Study On Micro Scale Pore Structure And Flow Mechanism Of Tight Oil Sandstones

Tight oil sandstones have some typical characteristics,which are rich pore types,pore-throat radius ranging several scales,and the unknown flow mechanism.In this dissertation,the micro-scale pore structure and flow mechanism of tight oil sandstones are studies.With the tight oil sandstone core plugs collected from the Yanchang formation in Ordos basin,experimental tests including X-ray diffraction analysis,scanning electron microscope,mercury injection capillary pressure,and porosity and permeability measurement were conducted to obtain the pore structure and petrophysical parameters.With the experimental results of CT and scanning electron microscope as constraint conditions,multi-scale digital cores that representing the characteristics of tight oil sandstones were reconstructed by a combination of Quartet Structure Generation Set algorithm(QSGS),Markov Chain Monte Carlo algorithm(MCMC),and process based algorithm(PB).By using the reconstructed digital cores as the input porous media,the single phase flow were simulated using both the pore scale lattice Boltzmann method and the generalized lattice Boltzmann method,and parameters affecting the permeability of the digital cores were discussed.After the study of the single phase flow,the pseudopotential multi-component-multi-phase lattice Boltzmann method was applied to simulate oil and water two-phase flow in 2D porous media,and parameters affecting the relative permeability curves were discussed based on the simulation results.The main conclusions and contribution of this dissertation are as follows.Qualitative description and quantitative characterization of the micro-scale pore structure of tight oil sandstones were accomplished by a combination of several experimental methods.The mineralogy of tight oil sandstones is mainly composed of quartz,feldspar and clays.The main pore types are residual interparticle pores,intraparticle pores of skeleton minerals,clay dominated pores,and micro fractures.The pore-throat distribution curves are poly-modal,and pore-throat radius ranges from 9.2 nm to 500 ?m.Nanometer pores(<1 nm)and mesopores(>62.5 ?m)are the main pores,among which nanometer pores dominates.The porosity is positive correlated with feldspar content,shows negative correlation with mixed layer of smectite and illite content and illte content,and has weak relationship with chlorite content.The three dimensional pore-throat structure of tight oil sandstones was unveiled,and the multi-scale digital cores representing the characteristics of tight oil sandstones were built.The results of CT experiment show that and the nanometer scale clay dominated pores bridges the micron scale interparticle pores.Micron scale three-phase digital cores(i.e.,skeleton phase,pore phase and clay phase)were built by QSGS algorithm and PA algorithm.And nanometer scale clay phase digital cores were reconstructed by QSGS algorithm and MCMC algorithm.The reconstructed multi-scale digital cores show excellent consistence with the real core plugs.The single phase flow in the single scale and multi-scale digital cores was simulated using the lattice Boltzmann method,and the effects of parameters on the permeability were analyzed.The single scale simulation results show that the permeability is positively correlated with porosity,and is negatively correlated with surface area and tortuosity.The multi-scale simulation results indicate that the permeability of the multi-scale digital core is positively correlated with the permeability and volumetric percentage of the clay,and is not related with reasonable volumetric percentage of the interpartice pores.Oil and water two-phase flow in the two dimensional porous media were simulated by the pseudopotential lattice Boltzmann method,and the effects of capillary number,oil and water viscosity ratio,and wettability on the relative permeability curves were analyzed.The relative permeability of oil phase(Knw)is positively correlated with capillary number.The relative permeability of water(Kw)shows positive correlation with capillary number under the neutral wetting condition,and shows no relationship with capillary number under the strong wetting condition.Knw is positively correlated with viscosity ratio under the strong wetting case,while Kw is insensitive to the viscosity ratio.When the porous medium converts from strong wetting to neutral wetting,Knw decreases gradually,and Kw increases slowly.This dissertation proposes novel methods to characterize the multi-scale pore structure and simulate the multi-scale flow of tight oil sandstone.The study accomplishes the qualitative description and quantitative characterization of the micro-scale pore structure of tight oil sandstones,reveals the effects of structure parameters and fluid parameters on the absolute permeability and relative permeability curves of oil and water two-phase flow.The conclusions have important theoretical values and practical significances to the successful development of tight oil reservoirs.