Gas Injection in Eastern Venezuela Field Increases Oil Recovery

Abstract

Abstract The West Guara field lies in the state of Anzoategui, Venezuela, and is one of a number of fields located in the Eastern Venezuela Sedimentary Basin. The F7 sand, largest producing sand in the field, is of Oligocene age and is found at an average depth of 6,020 ft. The sand itself is clean, very well sorted, medium grained, with permeabilities ranging from 1,000 to 20,000 md and porosities ranging from 25 to 35 per cent. The oil was originally saturated, and a small initial gas cap was found during development. Original stock-tank oil in place was estimated to be 84,710,000 bbl. Recovery as of Jan. 1, 1958, amounted to 51 million bbl or 60 per cent of the original oil in place. Evidence of gravity segregation was noted early in the life of the field and as a result, recovery under primary depletion, estimated at 59 per cent of the oil in place, would have been very good. However, early initiation of pressure maintenance by gas injection and exercising close control of producing operations will yield an ultimate recovery of approximately 70.5 per cent of the original oil in place. The ideal nature of the reservoir and its very light oil (42 API) are conducive to such control, and accurate prediction of performance using established engineering techniques is facilitated. Introduction The field produces from the Oficina formation of Oligocene age which consists of 3,000 ft of interbedded sands and shales. Of the 39 oil productive sands the F7 sand is the largest oil-bearing body in the field. Accumulation is controlled on the south by the West Guara fault, and on the north by an oil-water contact at 5,265 ft. The sand was deposited as a channel trending roughly north and south; thickness in the main channel approaches 100 ft. Thinning occurs to the east and west until on these flanks the thickness is approximately 10 to 20 ft. Fig. 1 shows pertinent structural details. Table 1 presents pertinent data concerning oil characteristics and other reservoir information used in analyzing the F7 sand reservoir. It became apparent early that reservoir recovery would be high. However, to supplement this recovery and to maintain the wells on a flowing status, it was decided to maintain pressure by gas injection. Methods used in predicting reservoir recovery without pressure maintenance and with pressure maintenance are presented. Also discussed are operating procedures used so that actual recovery will approach as nearly as possible the predicted recovery.