Fabrication of a Spontaneous Emulsification System for Low-Permeable Oil Reservoirs and Study of an Enhanced Oil Recovery Mechanism Based on the Nuclear Magnetic Resonance Method

Abstract

An emulsifier that can spontaneously emulsify oil by a gentle disturbance is an ideal displacement agent for enhanced oil recovery (EOR) in low-permeability reservoirs. Herein, a novel spontaneous emulsification (SE) system with a low concentration of alkali was fabricated in this work. An optimum SE system was screened by interfacial tension (IFT) measurement; the diameter distribution and microscopic morphology of emulsions were observed by an optical microscope and multiple light scattering; the EOR mechanism was studied by nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI); and the applicability of the SE system in heterogeneous reservoirs was further explored by parallel-core flooding tests. The result showed that the ion pairs formed as a result of the electrostatic attraction of surfactant head groups between cetyltrimethylammonium bromide (CTAB) and nonylphenol polyoxyethylene ether carboxylate (SS-231), which caused the IFT of the CTAB/SS-231 system to reach the ultralow level. The synergistic effect between alkali and surfactants occurred by adding only 0.1 wt % NaOH, which greatly enhanced the capability of emulsification. As a result, the IFT of the SE system (0.4 wt % CTAB/SS-231 + 0.1 wt % NaOH, with massCTAB/massSS-231 = 3:7) could reach 2.1 × 10–3 mN/m, and the SE system could emulsify the crude oil by inverting once. The crude oil in small pores and throats could be displaced by the SE system because the in situ emulsions formed by the SE system were small enough to flow through the small pores; therefore, they exhibited a higher EOR efficiency than the weak emulsification (WE) system. In addition, the SE system showed stronger capabilities in conformance control as a result of the deformation and migration of emulsions. This work revealed the EOR mechanism of the SE system from the level of the pore structure using NMR and MRI, which is important for evaluating the production efficiency of low-permeability reservoirs. Furthermore, the low concentration of alkali (0.1%) in the SE system avoids the scale formation of low-permeability reservoirs effectively.