Permian Basin CO2 Experience in Carbonate Oil Reservoirs

Sixty-six on going and abandoned CO2 projects in West Texas were evaluated. (13) As of December 2002 a total of 38 projects were considered to be economic. The reservoir characteristics of the 38 successful projects is summarized below:

• Average BHT = 108 degrees F (86 deg F –134 deg F)
  
• Average Viscosity = 1.52 cp (0.5 cp –2.6 cp)
  
• So at start of CO2 flood = 55% (35%-89%)
  
• Average porosity = 11% (7% - 13.5%)
  
• Average Permeability = 9 md (1.5 md – 62 md)
  
• Average Depth = 5,281 feet (4,500 feet – 8,000 feet)
  
• Average API = 33 degrees (28 deg. API – 41 deg. API)
  

The well spacing for the 38 West Texas CO2 successful carbonate reservoir floods was however substantially less than the North Dakota well spacing. The average well spacing for the 38 West Texas CO2 successful projects was 27.6 acres. Only one field exceeded 80 acre well spacing and was spaced on 130 acres per well. Thirty-three of the 38 fields were spaced on less than 40 acres.

Williston Basin Canadian CO2 Experience in Carbonate Oil Reservoirs

Weyburn Field:

Several CO2 projects have been conducted in Canada but the most important Canadian CO2 project relevant to North Dakota is the Weyburn field CO2 project that began in September 2000. Documentation and publications evaluating the Weyburn project are extensive and cannot be fully addressed in this limited report. The CO2 flooding analysis and experience at Weyburn does indicate the potential for successful CO2 application to the North Dakota Madison carbonate reservoirs.

Weyburn field was discovered in 1954 and encompasses an area of 52,000 acres. Weyburn field contains 723 wells, which include 179 horizontal wells, 221 injection wells and 323 vertical production oil wells. Approximately 146 wells have been shut in or abandoned. Well spacing averages 72 acres per well but horizontal wells decrease the effective well spacing below 72 acres per well. Current oil production rate is approximately 21,000 barrels of oil per day (bopd). Gas production is 2% hydrogen sulfide (H2S) with a sour oil API gravity of 25 to 34 degrees. Depth to the Mississippian Midale formation is 4,655 feet. OOIP is estimated at 1,400 MMbo with cumulative recovery of approximately 366 MMbo (26% of the OOIP). Estimated recovery from CO2 and water injection is 130 MMbo (10% of the OOIP).

Investment in the Weyburn project is $1.3 billion representing a finding cost of approximately $10/bbl. Water injection volume was 156,526 bopd in April 2003 with CO2 injection volume of 70-90 million standard cubic feet per day (MMcfpd). CO2 source is from the Great Plains Synfuels Plant in Beulah, North Dakota. Over the 25-year life of the project a CO2 volume equal to 30% of the reservoir HCPV will be injected. Water injection will then follow after an estimated 50 billion standard cubic feet (Bcf) of CO2 has been injected. The overall project will require injection of 10-12 thousand standard cubic feet (Mcf) of CO2 per barrel of oil produced. The peak oil production rate is projected to be 30,000 bopd.

The two principal carbonate reservoirs being CO2 flooded in Weyburn field are the Vuggy Limestone and Marly Dolomite of the Midale formation. The average net pay zone thickness is between 10 and 89 feet. The Marley Dolomite zone has a porosity of 26% with a permeability of 10 millidarcies (md). The Vuggy Limestone has a porosity of 15% with a permeability of 30 md. Original reservoir pressure was 2117 pounds per square inch (psi) with an original water saturation of 32% to 40%. (16) The bottom hole temperature is 151 degrees Fahrenheit. The Midale oil viscosity is 0.5 centipoise (cp) at reservoir conditions with an initial formation volume factor (Boi) of 1.46 rb/stb. The CO2 minimum miscibility pressure (MMP) for the Midale oil is between 1,912 psi and 2,408 psi (17)