Study on the microscopic occurrence forms of condensate oil in the pressure depletion process of condensate gas reservoirs

In order to research the formation mechanisms and microscopic occurrence states of condensate oil in the development process of condensate gas reservoirs, an experimental study was carried out on the pressure depletion of the condensate gas reservoir in the porous media of clastic reservoir and in the fractured-vuggy media of carbonate reservoir. Micrometer CT scanning was performed on the reservoir cores in different stages of pressure depletion. The oil/gas/rock skeleton in the CT scanning images were identified and divided to determine the distribution state of oil and gas in pores, fractures and vugs in the three-dimensional space. In addition, the occurrence forms and locations of condensate oil in different reservoirs of condensate gas reservoirs in different development stages were characterized by means of image processing and calculation result analysis. It is indicated that the condensate oil formed in the porous media of clastic reservoir during pressure depletion is existed mainly in the form of single pore and oil film (over 90%), the proportion of the condensate oil in the form of multiple pore is quite low and there is no network-form condensate oil. On the whole, oil phase is mainly dispersed in pores and throats. The condensate oil formed in the fractured-vuggy media of carbonate reservoir during pressure depletion is mainly existed in the form of network and multiple pore (over 85%). The distribution of oil phase is relatively concentrated. The condensate oil in the form of network and multiple pore is connected through oil film. The pressure depletion process is the transformation process of oil phase from multiple pore, single pore and oil film to network. In the process of pressure depletion, oil phase will approach the rock wall as the condensate oil increases. When the condensate oil is produced, the oil phase will move away from the rock wall. With the production of oil phase, oil phase reduces and condensate oil approaches the rock wall under the action of surface tension. This study provides the theoretical support for the formulation of condensate gas reservoir development scheme and the improvement of the recovery factor of condensate gas reservoir.