Performance of Seeligson Zone 20B-07 Enriched-Gas-Drive Project

Abstract

Abstract In 1957 Humble Oil and Refining Co. initiated a program in the Seeligson (Zone 20B-07) field, Kleberg County, Tex., that involved displacement of oil by enriched gas. Early evaluation showed that nearly 100 percent oil recovery was being obtained in the volume of reservoir swept by enriched gas. Subsequently, it was observed that the enriched gas way breaking through at producing wells earlier than expected, and that volumetric sweep efficiency was lower than anticipated. Simultaneous injection of water and enriched gas was begun in 1963 in an attempt to improve sweep efficiency. In 1965 it was determined that sufficient enriched gas had been injected into the reservoir, and the injection substance was converted to water and dry gas. The experiment program in Zone 20B-07 is largely complete. Cumulative production froth the reservoir has been 3.7 million bbl of oil, and oil recovery froth the Zone 20B-07 enriched-gas project has been 50 percent of the original ail in place. Production to date has greatly exceeded the native dissolved-gas-drive recovery that was predicted to be 22.3 percent of in-place oil. As another comparison, current recovery also exceeds expected waterflood recoveries in the range of 42 to 48 percent- estimates based on experiences in similar reservoirs in this area. This paper covers field results, particularly those regarding displacement and sweep efficiency and the effects of changes in injected fluids. Reasons for observed performance are discussed with regard to future application of miscible displacement. Introduction The Seeligson Zone 20B-07 enriched-gas-drive project is completed. The theory of miscible displacement, reasons for experimenting in the zone, and early performance data are well documented by earlier reports. This paper presents final results of the project and discusses factors that will assist future evaluations of enriched-gas-drive applications. Reservoir Description The Seeligson field area is one of several multipay anticlinal structures trending from southwest to northeast along the Vicksburg flexure in Southwest Texas. Seeligson Zone 20B-07 is encountered at approximately 6,000 ft on the eastern flank of the Seeligson structure in Kleberg County, Tex. The reservoir is bounded on the northeast and south by strandlines and to the west by faulting (Fig. 1). Drilling Once 1962 has expanded the reservoir limits and established an oil-water contact to the east. The average reservoir dip, west to east, is approximately 4 deg. Zone 20B-07 contains approximately 877 oil-productive acres and currently is developed with 16 wells. Fig. 2 is a net-sand isopachous map of the reservoir. Average sand thickness is about 12 ft, with the maximum sand development being 42 ft in the center of the reservoir. Reservoir oil was saturated at the original reservoir pressure of 3,010 psi and was associated with a small gas cap. The original dissolved GOR was 600 scf/STB at separator conditions of 300 psi and 75F. Gravity of the produced crude was 40 deg. API. Average sand characteristics include 100-md permeability, 19 percent porosity and 31 percent connate water saturation. Porosity was lowered from that previously reported, based on sonic logs of the recently drilled wells. The lower porosity values resulted in essentially the same original oil in place as previously reported (7.4 million STB) although the reservoir was increased in areal extend. JPT P. 369ˆ