Performance and Evaluation Methods of Colloidal Dispersion Gels In the Daqing Oil Field

Abstract

Abstract Polymer flood has been being used to provide a valuable tool for in-depth blockage of high permeability regions of the reservoirs in the Daqing oil field, where more than fifty thousand tons of polymers had been injected into reservoir for each year. But heterogeneity inside the reservoir is very serious in some areas and has caused channlling of polymer solution through high permeable zones, which not only wasted a lot of polymer but also affected the volumetric sweep. The colloidal dispersion gels (CDG) were made up of low concentrations of polymer, aluminum citrate and additive in water, and combined with polymer solution during polymer flood for in-depth blockage of high permeability regions1,2,3. This paper provides a study of the performances and characteristics of the gels that were made up of aluminum citrate, produced water and polymer made in the Daqing oil field. The parameters used to evaluate performances and characteristics of the gels in laboratory include resistance factor, residual resistance factor, viscosity, elasticity, and polymer molecular texture and flood efficiency. The experiment results showed that the cross-linking reaction among a few polymer molecules or inside a molecule had taken place, which caused performance difference between the gel and polymer solution, and the best gels can be formed with a polymer: aluminum: additive S ratio of 100:1:25. The cost of the gels formed with the above-mentioned ratio was below 5 dollars /m3, which was almost the same as polymer solutions. According to these experimental results, two pilot test units with seven injection wells have been selected in the central part of the oil field, in which there are three central injection wells. Introduction The Daqing oil field was discovered in 1959. In June 1960, a waterflooding pilot test unit was opened up in the central part of the oil field. After carefully studying the reservoir characteristics and availability of various well patterns, a development program of water-flood was adopted from beginning of the development. Up to 1976, the main oil-bearing regions had been basically put into development and its annual output had reached the planned peak level of fifty million tons. Since then, the oil field has been producing at such level. The Daqing oil field is located in the central depression of the Songliao basin which has 26 ×104 km2 in area. It is the largest in China and one of the few giant fields in the world. The productive zones of the oil field consist of the Saertu, Putaohua and Gaotaizi reservoirs. There are about 40 to 50 oil-bearing sands vertically distributed in a manner that high permeability ones are inter-bedded with the low, and thicker ones with thinner. The thickness of individual pay-zones varied from 0.2 m to 10.0 m. The effective permeability of the reservoir is generally 0.03 to 1.20µm2, and the Dykstra Parsons permeability coefficient from the reservoir change from 0.635 to 0.818. This shows that the vertical heterogeneity of the reservoir is very high. The porosity of the pay-zones was 20% to 30%; the original oil saturation is 70% to 80% in net oil area and 60% to 70% in oil-water transition area. At present, the Daqing oil field has entered a development stage of high water-cut level, where the water-cut is more than 80%. According to the requirement of our national economy, it is necessary to search for a longer stable production period. There are three methods used to keep production stable in the Daqing oil field. They include developing low or super low permeability reservoirs, drilling in-filling wells and chemical flood. The analysis of the investment showed that chemical flood specially polymer flood is more beneficial. It is predicted that the average oil recovery by water flood would be about 40% in the Daqing oil field, which shows that about 60% OOIP is left in the reservoir after water flood. According to prediction, the oil reserves of about 29.0 × 103 tons are available for EOR in Daqing oil field, which takes up 80% of all the oil reserves in China.