Microscopic Pore Structure Characterization And Production Characteristic Analysis Of Low-Permeability Sandstone Reservoir

Low permeability sandstone reservoir in China has geological characteristics of strong microscopic heterogeneity and low sedimentation maturity,microscopic features of small pore-throat radius,high capillary pressure and complex pore throat structure,and simultaneously,exists the contradictions of seepage as low movable fluid saturation,strong oil-water interference and low water-flooding displacement efficiency.JiYuan oilfield,located in the Ordos Basin,attributes to typical low-permeability sandstone reservoir in china.Its reservoir remains narrow pore-throats and variant pore-throat continuities.The seepage laws are synthetically controlled by sedimentation,diagenesis,pore structure heterogeneity,fluid properties,etc.,which results in the prominent contradictions and difficulties in reservoir development,and eventually,leads to the low recovery ratio and the accumulation of residual oil.In order to figure out the residual oil enrichment rule for making oilfield water-flooding development plan,this paper takes Chang 4+5 and Chang 6 reservoir of T area in JiYuan oilfield in Ordos Basin as examples,conducting experiments to microscopic pore structure characterization and production characteristic analysis in low permeability sandstone reservoir in both micro and macro aspects,including regular physical property,rock casting slice,SEM,high pressure mercury injection,constant speed mercury injection,NMR,oil-water relative permeability,real sandstone microscopic water-flooding.The microscopic coupling mechanism of different types of reservoirs is analyzed.The paper focuses on the analysis of the response relationship between reservoir petrological characteristics,microscopic pore structure characteristics,seepage mechanism and production performance characteristics of different types of flow units.The conclusions reached are as follows.?1?The major oil layer in Chang 4+5 reservoir is Chang 4+5₂²,which is a subdivided reservoir with the formation thickness of 26.5m in average.The major oil layer in Chang 6reservoir is Chang 6₁²,a subdivided reservoir with the formation thickness of 30.7m in average.The physical property of Chang 6 reservoir is better than Chang 4+5 and its lithology is mainly fine and very fine grained arcose gray and light gray with well sorting characteristics and medium compositional maturity.?2?On the plane,the distributions of porosity and permeability are closely related to the distribution of sand body.The larger the sandstone thickness,the less the mud content,the finer the reservoir physical property,the stronger the hydrodynamic performance,and accordingly,the value of porosity and permeability isoline gets higher.But in the area where the sand body is underdeveloped,such as distributary bay,the reservoir physical property is poorer,and the value of porosity and permeability isoline gets lower.?3?High pressure and constant speed mercury injection expriments showed that the continuity of pore structure is inversely proportional to displacement pressure.Higher displacement pressure corresponds poorer continuity and seepage capacity.Higher median pressure corresponds smaller pore and throat radii,and therefore the pore-throat heterogeneity become stronger.In the reservoir with finer pore structure,its larger pore and throat radii create larger effective reserving space and finer pore-throat continuity,and moreover,stronger reserving and seepage capacity.Contrarily,in the reservoir with poorer pore structure,its underdeveloped pore-throat creates smaller effective reserving space,poorer pore-throat continuity and stronger heterogeneity,which go against the reserve and seepage of fluid.?4?NMR and oil-water relative permeability experiments showed that microscopic pore structure heterogeneity is the major factor which affects oil and water distribution.Permeability affects reserve characteristic of movable fluid greatly.To a certain extent,the reserves of movable fluid are affected by different clay mineral content,which is a collective effect of various mineral content.Low permeability sandstone reservoir has poorer physical property and seepage capacity.The mutual interference between oil and water is strong and the area of oil-water co-permeation zone is narrow.?5?According to the production curve characteristic,the producing wells in the research area were classified into four types.I wells are the highest productivity and the oil production is rising.II wells are low water cut in the early stage of production and the oil production is rising steady and slightly.III wells oil production decreases and water cut increases.IV wells are low productivity and low efficiency,the water cut is high,producing a small amount of oil or no oil.?6?Utilizing Q-mode cluster analysis of SPSS software,the research area was divided into four different types of flow unit-E,G,M and P.The seepage and reserve capacity get poorer in sequence,that is to say,the physical properties are best in E flow unit,better in G flow unit,general in M flow unit,and worse in P flow unit.?7?Production performance characteristics adequately respond the correlativities among sedimentation,diagenesis,microscopic pore structure development,pore-throat configuration and seepage mechanism in different types of flow unit.In low permeability sandstone reservoir,the production characteristics of water-flooding development are simultaneously affected by rock characteristics,microscopic pore structure and seepage characteristics,and especially,microscopic pore structure characteristics occupy a predominant position.