Studies Of Water Displacement Experiments And Dynamic Network Simulation In Tight Oil Reservoirs

Different from conventional reservoirs,there are mainly micro-nano pores and nano throats developed in tight oil reservoirs.As the microstructure is so complex and strong heterogeneity that the geological and development characteristics are significant unconventional.In this paper,some related issues such as the unclear seepage law for water and oil during water flooding,as well as the residual mechanism of non-wet phase and enhanced oil recovery by water displacement were selected as the research topics.We took the typical tight oil reservoirs in northwest of China as a target block,microstructure and fluid distributions were studied by making full use of laboratory experiments.Then,combining the experiments of deionized water flowing in micro-nano quartz crystal tubes and simulation formation water flowing in tight oil reservoir cores,the non-linear flowing characteristics at the low rate were analyzed.Meanwhile,the dynamic network simulation was introduced to simulate the water displacement processes under different conditions.At last,T2 spectra during water flooding at different capillary number were also measured by a low field nuclear magnetic resonance unit to investigate the distribution of oil and water,and the experimental results were combined with the simulation data to validate the models which in order to guide a rational and efficient development in tight oil reservoirs.The main research works of this paper as following:(1)The microstructure and topology of pores and throats,as well as the microstructure parameters were obtained from the qualitative and quantitative characterization techniques wich included thin sections,scanning electron microscopy(SEM),constant-rate mercury injection(CRMI)and nuclear magnetic resonance(NMR).The NMR relaxation time T2 was translated to pore-throat radius by combining the fully water saturated T2 spectra with pore-throat size distributions by CRMI,Then the pore network was built based on the overall pore-throat size distributions.(2)Based on the high-speed centrifuge and spontaneous imbibition experiments by combining with NMR,movable and irreducible water distributions in tight oil reservoirs were characterized and the effect factors were also analyzed.The critical flow radius were obtained based on the converted T2 spectra during centrifuge and the average thickness of the film irreducible water was calculated based on a formula which deduced from the mercury injection theories in porous media.(3)The deionized water flowing in 1.0 and 5.0?m quartz crystal tubes and simulation formation water flowing in tight oil reservoir cores were measured to character the non-linear flow at low rate.By combining with some previous experimental data of micro-nano quartz crystal tubes,the equation of fluid motion in tight oil reservoirs which considering the effect of boundary layer was deduced based on the Navier-Stokes equations and the critical condition when threshold pressure gradient would affect the fluid flow in tight oil reservoir was clarified.(4)The microcosmic force during water displacement and the mechanism of non-wet phase remaining were analyzed theoretically.The calculation methods for related characteristic parameters for pore scale were also deduced in tight reservoirs.Meanwhile,the successive over relaxation method was introduced to accelerate the stable convergence of pressure field to improve the simulation efficiency.Then the dynamic network simulator was developed based on Qt5.4.0 and the simple cubic network was built based on pore-throat size distribution.The water flooding processes were simulated and compared with experiments under different pore-throat structure,displacement conditions,fluid viscosities and contact angles and their effects on improve oil displacement efficiency were also evaluated.(5)Water flooding experiments under different capillary number were designed by combining with a low field nuclear magnetic resonance unit.T2 spectra for different water injection volume and different displacement rate were measured to indicate the changing of fluid distribution and variation of displacement efficiency in different pore spaces.Then the mechanisms of improving displacement efficiency by changing capillary number and the secondary migration of residual oil were revealed.Based on the theoretical and experimental researches in this paper,the distribution of irreducible water and the flowing characteristics in mirco-nano quartz crystal tubes and tight oil reservoir cores were clearly presented.The flowing of water and oil and the changing of residual oil in pore spaces under different capillary number were clarified.It showed that the residual oil saturation could be effectively reduced by increasing the capillary number under certain conditions in tight oil reservoirs,which was meaningful for tight oil reservoirs development.Meanwhile,those experimental and theoretical researches for two phases flow had certain scientific significance to rich and develop the seepage theory in tight oil reservoirs.