New insight into imbibition micromechanisms and scaling model in fossil hydrogen energy development of tight reservoirs based on NMR

Abstract

Spontaneous imbibition is an important mechanism for developing fossil hydrogen energy. However, the research of imbibition micromechanisms and scaling methods in tight reservoirs with complex lithofacies remains limited. This study employed Nuclear magnetic resonance (NMR) monitoring to investigate imbibition at multiscale pores in tight sandy conglomerate samples. Then, a modified shut-in time-scale model was developed analyzing the imbibition modes and control factors. The results showed that the imbibition concludes three stages: suction, diffusion, and stable. The diffusion stage is crucial for promoting recovery. Two modes of imbibition in four lithofacies were distinguished: stable type (recovery>40 %) and sensitive type (recovery<20 %). Samples with medium permeability (0.2mD-0.6mD), bimodal pore distribution, and less illite/smectite mixed-layer (relative content<50 %) had high recovery. Nanopores provided foremost oil supply, while macropores facilitated fluid interchange. “Relay imbibition” from micropores to macropores was an important recovery mechanism. Hydrogen signals during clay expansion and microcracks propagation were identifiable by NMR-T2 after accounting for ferromagnets and temperature. The imbibition diffusion radius derived from T2 accumulation was found to modify the normalized time-scale model for complex lithofacies tight reservoirs. Consequently, the reasonable shut-in time for stable and sensitive reservoirs was concluded to be 32 and 11 days, respectively.