Pore Structure Characteristics And Their Control On Remaining Oil In Low-permeability Beach-bar Sandstones

The beach-bar sandstones are widely developed in the Upper Es₄ Member of Chunliang Area of Dongying Depression,and their permeability values are mostly distributed in the range of 0.1×10^-3-20×10^-3μm²,showing obvious low-permeability characteristics.Moreover,the wide distribution,high oil geological reserves and great exploration and development potential make the beach-bar sandstones among the most important petroleum exploration targets in this area.Different from conventional sandstones,the beach-bar sandstones are featured by complex pore structures with multiple types and multiple scales of pore throats.The geometry and connectivity characteristics of the pore throats both show strong heterogeneity.The heterogeneous pore systems also lead to different seepage laws of the oil and water phases,which in turn affect the formation,distribution and occurrence of microscopic remaining oil and even the crude oil production during oilfield development.Therefore,it is of important theoretical and practical significances to investigate the pore structure characteristic and its control on remaining oil for revealing the formation mechanism of remaining oil and guiding the tapping of remaining oil in low-permeability beach-bar sandstones.Various experiments,image analysis and processing technology,oil-water flow simulation method and machine learning algorithm are comprehensively utilized to systematically investigate the pore geometry characteristics,pore connectivity characteristics,pore structure type classification and genetic analysis,the control of pore structure heterogeneity on residual oil,the prediction and tapping of residual oil in typical Areas.Important conclusions have been obtained as follows:The pore-throat types,pore-throat size and distributions,pore geometry and geometric heterogeneity characteristics of low permeability beach-bar sandstones are subtly described.The pore systems of low-permeability beach-bar sandstones are mainly composed of residual intergranular pores,secondary dissolved pores,micro-pores and micro-fractures.The pore and throat size distributions both show multi-scale characteristics of multi-peak or double-peak.The volume fraction of macropores in sandstone with better physical properties is higher,and the multi-scale characteristics and heterogeneity of pore-throat size distributions are more significant.The macropores have more irregular geometry and stronger geometric heterogeneity than micro-small pores.The pore connectivity characteristics of low-permeability beach-bar sandstones are clarified,and the impact of pore connectivity on the macroscopic reservoir quality,permeability contribution,and fluid mobility are revealed.The coordination number of macropores in low-permeability beach-bar sandstones is larger than that of micro-small pores.Locally connected pore networks are more likely to be formed in macropore systems,and the tortuosity of macropore systems is also lower.The pore throat radius exerts a strong linear correlation with the absolute contribution of pore throats to permeability.The pore systems dominated by the pore throats with radius>1μm tend to show the characteristics of“low porosity and high permeability”,while the pore systems dominated by the pore throats with radius<0.1μm are prone to“high porosity and low permeability”.The bound fluid pores in low permeability beach-bar sandstones mainly correspond to the micro-pore systems with radius<1μm.With the increase of the average pore radius of pore systems,the pore connectivity becomes better and the fluid mobility increases.The pore structure types of low-permeability beach-bar sandstones are divided,and the genesis of different types of pore structures are elucidated.According to the critical pore structure parameters,the pore structures of low-permeability beach-bar sandstones can be divided into six categories.Type I pore structures,characterized by“low porosity and high permeability”,are closely related to the development of micro-fractures.Type II pore structures,composing of residual intergranular pores and secondary dissolved pores,are mainly developed in fine sandstone with strong anti-compaction ability and pervasive feldspar dissolution.Type III pore structures are mainly constituted of secondary dissolved pores and a small amount of residual intergranular pores,which are common in siltstone.Type IV pore structures are mainly composed of secondary dissolved pores,in which pore-filling carbonate cements are commonly observered.The type V pore structures are characterized by strong carbonate cementation,and the pore systems are dominated by micro-pore throats with a small amount of secondary dissolved pores.The high content of argillaceous matrix in the type VI pore structures results in the pore systems being dominated by micro-pore throats,and the appearance of"high porosity and low permeability"often occurs.The controls of pore structure characteristics and heterogeneity on remaining oil distribution have been revealed.The water sweep efficiency in water-wet pore systems is significantly higher than that in the neutral or oil-wet pore systems,and the water flooding recovery is also higher in water-wet pore systems.The water-wet pores with complex geometry are more likely to be sweeped by water,so it is not easy to form remaining oil at pore corners,while the oil-wet pores with complex geometry have strong adsorption force on oil,which may cause the formation of remaining oil at pore corners.Macro-heterogeneity of pore structures affects macro sweep efficiency,while micro-heterogeneity of pore structures controls the micro sweep efficiency and crude oil utilization in the sweeped area.The pore networks with better connectivity have stronger heterogeneity in pore throat size distribution and connectivity,and it is easier to form dominant flow channels in these pore networks,resulting in reduced micro sweep efficiency and formation of micro remaining oil.The spatial distribution of pore structures of low-permeability beach-bar sandstones in typical blocks is clarified,and the favorable tapping intervals and zones of remaining oil are predicted after expounding the remaining oil potential of different types of pore structure.The pore structures of Type III and II in the 1³ th and 1⁴ th sublayers of the 1st sand layer in F151Block account for a relatively high proportion,with concentrated spatial distribution,resulting in the enrichment of remaining oil.The subsequent oilfeld development should focus on tapping the potential of these remaning oil enriched sublayers.Well F151-1 area,well F151-P1 area,well F147-P1 area and well F151-12 area are rich in remaining oil in 1³ th and 1⁴ th sublayers of the 1st sand layer,which are favorable zones for tapping the potential of remaining oil.The water sweep efficiency and oil displacement efficiency can be improved by reducing oil-water interfacial tension,maintaining water injection,increasing displacement speed,and increasing water phase viscosity,so as to tap the potential of remaining oil.