A comparison study between N-Solv method and cyclic hot solvent injection (CHSI) method

Abstract

Abstract Steam injection is one of the major recovery methods to exploit heavy oil reservoirs. Replacing steam with hot solvent is an innovative idea to immensely decrease environmental pollution and energy consumption in steam-based recovery methods. The current hot solvent injection method attracts attention to the vapor solvent extraction (VAPEX) process. In comparison with the VAPEX process, cyclic solvent injection (CSI) process has been acknowledged as a high oil rate method due to the driving force of foamy oil flow. Hot solvent injection method is potentially applicable in the CSI process, which may increase the oil production rate and avoid the disadvantage of the VAPEX method. In this study, two experiments have been conducted to compare the production performance between the hot solvent-based CSI method and the hot solvent-based VAPEX method. The hot solvent-based CSI method is named the cyclic hot solvent injection method (CHSI). The hot solvent-based VAPEX method refers to the N-Solv method. N-solv is a patented method where the solvent is injected at the dew point temperature under the reservoir pressure condition. Due to the temperature difference between the reservoir and injected solvent, the injected solvent will condense in the reservoir and the condensed solvent will generate latent heat that can heat the reservoir chamber and thus reduce the oil viscosity (Nenniger and Gunnewiek, 2009). In this study, the N-Solv method has been studied in a laboratory test for the first time in N-Solv research history (Cao, 2011; Nenniger, 2005; Nenniger and Gunnewiek, 2009). According to experimental results, either of the two methods demonstrates some advantages in comparison with the other method from various aspects. From the production performance point of view, the recovery factor of the CHSI method is nearly 10% higher than that of the N-Solv method. The longer high oil rate period of the CHSI method in contrast with the N-Solv method indicates that the foamy oil behavior is more beneficial to the oil production compared with the gravity drainage and the solvent diffusion effect. In addition, it is shown from the solvent chamber growth difference that the gas flooding effect can enhance oil production in the CHSI process because the oil from gas-flooded area in the reservoir is clearly extracted out. However, according to the production GOR data, CHSI is a solvent-consuming method where a tremendous volume of free gas tends to be trapped in the solvent chamber as a result of gas flooding, which may offset the positive effect of gas flooding due to large consumption of solvent. The phenomenon of free gas trapping has not been found in the N-Solv process.