Modified Starch Gel Foam: Research and Application in Naturally Fractured-Vuggy Carbonate Reservoir

Abstract

Abstract Tahe oilfield is one of the most important naturally fractured-vuggy carbonate oil fields in China. The characteristics of this type of reservoirs are greatly complex due to its multiple flow units: matrix, fractures and vugs. Gas flooding is a practical way of enhancing oil recovery (EOR) for fractured-vuggy carbonate reservoir. The main problems during gas flooding include the complicated oil-gas relationship and gas channeling rapidly. Gel is an effective measure to control channeling. However, ordinary gel cannot adapt to the reservoir conditions in which salinity is 220000mg/l and temperature is 120°C. Based on Tahe oil field reservoir conditions, we develop Modified Starch Gel Foam (MSGF) which has the function of salt-tolerance and temperature-resistance on the basis of laboratory experiments. In order to study the static performance, different foam volume and half-life period were recorded by varing temperature, mineralization and pressure. We also observed microscopic morphology and bubble distribution of MSGF. To evaluate the dynamic effect, fractured-vuggy physical model was designed and fabricated to present the dynamic characteristic in fractures and vuggs media. Experimental results show that the even distribution, similar diameters of bubbles, thick liquid membrane and high viscosity prove MSGF is stable and available in the water with the salinity of 220000mg/l and temperature of 120°C. The liquid membrane including the continuous structure formed by macromolecular gel protects the bubbles from the harmful influence of ions. After the defoaming behavior, the rest solution becomes gel again, which benefits controlling gas channeling. The item which contributes to this phenomenon is proved to be the foaming stabilizer. MSGF was able to control nitrogen mobility and channeling. Because of the salinity and temperature tolerance, MSGF can be applied in high salinity and high temperature reservoirs, especially in fractured-vuggy carbonate reservoirs. This kind of reservoirs does not have porosity which will make it possible for normal foaming system to foam again. Besides, MSGF can achieve profile control after the foaming through secondary gelation. This study could provide a constructive guide for the fractured-vuggy carbonate reservoir to improve production performance.