Mobilization of tight oil by spontaneous imbibition of surfactants

A series of spontaneous imbibition (SI) tests of tight oil were performed, together with oil distribution scans by computed tomography (CT) and nuclear magnetic resonance (NMR). Thus, the best surfactants to optimize the SI effect were obtained, the basic requirements to surfactants for efficient SI were determined, and the oil mobilization by SI revealed. The results show that anionic surfactants significantly outperform non-ionic, cationic, and zwitterionic ones in SI process. Excellent systems can be further obtained by mixing anionic surfactants with others (e.g. 1:1 mixtures of AES:EHSB). The requirements to interfacial properties of surfactants for achieving efficient SI at permeabilities of 0.05, 0.5, and 5.0 mD are as follows: 10⁰ mN/m, < 40°; 10⁻¹–10⁰ mN/m, < 55°; and 10⁻¹–10⁰ mN/m, < 70°, respectively. Although a high oil recovery of 38.5% by SI was achieved in small cylindrical cores (ϕ2.5 cm × 3.0 cm), the joint SI and CT tests in larger, cube-shaped cores (5.0 cm × 5.0 cm × 5.0 cm) showed that the SI process could only remove the oil from the outermost few millimeters of the cores with permeabilities of 0.05 and 0.1 mD, indicating the great difficulty encountered for their development. The NMR showed that the SI treatment preferentially removed oil from smaller pores rather than medium or large pores.