Analysis of imbibition mechanism and influencing factors of surfactant displacement in shale oil reservoirs

Abstract

Cores from shale reservoirs in Subei Oilfield were taken as sample to study the effects of surfactants on the oil displacement efficiency and the main controlling factors of imbibition in shale reservoirs. The TOC analysis and scanning microscope technique （SEM） were used to characterize the pore structure of the core samples. Imbibition experiments under the influence of multiple factors were conducted using nuclear magnetic resonance （NMR） technology. Then the effects of factors were evaluated such as wettability， fractures， porosities， permeability， surfactant concentrations， and types on the imbibition efficiency of shale oil reservoirs， and the distribution characteristics and producing conditions of the oil in different pores were clarified during the imbibition process. The results showed that shale imbibition could be divided into three stages： early stage， middle stage， and late stage. The imbibition rate was high in the early stage， and the imbibition efficiency increased rapidly. The middle stage accounted for the longest period， and the imbibition rate slowed down in the late stage， with the imbibition efficiency tending to be stable. By comparing the effects of different factors on imbibition， it was found that a wetter core indicated more fractures and higher imbibition rate and efficiency. Moreover， a higher porosity and lower permeability led to higher imbibition efficiency. The imbibition efficiency showed an increasing trend with the concentration of surfactants decreasing. The NMR experimental results demonstrated that in the process of shale imbibition， the produced oil of micro-pores was more than that of macro-pores， and changing the imbibition conditions could improve the producing percentage of oil in various pores.

Keywords

• shale oil reservoir
• spontaneous imbibition
• nmr
• pore structure
• surfactant