A Field Comparison of Wet and Dry Combustion

The injection of water during an in-situ combustion process has been shown with laboratory studies to be more efficient than dry combustion. This paper substantiates the superiority of wet combustion over dry burning with field data from the Bodcau in-situ combustion project in Bossier Parish, LA. Introduction Cities Service Co. operates an in-situ combustion project in the Bellevue field of Bossier Parish, LA (Fig. 1). The field was discovered in 1921 and covers approximately 900 acres (3.64 km2). Production is from the Upper Cretaceous Nacatoch sand occuring between 300 and 400 R (91 and 122 m). Porosity and permeability of the unconsolidated reservoir average 33.9% and 700 md, respectively. The 19 degrees API (931-kg/m3) gravity crude has a viscosity of 676 cp (0.68 Pas) at the reservoir temperature of 75 degrees F (24 degrees C). Due to the low reservoir pressure and high viscosity of the oil at reservoir temperature, typical producing rates are 1 or 2 BOPD (0.16 to 0.32 m3/d) per well. Reservoir properties are summarized in Table 1.Dry forward combustion was tested by Cities Service in a pilot started in 1971. Other production stimulation techniques had been tried in the field, but none were economically successful. Production from the four 2.5-acre (10 117-m2) inverted nine-spot patterns during the 3-year combustion phase was in excess of 167,000 bbl (26 550 m3). The pilot area is still under waterflood, and ultimate recovery will exceed 40% of the original oil in place. Although the pilot test demonstrated that dry combustion could recover the low-gravity crude economically, laboratory studies indicated that wet combustion would be superior to dry combustion in the Bellevue field. To compare the two combustion processes directly, a field test was begun in 1974.Successful results from the field test led to the development of eight additional patterns using wet combustion, including five operating under a contract with the U.S. DOE. The DOE project demonstrates the technical efficiency and economics of a commercial combustion project. Eight more patterns have been drilled and presently are being developed, while additional acreage is being prepared for future fireflooding operations (Fig. 2). Description of Test Area Two 2.5-acre (10 117-m2) inverted nine-spot patterns were developed as shown in Fig. 3. Pattern 6 was to be processed by dry combustion, while Pattern 5 would make use of a type of wet combustion specifically designed for this reservoir. To monitor the progress of the combustion and water fronts, 18 temperature observation wells were drilled within two patterns.Producing wells were completed with 7-in. casing cemented to surface. The entire pay interval was perforated at a density of two hots per foot, and the wells were equipped for beam pumping. Injection wells also were completed with 7-in. (18-cm) casing, but only the bottom 15 ft (4.6 m) of pay were perforated for air injection. JPT P. 1523^