A Method for Engineering In-Situ Combustion Oil Recovery Projects

Abstract

An oil-recovery/volume-burned relationship provides a method for engineering in-situ combustion projects. Oil rates, oil recovery, and air/oil ratios can be determined as a function of air injected. Economic limit can be determined. The results match both laboratory and field data better than frontal displacement calculations. Introduction Engineering calculations for in-situ combustion often are made with the assumption that oil is recovered only by frontal displacement from the sand volume burned. Both laboratory combustion tube runs and field applications indicate that oil is recovered more rapidly in the early life of burns that have low initial gas saturations than is indicated by simple frontal displacement. Additional oil is recovered early from the unburned volume ahead of and adjacent to the burned volume. Recognition of the actual behavior of the combustion oil recovery process led to development of a method called the "oil-recovery/volume-burned" method, as opposed to the older "frontal displacement" method. Ideally, both the frontal displacement calculation and the oil-recovery/volume-burned method have the same total oil recovery and total air required at 100% volume burned, assuming zero excess air.The frontal displacement method assumes constant air/oil ratios throughout, whereas the oil-recovery/volume-burned method and field results indicate that the air/oil ratio drops rapidly to a minimum early in an application (in a field with low initial gas saturation) and rises thereafter. This means that the investment in compressors and the operating expense would be much lower than that indicated by the frontal displacement method for a given target oil rate during the early life of in-situ combustion in a field from which little oil has been produced. During the latter, less-profitable stage produced. During the latter, less-profitable stage when oil recovery is high, compressor operation could be terminated before the reservoir volume is burned completely. Thus the average air/oil ratio also would be less than indicated by the frontal displacement method.Initial data on oil recovery and volume burned from the South Belridge thermal recovery experiment were published by Gates and Ramey. A summary of subsequent data and a complete analysis leading to the oil-recovery/volume-burned curve for the experiment were presented by Gates et al. This paper presents: (1) substantiating laboratory combustion presents:substantiating laboratory combustion run data,results of a study of the influence of initial liquid saturation on the oil recovery curve as indicated by laboratory combustion tube runs,the role of fuel concentration in the process,the method of determination of air/oil ratios from the curve, anda summary of the engineering method. The oil-recovery/volume-burned method provides a means for making engineering and economic evaluations for the design and monitoring of in-situ combustion projects. Performance can be computed to obtain oil rates, air/oil ratios, oil recovery, and economic limit. The reliability of the method likely will prove best in reservoirs with characteristics similar to South Belridge: heavy oil, high permeability, high porosity, and high oil content. permeability, high porosity, and high oil content. JPT p. 285