A method for estimating the drainage depth of oil well in ultra-deep fault-karst reservoirs and its guiding significance to oilfield development

Ultra-deep fault-karst reservoirs are a kind of fault-controlled fracture-cavity carbonate reservoir discovered in the Tarim Basin in recent years. During the production of oil well, both oil and gas under the bottom of well flow to the bottom hole along the high-conductivity flow channels of faults. To characterize the vertical drainage range of oil well in ultra-deep fault-karst reservoirs, this paper proposes the concept of "drainage depth", and establishes a method for estimating the drainage depth of oil wells based on well temperature. "Drainage depth" is defined as the vertical distance from the position of fluid under the bottom hole of oil well starting to flow upwards to the bottom hole during the production of oil well in ultra-deep fault-karst reservoirs. At the vertical drainage position of the oil well, vertical driving pressure gradient is equal to flow resistance, the vertical flow velocity is equal to 0 and the flow temperature is equal to static temperature. Well temperature method can be used to calculate the drainage depth of oil well. The essence of estimating the drainage depth for oil well using well temperature method is to determine "heat source" location based on non-isothermal flow characteristic of ultra-deep fault-karst reservoirs. The results show that the denominator of the governing equation of drainage depth for oil well based on the temperature-depth linear relationship is the difference between the static temperature gradient and the flow temperature gradient. The larger the nozzle size is, the deeper the drainage depth of oil well is and the lower the fluid heat loss is, which both lead to bigger measured temperature difference between flow temperature and static temperature at the same depth. The concept of drainage depth of oil well and its estimation method are of great significance to the understanding of oil column height recognition, reserves calculation and driving way optimization in ultra-deep fault-karst reservoirs. The initial static pressure of ultra-deep fault-karst reservoirs is not "static". The calculation method of reservoir engineering evaluation index based on oil well pressure difference and reservoir pressure drop in ultra-deep fault-karst reservoirs needs to be re-established. © 2021, Editorial Office of ACTA PETROLEI SINICA. All right reserved.