Pore Structure And Spontaneous Imbibition Characteristics Of Shale Oil Reservoir Of Fengcheng Formation In Mahu Sag

The Mahu sag in China is rich in shale oil resources,and the Fengcheng formation is the main production layer.However,the pores of shale are mainly micro-nano size and widely distributed,which leads to complex pore structure and diverse physical properties,restricting the development of shale oil resources.Therefore,how to realize the efficient development of shale oil is a key issue concerned by academia and industry.Hydraulic fracturing is the necessary technique to develop shale oil and gas.The imbibition of fracturing fluids into shale to replace crude oil is a critical mechanism to recover shale oil,and strengthening the imbibition displacement is an effective way to improve shale oil recovery.Consequently,it is of great practical significance and theoretical value to determine the pore structure and imbibition characteristics and clarify the main controlling factors of imbibition capacity of Fengcheng Formation for the efficient development of shale oil resources in Mahu sag.This article first obtained basic data on shale porosity,permeability,wettability,mineral composition,organic matter abundance,maturity,and other aspects through experimental testing.A combination of scanning electron microscopy(SEM),N2 adsorption,and nuclear magnetic resonance(NMR)techniques was then utilized to investigate the multiscale pore structure of Fengcheng shale.The pore types of Fengcheng shale were defined,and the pore morphology and pore size distribution characteristics were analyzed.In addition,fractal dimensions were determined form N2 adsorption isotherms and applied to characterize the complexity of shale pore structure.Subsequently,the spontaneous imbibition of water in Fengcheng shale was investigated using a precision analytical balance and NMR spectroscopy.The mass and NMR T2 distribution of shale samples were recorded during imbibition,and the NMR T1-T2 maps were also acquired to study the fluid displacement in the Fengcheng shale.The effects of pore structure,porosity,permeability,wettability,mineral composition,and secondary imbibition on imbibition capacity of Fengcheng shale were analyzed.Meanwhile,the key influencing factors of imbibition capacity were identified by grey correlation analysis.Finally,the pore connectivity of Fengcheng shale was quantitatively evaluated according to the characteristics of spontaneous imbibition.The results show that the pore types of Fengcheng shale in Mahu sag mainly include intergranular pores,intra granular pores,organic pores and micro-fractures.Micropores and mesopores are commonly developed,and mesopores are the main imbibition channels.NMR T1-T2 maps indicate that kerogen,asphalt and adsorbed oil are present in organic micropores before imbibition,meanwhile residual mobile oil and surface water exist in organic mesopores and inorganic micropores,respectively.After imbibition,the signal of movable oil in shale samples is weakened obviously,while the signal of free water is strengthened and the signal of adsorbed oil of rock samples is also strengthened.The spontaneous imbibition curves of shale present a three-stage distribution feature.Imbibition capacity is positively correlated with porosity,total pore volume,specific surface area,content of quartz and clay,and imbibition times,and negatively correlated with reservoir quality index(RQI),wetting angle,TOC and Ro.Porosity,specific surface area,total pore volume and clay mineral content are the main factors affecting imbibition capacity.The pore connectivity of Fengcheng shale is generally good,and the slopes of spontaneous imbibition curves are widely distributed,indicating the strong heterogeneity of shale pore structure.Moreover,the connectivity of oil-wet pores is better than that of water-wet pores.