Reservoir Oil Resaturation with Gas During Pressure Build-up

Abstract

Abstract A sand-packed cell was used to determine the pressure-volume relationship of confined oil and gas by applying both increasing and decreasing pressures. Then the same hydrocarbon fluid was given a routine PV analysis over the same pressure and temperature ranges. The relationship between pressure and hydrocarbon volumes proved to be practically the same for all runs. Equilibrium conditions between phases were very rapidly established within the sand-packed cell. A small, closed reservoir was produced until free gas developed in the oil leg; then the reservoir was gas-injected to restore a third of the pressure decline. By volumetric balance considerations it was indicated that as the pressure was raised the oil dissolved gas; and within experimental error, the gas maintained the oil saturated. It was concluded that for both the laboratory and reservoir tests, the dispersed gas in the oil maintained the oil saturated during pressure increase. Introduction In Eastern Venezuela, the question of resaturation of reservoir oil in the oil leg through pressure build-up has often arisen. Should oil leg resaturation take place, increased effective permeability will develop with increasing pressure. Decreased viscosity and increased oil formation volume factor from increased solution of gas in oil, would lead to reduced residual oil. These benefits would be reflected in increased ultimate recovery and improved well productivity. However, they could be limited, e.g., active water or solution gas drives. In the case of water drive, loss by backflow of oil into the water leg might result from pressure build-up. With an active solution gas drive that has led to a relatively high free gas saturation, to attain equal reservoir volumes of residual oil through gas injection, higher abandonment gas:oil ratios are necessary at higher pressures. The cost of handling greater volumes of gas could outweigh the advantages of an increased formation volume factor associated with a higher pressure. There are a number of conflicting articles (pro and con) in the literature concerning resaturation.