Study On The Influence Of Tight Reservoir Micro Pore Structures On Seepage Rule

Tight oil as a new hot spot for of the global unconventional oil and gas resources,following shale gas,shows great development potential.But due to the the tight lithology,tiny pore throats,and strong micro heterogeneities,the oil-water seepage in tight sandstone reservoir is totally different from that in conventional reservoir.The particular micro pore structure is the essential reason for the unusual oil-water seepage.Understanding the pore structure and seepage features,and the impacts of the micro pore structure feature on the oil-water seepage rule is very significant to the fine evaluation and deveolopment of reserovir.In this paper,Chang 6 reservoir in Huaqing and Chang 8 reserovir in Maling of Ordos basin were taken as examples for analysis.Pore structures were qualitatively and quantitatively characterized by automated scanning electron microscope?QEMSCAN?,optical microscope,laser scanning confocal microscope?LSCM?,CT scanner,field emission scanning electron microscope?FESEM?and mercury intrusion porosimetry.The results showed that these two reservoirs has similar mineral compositions and pore structures.The content of quartz and feldspar were the highest.There were abundant primary intergranular pores and intergranular pores.Meanwhile,some intragranular pores and moldic pore could also be observed.The pore connectivities were good for both reservoirs,and the sorting of pore throats was fair.For Chang 8 reservoir,the chlorite developed around the pores protected the primary pores to some extent,therefore,there were a lot of throats larger than 0.1?m,and some throats are even larger than 1?m.Based on the principle of NMR,the NMR T₂ distribution was converted into pore throat radius distribution in combination with mercury intrusion technique,and the distribution rule of movable fluid and oil in Chang 6 reservoir of Huaqing and Chang 8 reservoir of Maling were studied.The results showed that the saturations of movable fluid were high?approximately 80%?for both reservoirs,and the cut off values of the pore radius for movable fluid were ranging from 0.003⁰.02?m,with an average value of 0.013?m.The saturations of movable oils were in the ranges of 24.438%to 58.926%and 40.372%to 63.585%respectively.In addition,the distribution rules of movable fluid and oil were in accordance with that obtained by NMR method,and the radiuses were in the ranges of 0.01⁰.1?m and0.1¹?m.For cores from the same reservoir,as the permeability increased,the amounts of movable fluid and oil in pore throats with the radiuses of 0.1¹?m,1¹0?m?chinese abstract does not have such information?were increased,and that in pore throats with the radiuses of 0.01⁰.1?m,0.001⁰.01?m and and smaller than 0.001?m were reduced.For the same core,the ratio of movable fluid and oil in large pore throats was high.The seepage rule of oil(S_wc)in tight reservoir was studied by core displacement experiments.The results showed that the minimum starting pressure gradients of Swc in Chang 6 and Chang 8 were in the ranges of 0.0054⁰.0167 MPa/cm and 0.0014⁰.081MPa/cm.Under the same condition,the minimum starting pressure gradients of S_wc in Chang6 was higher than that in Chang 8,and it was negatively correlated with the permeability and the largest pore throat size.The seepage rule of oil in tight sandstone reservoir was that there was a flexuring section,and as the core permeability decreased,the throat distribution curve was shifted to the left.Under the same pressure gradient,the fluid boundary layer was thick,higher pressure was needed to drive the fluid in all pore throats,and the nonlinearity section of the seepage curve became longer.Finally,the minimum starting pressure gradients and nonlinear seepage feature were considered,and the calculation model of seepage rule of oil-water in tight reservoir was established.The oil permeability obtained from the improved JBN method was higher than that obtained from the conventional JBN method,and the water permeability was lower,and the isopotal point was shifted to the left.The effects of mineral composition,wettability and pore structure on the shape and eigenvalue of oil-water relative permeability curve were analyzed,and the results showed that the radius and heterogeneity of pore thorats were the key influence factors on oil-water relative permeability curves.