Conformance Control and Improved Oil Recovery Mechanism of Branched Preformed Particle Gel in Fractured Reservoirs Utilizing Nuclear Magnetic Resonance Technology

The limited sweep improvement in fractured reservoirs has always been the key factor restricting the enhancement of their recovery factor. Particle plugging agents have attracted widespread attention due to their excellent performance in strongly plugging fractures. Among them, branched preformed particle gels (B-PPG) have been widely applied in conformance treatment in recent years because of their excellent elasticity and deformability. Nevertheless, the precise microscopic mechanism behind their ability to enhance oil recovery and improve the conformational effect within fractured cores remains enigmatic. In this paper, a comprehensive evaluation of B-PPG, encompassing static particle gel swelling performance evaluation, rheological property tests, and dynamic injection and plugging performance experiments in fractured core samples, was conducted. Furthermore, the impact of varying core permeabilities and injection rates on the injection and plugging efficacy of B-PPG in fractured cores was studied by combining low-field nuclear magnetic resonance (LFNMR) technology. Results showed that B-PPG had outstanding water absorption and swelling capabilities, with a swelling multiple in brine reaching 40 times. Post-swelling, B-PPG also exhibited exceptional viscoelastic properties, achieving a G′ value of 70 Pa. Additionally, the injection rate significantly affected the injection efficiency and plugging strength of the B-PPG. LFNMR analysis confirmed that, at lower injection rates, B-PPG penetrated deeper into the matrix under high pressure. Meanwhile, it formed a more robust plugging zone within the fractured core. B-PPG was particularly well-suited for fractured reservoirs with a matrix permeability exceeding 0.1 × 10–3 μm2. However, in reservoirs with an excessively low matrix permeability, B-PPG could inflict irreversible damage to the matrix, potentially hindering subsequent water-flooding operations. The insight from this research elucidated the migration mechanisms of B-PPG in fractured reservoirs and offered theoretical support for the design of B-PPG injection and plugging strategies tailored to fractured reservoirs.