The thermal stability mechanism of MBA-crosslinked PPGs: insights from macroscopic phenomena to chemical reactions

Preformed particle gels (PPGs) is one of the most efficient enhanced oil recovery techniques used for conformance control. Evaluating the thermal stability of PPGs and understanding their thermal stability mechanism are crucial for the development and application of high-temperature-resistant PPGs. We focused on conventional N, N'-methylene-bis-acrylamide (MBA) crosslinked PPGs, systematically analyzing the macroscopic state changes, microstructural changes, and chemical reactions occurring during the ageing process in 1% NaCl solution at different temperatures, and delved into their thermal stability mechanism. Our findings revealed that the swelling ratio (SR) of PPGs during ageing is correlated with temperature, showing three distinct patterns: a stable SR at low temperatures (<60°C), a period of unchanged swelling followed by a continuous increase within the intermediate temperature range (70-90°C), and a rapid swelling leading to sol conversion at high temperatures (>95°C). Scanning electron microscope observations and elastic modulus analysis confirmed that the microstructure of PPGs remains stable when the SR was unchanged, while continuous increases in the SR indicated damage to the gel's micro-network structure. X-ray photoelectron spectrum and carbon nuclear magnetic resonance spectroscopy indicated that no chemical reactions occurred in PPGs at temperatures not exceeding 60°C. When temperature was 70-100°C, a small amount of acrylamide (AM) hydrolysis occurred initially, which then sequentially triggered the hydrolysis of MBA crosslinkers and 2-Acrylamido-2-methylpropane sulfonic acid (AMPS) due to the catalytic effect of undissociated carboxyl. At temperatures above 100°C, hydrolysis reactions occurred simultaneously in AM, MBA crosslinkers, and AMPS. The thermal stability of MBA-crosslinked PPGs is indeed related to the thermal stability of the MBA, but it can be reduced by the influence of other non-temperature-resistant monomers, such as AM. This study provides insights into the thermal stability mechanisms of PPGs and guidance for the development of high-temperature-resistant PPGs.