Experimental Study on Carbonated Water Flooding Enhanced Volatile Reservoirs Recovery Based on NMR Technology

Abstract Volatile reservoirs possess intricate pore-permeability structures and high connate water saturation. Water flooding development is impeded by a strong water-lock effect, while gas injection development faces challenges with significant gas channeling, resulting in a low recovery rate. To address these issues and capitalize on the synergies between water and gas flooding, this study explores the utilization of carbonated water (CW) flooding to enhance the recovery factor in volatile reservoirs. This study investigates the feasibility of CW flooding in a practical volatile reservoir in Kazakhstan. Employing online nuclear magnetic resonance (NMR) displacement technology, we systematically explore the impact of different displacement media (formation water, surfactant, and CW) on the recovery factor in volatile oil reservoirs. Additionally, through the integration of high-pressure mercury data with NMR T2 spectra, we ascertain the lower limit of pore diameter under different displacement methods. Finally, NMR imaging is applied to analyze the oil recovery and core microscopic pore throat producing characteristics within the rock cores. Analysis of converted NMR T2 spectra reveals that conventional water flooding and surfactant flooding are more effective in producing crude oil in microscale micropores and microscale macropores, facing challenges in nanopores and submicron pores. CW flooding compensates for the limitations of water and surfactant flooding in producing crude oil from nanopores and submicron pores. Further data processing allows for the calculation of recovery rates and lower limits of pore diameter for different displacement methods. The pore diameter lower limits for conventional water flooding and surfactant flooding are approximately 0.1 |-im. with CW flooding exhibiting a lower limit of about 0.08 ^m. The recovery rate for CW flooding is approximately 5% higher than the other two displacement methods. Moreover. an unexpected observation from core imaging reveals that. for fractured cores. regardless of the displacement method. primary producing predominantly occurs around the fractures. As displacement progresses. crude oil in the matrix migrates towards the fracture ends. resulting in the producing of matrix oil. Through a series of online NMR displacement imaging experiments. we investigated the impact of conventional water flooding. surfactant flooding. and CW flooding on the oil recovery and core microscopic pore throat producing characteristics in volatile reservoirs. The study determined the feasibility of CW flooding to improve the recovery efficiency of volatile reservoirs. presenting promising application prospects. Moreover. it is considered an environmentally friendly method for reducing greenhouse gas emissions.