Water-Oil-Gas Settling Interface Calculation in Shut-In for Undulating Well

Abstract

Abstract Modern directional wellbore trajectories are more complex now with some wells having an undulating trajectory. When installing artificial lift devices such as an electric submersible pump (ESP), the pump must be submerged in the liquid phase of the hydrocarbon in order to avoid gas-lock issues, including in shut-in conditions. Incorrect placement of such device would require a very costly remedy. In an undulating trajectory well, it is possible that the fluid in the well is in the gas phase at some of the downhole upper elbow locations during shut-in. This paper presents a method to calculate the water/oil/gas settling interface during shut-in conditions for an undulating trajectory well. In this method, the well is divided into multiple uphill and downhill sections. For each section, it is assumed that the GOR (Gas Oil Ratio) and WOR (Water Oil Ratio) are the same as the production GOR and WOR. The pressure at the elbow turning point of two uphill and downhill sections is assumed to be the same to ensure the pressure continuity. Then, by the given surface or bottom (reservoir) pressure, the gas/oil and oil/water interfaces are calculated with iterations with a numerical method to match the GOR and WOR considering the wellbore pressure and temperature effect for each section. Once all the sections are calculated, the final results are combined to achieve the global gas/oil and oil/water interfaces and the pressure and temperature profiles. This method has been implemented in an advanced casing and tubing design application to calculate the well pressure and temperature profiles, and then these profile results are applied later for further stress analysis. A typical undulating well is presented for case study to explain the calculation steps and results. As expected, the gas/oil and oil/water interfaces are obtained where the water is settling at the lower elbow of the well, the gas phase is accumulated at the upper elbow of the well, and the oil phase stays in the middle between the gas and water. The pressure profile is achieved showing different pressure gradients of the gas/oil/water settling intervals and pressure is continuous along the whole well. The temperature profile is also obtained, showing different temperature behavior for gas, oil, and water intervals with different shut-in duration, which is useful for stress analysis in the string at different depths. This paper fills a gap that has not yet been discussed in the industry literature. It is an important for field application, especially for choosing the artificial lift device installation depth. Further, the calculated pressure and temperature profiles are useful for casing and tubing design stress analysis purposes.