In the development of tight gas reservoirs, effective flowback of fracturing fluids is crucial for enhancing production. However, water blocking damage caused by fluid retention significantly affects reservoir performance. This study utilizes nuclear magnetic resonance (NMR) T 2 and T 1 – T 2 techniques to analyze water blocking damage during flowback, aiming to characterize fluid retention and the degree of water blocking damage. Reservoirs are classified into Types I, II, and III, based on the physical properties, pore-throat structure, and mineral composition of core. By integrating high-pressure mercury intrusion with NMR T 2 spectra, pores are categorized into macropores, mesopores, and micropores. The results indicate that micropores and mesopores are primary regions for fracturing fluid retention. As the flowback pressure differential increases, water blocking damage decreases, with Type I cores exhibiting lower water blocking damage compared to Types II and III. Furthermore, the integration of NMR T 2 and T 1 – T 2 techniques enabled the establishment of distribution and occurrence charts of hydrogen-containing substances (hydrogen water, bound fluid, and free fluid) in three types of reservoirs. The T 1 – T 2 spectra visualize pore development and fluid retention. During flowback, significant signal changes in bound water region indicate less retained fluid. The macropores have the lowest water blocking (<90%), while mesopores and micropores exceeds 90%. After flowback, mesopores exhibit most significant changes, while micropores have highest degree of water blocking damage, potentially resulting in long-term or permanent water blocking damage. This study elucidates the mechanisms of fluid retention and water blocking across pore scales, providing a scientific basis for optimizing fracturing flowback.
Li et al. (Sun,) studied this question.