Fluid flow behaviors in tight reservoirs of the Middle Jurassic Shaximiao 1 Member,Gongshanmiao oilfield,Sichuan Basin
Tight oil reservoirs are characterized by complex pore structure,high resistance in fluid flow,and low production,resulting in poor development benefits.Thus,taken the low to ultra-low permeability tight sandstone oil reservoirs of of the Middle Jurassic Shaximiao 1 Member,Gong-shanmiao oilfield,central Sichuan Basin,as examples,numerous experimental studies,such as low-velocity fluid flow,constant-rate water flooding and spontaneous imbibition,were conducted on the basis of another one experiment on rock wettability.Experimental results show that(i)generally,the reservoir rocks of Shaximiao 1 Member represent a weak hydrophilic feature.When core samples are fully saturated with formation water,the water phase flow curve assumes a straight line passing through the coordinate origin.There is no start-up pressure or start-up pressure gradient in the forma-tion-water single-phase flow which is consistent with the Darcy's law;(ii)when establishing an irreducible water saturation via oil displacing water,water membrane adsorbed on pore surface may reduce oil-phase flow channels,whereas the Jamin's effect in two-phase flow may increase additional resistance to oil-phase flow.The low-velocity flow curve of oil phase does not pass through the coordinate origin,indicating the existence of start-up pressure or its gradient.The lower the reservoir permeability,the higher the start-up pressure or pressure gradient;(iii)both displacement rate and rock permeability exert great impacts on the performance of water flooding.At a higher rate,not only fingering of injected water in some larger pores but water channeling along fractures are the main controls on the decline of flooding efficiency;(iv)rock permeability may affect imbibition displacement obviously.And for fracture samples,the imbibition displacement efficiency is bigger than that in matrix samples;and(v)after large-scale fracturing in tight oil reservoirs,the induced fractures with high permeability can reduce start-up pressure and its gradient with effect.And as an important means,the single-well huff and puff(water-oil exchange by imbibition)may also ensure these reservoirs in the long-term stable production.The research results are significantly referential for the development of tight reservoirs or shale oil.
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