The Study On Pore Structure Character And Remaining Oil Distribution Law Of Low Permeability Layer

For researching the differences of pore configuration between low permeability reservoir and high permeability reservoir, we found out the reasons of low permeability reservoir with low flooding efficiency from the microscope, the paper was on the base of five experiments(the pressing mercury experiment of the natural core, the scanning electron microscope experiment, the wetting experiment, the driving oil experiment and the micro-dissection experiments of the core ) and combined with statistical analysis and number value analysis, the distribution law of the pore structure in the natural cores with different permeability and wettability, the law of the micro surplus oil distribution, and the relationship between the saturation of remaining oil and micro parameters of the pore structure was studied; The pore-scale modelling which is suitable of the deluge low permeability cores has been set up and the relationship between the relative permeability curves, the saturation of remaining oil and micro parameter of the pore structure was also studied. The results showed as follow:According to the statistical analysis of the mercury penetration data of coring well from Daqing outside oil fields with low permeability, we obtained the relationship between permeability and porosity. As can be seen through the mapping, the average radius of reservoir, the median radius and the mean radius increased with the permeability increasing, and it presented apparent regularity; The maximum mercury saturation increased gradually with grading factor increasing; At the same time,pore configuration characteristic parameters and reservoir permeable parameters also increased gradually with permeability increasing, within low permeability reservoir( 0.01×10 ^-3μm 2 50×10 ^-3μm²), reservoir permeable parameters had a relatively rapid growth .Taking of natural cores with different permeability levels from Daqing Oilfield, first of all, we carried out indoor experiments to confirm some parameters such as porosity, permeability, initial oil saturation, water flooding recovery and so on, then we coped with the cores using flooding experiments and processed them with scanning electron microscope, at last we obtained the characteristics of pore throat pictures which can be observed clearly under different magnifications. Through statistical analysis of particles, pores, throat and other parameters, we can analyze their distribution.The experiment results showed that the average rock particle radius increased rapidly with permeability increasing, and the ratio between the average particle area and perimeter increases when the permeability increased; The pore area, the pore perimeter and the ratio between them presented irregular changes but the pore radius increased slowly with permeability increasing; The throat area, the throat perimeter and the ratio between them had no direct relationship with permeability, and throat radius increased with permeability increasing; The average coordination number of low permeability core was less, and the average coordination number increased with permeability increasing; The average pore throat ratio increased rapidly and the average shape-dependent constant became small when the permeability reduced; As the pore radius, the particle radius and the throat radius increased, the water flooding recovery has increased trend, then increasing throat radius had an obvious effect on water flooding recovery; The water flooding recovery increased with the average coordination number, the average shape-dependent constant and the ratio between the average pore area and perimeter increasing, but it had no obvious relationship with the ratio between the average throat area and perimeter.By the combination of depth of field expansion software and measurement software with the fluorometric analysis technique, the measurement on microscopic parameter such as pore radius, the pore throat ratio, the coordination number, the tortuosity, the shape-dependent constant and the measurement on remaining oil saturation of natural cross section in low permeability cores can be realized.The wettability experiment was carried on 4 pieces of low permeability cores in Daqing peripheral oilfield and 2 pieces of low permeability cores in Changqing oilfield, and the flooding experiment, the core micro-anatomy experiment was carried in turn. The result indicated that the low permeability cores were more water wetting, and that the throat radius distribution of low permeability cores were relatively concentrated with high peak and smaller throat radius, which were the significant features of the low-permeability reservoirs different from the high-permeability reservoirs in Daqing oilfield. In the cores of different permeability pore radius were not very different, while the throat radius had big differences, so with the decrease of permeability, the pore throat ratio increased sharply; And with the decrease of permeability, the distribution frequency of the cores with smaller coordination number or with smaller shape-dependent constant or with bigger tortuosity were larger. The coordination number of different permeability cores was concentrated between 2³ and whose curve shape was closer in Daqing oilfield, while the coordination number of the two cores in Changqing oilfield was relatively bigger with good pore connectivity. Comparing the cores of the same permeability level in Daqing oilfield with that in Changqing oilfield, the tortuosity was lager and the shape-dependent constant was smaller and the pore shape was more complex.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 indicated that: In the water wetting cores the pores containing remaining oil proportion decreased with the decrease of the pore radius, while in the intermediate wetting cores the pores containing remaining oil proportion decreased with the increase of the pore radius; With the increase of the pore throat ratio, the pores containing remaining oil proportion increased; With the decrease of the coordination number, the pores containing remaining oil proportion increased; With the increase of the tortuosity, the pores containing remaining oil proportion all increased; With the decrease of the shape-dependent constant, the pores containing remaining oil proportion increased. Comparing the two cores of Changqing oilfield with the same level permeability cores of Daqing oilfield, under the situation of the same pore throat ratio, plane tortuosity or shape-dependent constant, the proportion of the pores containing remaining oil were smaller, which showed the displacement effect was better.In order to study the relationship between permeability ratio curve and remaining oil saturation and pore structure microcosmic parameters, the pore-scale modelling fitting for the water wetting low permeability cores was established , and the simulation calculation was carried out. The results showed that: under the water wetting condition, capillary force was a kind of driving force for water-flooding in oil displacement process; With the decrease of the radius, the residual oil saturation decreased, and the two-phase region become larger, and the water-flooding recovery efficiency increased; With the increase of the pore-throat ratio, the residual oil saturation increased, and the two-phase flow region become smaller, and the water- flooding recovery efficiency decreased; With the increase of the plane coordination number, and the two-phase flow region become larger, and residual oil saturation decreased, and the relative permeability along the saturation of end point increased. Relatively speaking, the influences on the non-wetting oil phase were larger, and oil-dripping flow channel increased, which improved the oil permeability, and the opportunity of fluid being captained decreased, so the forming probability of remaining oil decreased, and water-flooding recovery efficiency increased; With the decrease of the shape factor, the residual oil saturation increased, and the two-phase flow region decreased, and water-flooding recovery efficiency increased.