 9 Md (1.5-62) 10.2 Md Depth 5,281 feet (4500-8000) 7,500 feet API 33 degrees (28-41) 38.7 degrees BHT 108 degrees F (86-134) 201 degrees F Viscosity 1.52 cp (0.5 –2.6) 1.54 cp So at start of CO2 flood = 55% (35%-89%)

Reservoir Parameters of Carbonate CO2 Floods Well Spacing for 38 Successful CO2 Floods: 1 Field at 130 acres 2 Fields at 75 acres 2 Fields at 50 acres 32 Fields < 40 acres 38 Well average was 27.6 acres

Favorable Reservoir Characteristics for North Dakota Carbonate Reservoirs MMP is Easily Achievable Depth, API Gravity and BHT all Favorable for CO2 Flooding. Porosity, Permeability and So are all within Range of other Successful Carbonate Reservoirs. H2S in Sour Crude in not a constraint. Deeper Reservoirs offset Higher BHT for MMP

Unfavorable North Dakota Reservoir Characteristics Large Well Spacing averages 160 acres. Many Reservoirs are Vertically Fractured. Depth Increases Well Costs for Infill Drilling New Wells : $100/ft Sour Gas Recycling Costs. Heterogeneity limits sweep efficiency.

Weyburn CO2 Project, Saskatchewan, Canada Started in 2000. $1.3 billion Encana project. $100 million DGC project. 25 year life. 1,300 MMbo in place. CO2 recovery 130 MMbo. 10% OOIP from CO2 flood. 723 wells, 179 horizontal, 221 injection. Injecting 70-90 MMcfpd CO2. 95% Quality CO2 from Great Plains in Beulah. 198 mile pipeline 4,655’ deep. 10’-89’ thick. Sour 25-35 API gravity. Vuggy Limestone: 15% porosity 30 md. Marly Dolomite: 26% porosity 10 md. SSWAG injection due to fractures.

CO2 Reservoir Excel Database NDIC Derived Data Author’s Data Input Unit Name/ Pool/ OOIP CO2 Oil Recovery (8% OOIP) Peak Rates, CO2 Oil Reserve Category Primary EUR, RF Favorable/Unfavorable Secondary EUR, RF CO2 Flood Characteristics Unit size Porosity, Permeability, Boi Effective Date uo, uw, M, h, BHT, API, Rw WOR, Soi, Soirr Swirr, P*, Pbp, Non-Unitized Fields Distance to CO2 Pipeline, Fracture Description

North Dakota Highest Ranked Reservoirs for CO2 Flooding Mississippian -Madison Group Mission Canyon Glenburn Sherwood Ratcliffe Devonian - Duperow Permian/Triassic -Spearfish Pennsylvanian -Tyler “Health”

North Dakota Lowest Ranked Reservoirs for CO2 Flooding Silurian - Interlake Devonian - Winnipegosis Madison with Cross-seal Fractures Ordovician - Red River B with Cross-seal Fractures

CO2 Potential Oil Reserve Classification Probable, Possible, Unfavorable Probable (>2MMbo and <2MMbo): Highest probability of success based upon empirical analysis and comparison to other successful projects. Possible (>2MMbo and <2MMbo): Feasible but have less favorable reservoir characteristics. May have lower oil recovery. Unfavorable: Significant reservoir problems. Projects with greater than 2 MMbo recoverable best candidates.

Probable Top Ten North Dakota Units for CO2 Flooding Beaver Lodge - Madison 17.6 MMbo Tioga - Madison 17.2 MMbo Big Stick - Madison 13.3 MMbo Fryburg - Heath (Tyler) 12.4 MMbo Beaver Lodge - Devonian 11.1 MMbo Newberg - Spearfish & Charles 7.7 MMbo Wiley - Glenburn 7.6 MMbo Blue Buttes - Madison 7.4 MMbo North Tioga - Madison 7.2 MMbo Charleson North - Madison 6.4 MMbo

Possible Top Ten North Dakota Units for CO2 Flooding Cedar Hills South - RRB 28.8 MMbo Cedar Hills North - RRB 22.2 MMbo Antelope - Madison 8.0 MMbo Cedar Creek - Ordovician 7.7 MMbo Medicine Pole Hills West - RRB 3.4 MMbo Medicine Pole Hills - RRB 3.2 MMbo Medicine Pole Hills South - RRB 3.1 MMbo Eland - Lodgepole 2.8 MMbo Lignite - Madison (PA) 2.6 MMbo Rough Rider East- Madison 2.3 MMbo

Unfavorable North Dakota Units for CO2 Flooding Horse Creek Red River Red Wing Madison Fryburg Madison TR Madison (South)

Constraints to North Dakota CO2 Development Most oil reservoirs are 7,500 feet and many have well spacing >80 acres requiring high cost infill drilling. Only Nesson Anticline fields are less than 10 miles to existing CO2 pipeline. Fields not on the Nesson anticline will be burdened with large pipeline installation costs. Most existing waterflood units are in mature waterflood stages at high water cuts. Low permeability and highly heterogeneous. Many reservoirs have strong water influx from vertical fractures outside of pay zone boundaries.

Conclusions 55 fields in North Dakota have Probable CO2 recoverable reserves of 171 MMbo. 26 fields in North Dakota have Possible CO2 recoverable reserves of 106 MMbo. One successful CO2 pilot has been conducted to date. Comparison of North Dakota reservoir characteristics to successful Texas and Canadian reservoirs utilizing CO2 flooding is favorable. Future North Dakota CO2 development constraints are principally economic and will require large capital investments over long time periods plus the ability to manage both oil price and CO2 price risks. A long term economic perspective will be required since CO2 projects require long payouts on investment.