An important method to increase the recovery of reservoirs is ASP (alkaline/surfactant/polymer) flooding to lower the IFT (interfacial tension) between the crude oil and displacing fluid to an ultra-low value (<10-3 mN/m) to increase the displacement efficiency and at the same time to increase the viscosity of the displacing fluid in order to increase the volumetric sweep efficiency. However, the alkaline in the ASP fluid makes it difficult to increase the viscosity by polymers. Therefor, experiments were performed by adding natural gas to the ASP fluid to form an ASP foam (ASPF) displacing fluid. This ASPF fluid should have the characteristics of an ultra-low IFT, high displacement efficiency and also low mobility, which should increase the volumetric sweep efficiency also.
This paper introduces the results of ASPF tests in the laboratory and field. The experimental results in the lab include the foaming ability of the ASPF, the stability of the foam, its IFT, resistance to flow in cores, the incremental recovery of cores of different heterogeneity and the relationship between incremental recovery and residual oil saturation and polymer concentration. The field results include reservoir description, well pattern, composition of the injected ASPF, injection and production conditions and problems encountered.
Good results were obtained in the field. The current recovery factor is already 65.6% OOIP. The final recovery is estimated to reach 71.3% OOIP.
Introduction
In order to increase the oil recovery, experiments were performed both in the lab and field to inject ultra-low IFT (<10-3mN/m) displacing fluid (for example ASP flooding) to increase the displacement efficiency and also to inject low mobility foam displacing fluid to increase the volumetric sweep efficiency. Both methods obtained certain beneficial results.
However, for ASP flooding, usually the displacement efficiency is high, but because there is alkaline in the ASP fluid, its viscosity is rather low, and the volumetric sweep efficiency is low also. Very little has been reported in the literature on ultra-low IFT foam flooding (especially in the oil field), the IFT of most foam fluids is usually rather high.
This paper introduces the results of increasing the recovery in the lab and field by injecting ultra-low IFT ASPF displacing fluids.
Results of Laboratory Experiments
Foaming ability of ASPF.
The foaming ability of ASPF was tested by using natural gas, water of different salinities, different concentrations of polymers, polymers of different molecular weight, different surfactants, surfactants of different concentrations and at different temperatures.
To measure the foaming ability of the ASPF system, a coefficient Fq was introduced:Fq=3/4t1/2.qo
In whichFq—foaming ability coefficientt1/2—half-life period of the foamqo—initial quantity of the foam
Effect of different types of surfactants.
Alkyl benzene sulphanate surfactants (A), non-ionic surfactants (N), mixtures of the two surfactants and ASP (using a type of A as the surfactant) fluids were chosen to compare their foaming abilities. The results are shown in Table 1.
By comparing the foaming ability coefficients, it can be seen that from the best to worst, the order is ASP fluid, mixture of A and N surfactants, A surfactant and the last is N surfactant.
Influence of concentration of surfactant A on foaming ability coefficient can be seen in Figure1.
From Figure 1 it can be seen that when the concentration is lower than 0.3%, the foaming ability of the fluid increases rapidly with the increase of concentration of surfactant A. After that, it decreases slowly and gradually reaches a constant value. This shows that there is an economical and technical surfactant concentration limit for a given foam composition.
