Study On Key Scientific Problems Of Percolation And Oil Displacement In Tight-low Permeability Reservoirs

The theory of oil displacement and percolation can not be applied to conventional reservoirs.For example,The Problems of Nonlinear Percolation Problem or abnormal capillary force and so on.The new theoretical foundation of tight oil reservoirs are required.Aiming at these problems,A new method is studied to study the mechanism of seepage and displacement in low permeability/tight reservoirs.There are three important scientific discoveries of micro-scale seepage in tight reservoirs.1.Discovery of amorphous solid-like water in nanotubes at room temperature,atmospheric pressure or high vacuum.TEM sections made by dual-beam scanning electron microscope at room temperature.Observation of liquid state in microtubules by TEM.Analysis of structure by electron diffraction.Liquid water becomes amorphous solid-like water only because of the change of scale.This solid-like water can exist stably at room temperature(20 C),high vacuum(10^-5 Pa)or atmospheric pressure.This is the essential mechanism of liquid"start-up pressure"in tight/low permeability core.This amorphous solid at room temperature is a new structure of water.2.Discovery of wall fluid layer.Using of Cryo-FIB-SEM,Cryo-TEM and SAED.There is a layer of structural anomaly in pure water on the wall of a circular pipe.The thickness increases with the decrease of pipe diameter.The wall fluid layer filled with microtubules in the nanotubes(d<1?m).This is the essential mechanism of non-linear seepage.3.Discovery of negative slip of high pressure gas.The phenomenon of"negative slip"flow of high pressure gas contrary to the theory of gas slip in tight cores has been found by invented high pressure micro differential gauge and high pressure micro flowmeter.The apparent permeability increases monotonously with the pressure gradient and tends to an extreme value.This reveals the mechanism of abnormal percolation of high pressure gas in tight reservoirs.The micro-scale seepage mechanism model of tight reservoir(Y-MS model)is established.This model is established by"Amorphous Solid Water","Wall Water Layer",early related micro-scale flows and experimental results of seepage in tight cores.All four parameters in the model clearly represent the essential mechanism of"abnormal"seepage in tight-low permeability cores,and have clear physical significance.It can fit non-flow region,low-speed non-linear seepage region and linear seepage region.This provides a basic model for constructing seepage theory of tight-low permeability reservoirs.An important scientific discovery of the displacement mechanism of crude oil in tight reservoirs has been made.The micro-scale interface phenomena in microtubules were found.The oil-water meniscus in microtubules was observed by optical microscopy,cryo-FIB-SEM and cryo-TEM.Under the same material and roughness,the strong change of oil-water two-phase wettability reversal caused by the decrease of spatial scale and the abnormal phenomena of two three-phase contact points caused by the existence of wall fluid layers.This reveals the abnormal mechanism of capillary force,imbibition and displacement characteristics in compact-low permeability reservoir matrix.A method for evaluating seepage capacity of tight cores and an apparent permeability tester have been developed.Based on Y-MS model and"negative slip"seepage law of high pressure gas as testing principle.The core technology is the invented high-pressure micro-differential pressure meter,high-pressure micro-flowmeter and other instruments.The apparent permeability tester for tight cores has been developed,and the testing and evaluation methods for liquid seepage capacity and high pressure gas seepage capacity of tight cores have been established.This provides a scientific method and instrument for objectively characterizing and evaluating seepage capacity in tight-low permeability reservoirs.