Research On Pore Structure Characteristics Of Tight Oil Reservoirs

The tight oil reservoir is not only the key point to oil exploration and development in recent years, but also the trend in the future. The tight oil research is still in the initial stage in China. Research on the pore structure of tight reservoir and occurrence mode of the oil phase, as the basic element in the field development, have great impact regardless of reservoir properties, oil and water flow characteristics or development mode selected. By some experiment such as scanning electron microscopy, constant pressure mercury, high pressure mercury, low temperature adsorption and NMR etc., analyzing the pore structure to two large tight oil regions, Changqing and Sichuan, two lithological reservoirs, sandstone and limestone from five aspects that is the reservoir characteristics, mineral composition, cracks-micro cracks, pore-nanometer pore characteristics, the oil phase content in the pores and the available stock.Routine physical experimental analysis about the 311 cores in 2.5cm diameter shows that the porosity and permeability parameters of the core from target oil zones limestone, sandstone reservoir are in line with tight oil reservoir parameters characteristic. Relative in comparison, the porosity of the cores from Changqing tight sandstone reservoir core are relatively higher, the permeability are relatively lower, permeability distribution are concentrative and homogeneity are better than the cores from Sichuan tight limestone reservoir. The performances of the cores from Sichuan limestone and sandstone reservoirs are opposite. The porosity averages of the core from Changqing tight sandstone reservoir, Sichuan tight limestone reservoir, Sichuan tight sandstone reservoir are 8.29%　and 1.78% 4.01%, the permeability averages are 0.13mD,0.29mD and 0.32mD. The ranges of the permeability average are mainly distributed 0.01mD～1mD, less than 0.01mD, greater than 0.1mD and less than 0.01mD. The porosity and permeability properties relations in the same reservoir and distinguish in three types of reservoir are determined by the different pore structure.Three aspects to pore characteristics were studied including mineral composition of core, cracks-microcracks characteristics and pore-　nanometer pore characteristics. Researches have shown that:the minerals of the cores from Changqing sandstone reservoir are dominated by quartz, followed by clay minerals. The main pore type is intergranular pore. Relative to the other two reservoirs, the pore throats number of the cores from Changqing sandstone reservoir are much, the pore radius are large and the throat radius are small. The pore space is mainly controlled by the throat which radius is 0.05μm～0.5μm. The mineral composition of the cores from Sichuan sandstone reservoir is similar with the cores from Changqing sandstone reservoir. The main pore type is slit-like pore (intergranular joints). That exhibits relatively large pore throat number, the larger pore radius, the large throat radius. The pore space is mainly controlled by the throat which radius is greater than 0.1μm. The minerals of the cores from Sichuan limestone reservoir are dominated by calcite, aragonite-based. Clay content is extremely low. The main pore type is slit-like pore. The numbers of pore throats are small, pore radius are small, and throat radius are large. The pore space is mainly controlled by the throat which radius is larger than 1μm.The stock of the oil phase and mainly utilized space to the conventional and nanoscale pore(less than 0.10μm throat radius control) were studied by the initial oil saturation experiments and the movable oil experiment based on NMR. Changqing sandstone reservoir initial oil saturations are between 39.03%-53.17%, three target reservoir simulation reservoir oil saturation were approximately 56%. Conventional porosity movable oil saturations are greater than nano-pores movable oil saturation. The major movable oil space to Changqing sands reservoir cores is control by 0.02μm～1μm throat, oil concentrated in middle pores, continuous occurrence. The major movable oil space to Sichuan limestone reservoir cores is control by greater than 1.0μm and 0.05μm～0.10μm throat, oil concentrated in big pores or nano-pores, the occurrence is not continuous.