Evaluation of an In-Situ Combustion Process by Postburn Core and Log Analysis

Abstract

Summary Evaluation of the performance of an in-situ combustion process applied by Cities Service Co. in the Bellevue field was made possible by analysis of postburn cores and logs from wells drilled within the flooded zone. postburn cores and logs from wells drilled within the flooded zone. Temperature-observation-well data and production performance were inadequate to describe the coverage and efficiency of the simultaneous air- and water-injection method applied. Introduction An attempt was made to monitor the coverage of the burn fronts generated by five injection wells in the Bellevue field using one temperature-observation well per injector. After 4 years' continuous injection, only one observation well ever had shown the high temperatures seen in laboratory combustion tube studies, and an accurate description of the burned zone could not be determined. After air injection was terminated, four wells were drilled within one pattern, obtaining openhole logs and cores of the producing formation. pattern, obtaining openhole logs and cores of the producing formation. Results from this postburn drilling program were used in constructing an approximate burned zone for the pattern, enabling better understanding of the combustion process and of the efficiency of the field project. Reservoir and Laboratory Data The combustion project is being conducted in the Bellevue field of northwestern Louisiana. This field, discovered in 1921, produces a 19API (0.9 g/cm3) crude from the Upper Cretaceous Nacatoch sand. Primary production from the field is estimated to have been less than 15% of the production from the field is estimated to have been less than 15% of the original oil in place (OOIP). The field is a 900-acre (4 × 10 6 m2) low-relief dome with pay thickness controlled by a water/oil transition at 212 ft. (64.6 m) subsea. The project area covered by this report is a 19-acre (77 × 10 3-m2) five-pattern demonstration conducted by Cities Service Co. under a cost-sharing contract with the U.S. DOE (Contract DE-AC03-76ET12057). In this area of the field the top of the Nacatoch is encountered between 109 and 174 ft (33.2 and 53.0 m) subsea, with dip to the northeast at 4.5 0. At this shallow depth, the Nacatoch is a highly unconsolidated sand with numerous thin, dense, fossiliferous limestone layers. These limestone layers are identified easily from bulk-density logs (Fig. 1) and essentially subdivide the pay section into four principal sand intervals (S1, S2, S3, and S4). The average porosity and permeability of the sand intervals are 33.9% and 700 md, with the sand having a 27.4% water saturation. Average reservoir pressure is only 40 psig (280 kPa). JPT p. 827