Study On Pore Structure Chracterization And Spontaneous Imbibition Flow Mechanism Of Tight Oil Sandstones

The study targets at investigating the problem of tight sandstones,including complex pore types as well as large numbers of multi-scale pore throats,ambiguous mechanisms of spontaneous imbibition.In addition,the applicability of nuclear magnetic resonance(NMR)technology on representing pore throat structures and spontaneous imbibition experiments is not identified.Considering the above tough problems,micro/nano pore throat structures and imbibition regularity are investigated in detail.A series of physical simulations are firstly carried out to represent complex pore throat structures such as X-ray diffraction mineral composition analysis(XRD),Scanning electron microscope experiment(SEM),High pressure mercury injection(MIP),NMR,NMR cryoporometry(NMRC),nitrogen adsorption method and so on,using cores from Yanchang and Linxing tight reservoirs,Ordos Basin.In particular,the combination of multistage centrifuges and NMR technique provides a novel idea to characterize complex pore throat structures and fluid mobility.At the same time,not only the volume method,but also the NMR technique are performed to analyze influencing factors affecting spontaneous imbibition in tight oil.In addition,a lot of geochemical experimental methods are employed to study the effect of a series of common ions on reservoir surface properties,based on the low salinity effect mechanisms.Pore morphology and pore throat structures of tight sandstone were observed and its mineral compositions were obtained.a novel method determining the conversion factor between NMR T₂ and pore size was recommended based on the centrifuge results combined with NMR,which provide a better idea to characterize the pore structure and mobility properties.Pore radii of tight sandstones range from 3 nm to 100?m,and the larger the permeability,the wider the range of pore radii distribution.The mobile fluids corresponding to different centrifugal force intervals were obtained,which corresponding to the interval of different centrifugal radii.Therefore,the mobile fluid can be regarded as fluid controlled by different intervals of pore throat radius.The flow ability evaluation of tight reservoirs was carried out.According to the NMR T₂spectra at different centrifugal forces,a new method for determining the T₂ cutoffs is proposed,and the T₂ cutoff values of the tight reservoir sandstone samples and the tight sandstone outcrop were analyzed and compared.The applicability of the permeability prediction model in tight reservoirs can be analyzed.There are few or no mobile fluids controlled by pore throats with radii larger than 1?m.The Coates model based on is more suitable than the SDR model for tight formation permeability prediction.Taking the imbibition recovery and imbibition front as indicators,the effects of boundary conditions,core length,pore size distribution,initial wettability,etc.on spontaneous imbibition were analyzed.The dynamics force of spontaneous imbibition under different boundary conditions were analyzed.The imbibition under the all face open boundary condition is mainly dominated by capillary force,but the co-current imbibition by gravity occurs at the same time,while the former is disturbed by the latter.The highest spontaneous imbibition recovery does not correspond to the most hydrophilic wettability state.At the mobile peak,as the pore radius increases,the percentage of imbibed mobile oil increases.At the immobile peak,as the pore radius increases,the percentage of imbibed mobile oil decreases.Based on the low salinity effect mechanisms,a series of solid-liquid reactions were carried out with varying ion types/mineral types,the changes of p H and silicon dissolution were recorded as changing indicators of rock surface properties,influenced by the mineral types,ion types,etc.Based on the results above,Zeta potential tests and wetting angle measurement experiment were carried out to obtain a solution ion composition,which is helpful to the spontaneous imbibition of tight reservoirs.This study carried out research on the characterization of tight oil reservoir pore structure and the spontaneous imbibition mechanisms of tight oil,using a variety of experimental methods.In addition,mobile fluid distributions and parameters of tight sand stones were evaluated,and the effect of ion types and minerals on the surface physical properties of tight sandstones under low salinity level were analyzed,which has important theoretical value and practical significance for the efficient development of tight reservoirs.