| In order to learn the difference of pore structure between low permeability layer and high permeability layer, find out the reason why the displacement efficiency in low permeability layer is lower than that in high permeability layer, the character of pore structure, distribution law of microscopic remaining oil along with its relation with pore structure in cores with different permeability and different wettability was researched based on the wettability experiment, oil displacement experiment and the core dissecting experiment,. The outcome of this study is as bellowed.The technique of core micro-anatomy experiment was developed and the equipment assembly was completed. By the combination of fluorescence microscope and the metallography microscope, by the union of depth of field expansion software and measurement software and the fluorometric analysis technique, this method can realize the measurement on microscopic parameter such as pore radius, the pore-throat ratio, the plane coordinate number, the plane circuitous degree, the shape factor and the measurement on remaining oil satruation of natural cross section in low permeability cores.The wettability experiment was carried on 5 piece of low permeability cores, and the flooding experiment, the core micro-anatomy experiment was carried in turn. The same experiment was carried on 5 piece of high permeability cores in order to contrast the pore structure between low permeability cores and high permeability cores.Wettability of 10 piece of natural cores that after and before washing oil was measured.The result indicated that the often used oil washing method may change the wettability of cores seriously, which causes it more pro-oil bias. Through flooding of a certain time, this impaction can be basically eliminated, and the wettability of the cores return to the initial state. Then the experiment should be carried on before the procedure of washing oil or after water flooding when the wettability of cores are studied.After the measurement on wettability, the oil displacement experiment was launched before the core dissecting experiment. Both metallographic picture and fluorescent picture of nature fractured surface in cores were shoot, the parameters of cores including pore radius, throat radius, ratio between pore and throat, plane coordinate number, plane tortuous degree and form factor were measured. The result of experiment shows that: (1) With the increase of permeability, the average pore radius of cores increase, but this trend was not apparent which indicates the impact of the pore radius on permeability was not significant; (2) with the increase of permeability, the ratio between pore and throat decrease. The classification feature is obvious, there is a significant differences in ratio between pore and throat between low permeability cores and high permeability cores; (3) the greater the permeability is, the greater the plane coordinate number is, which indicates that low permeability cores have poor pore connectivity; (4) With the increase of permeability, plane tortuous degree become smaller, which indicates that the distribution of pore is more complex in low permeability cores than that in high permeability cores; (5) the form factor in low permeability cores is much smaller than that in high permeability cores, which shows that the pore-throat section shape in low permeability cores is too complex to allow the flowing of water and oil in it; (6) the most significant difference between the low permeability and high permeability cores is that the ratio between pore and throat is greater and form factor is smaller in low permeability cores.According to the comparison between two kinds of photographs of the metallographic type and the fluorescent type, the distribution law of the microcosmic remaining oil in cores was studied. The result indicates that: (1) In the oil-wet and intermediately wet cores, the smaller the pore radius is, the larger the probability of the existence of remaining oil is. However, the remaining oil in the water-wet cores is mainly contained in large pores. (2) Whether the cores are water-wet or oil-wet, with the increase of the pore-throat ratio, the probability of the existence of remaining oil in the pore would both increase. (3) Whether the cores are water-wet or oil-wet, with the increase of the coordinate number, the probability of the existence of remaining oil would both decrease. It indicates that improving the microcosmic connected conditions of pores is conducible to enhance oil recovery. (4) Whether the cores are water-wet or oil-wet, with the increase of tortuous degree, the probability of the existence of remaining oil would both increase. It shows that it is much harder to displace the remaining oil when the pore shape is more complex. (5) The smaller the shape factor in the water-wet cores and intermediately wet cores is, the larger the probability of the existence of remaining oil. While in the oil-wet cores, as the shape factor decreases, the probability of the existence of remaining oil would also decrease. (6) As the pore throat in the low permeability layer is narrow and the shape is complex, the resection of oil globule in the channel is the primary form to generate remaining oil. (7) Large pore-throat ratio and small shape factor is the main reason for the low displacement efficiency in the low permeability oil layer.When the oil droplet moves from the pore throat to the pore, capillary pressure gets bigger and is the driving force all through the displacement. Discontinuous oil droplets tend to be easily captured and become remaining oil when moving from the pore throat into the pore. Once remaining oil is formed, it is impossible to establish effective pressure gradient to drive it out. The size of the pore is very small in the low permeability layer and the remaining oil in the form of the oil droplets due to capture accounts for a big percentage. Disconnection due to the big ratio in size of the pore to the throat is the main reason for the low displacement efficiency in the low permeability layer.
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