Development of a Computational Tool for Worst-Case Discharge Rate
Authors: Elgaddafi Rida , Kiran Raj , Ahmed Ramadan , Salehi Saeed , Griffith Craig 
Published: October 2020

After a blowout incident, the wellhead conditions (hydrocarbon flow rate and pressure) are difficult to predict. The worst-case discharge during an offshore blowout has always been a concern for the oil and gas industry. When a blowout occurs, the two-phase flow establishes various flow patterns along the wellbore depending on several factors such as wellbore and reservoir conditions as well as flowing fluid properties. The multiphase (gas-liquid) fluid flows upward through the wellbore at an extremely high velocity that may reach a sonic condition at the wellhead. One of the challenges for the worst-case discharge rate calculation is predicting the pressure drop and velocity of the influx in subsonic and sonic conditions. This paper presents a novel computational tool which is developed to characterize the well control events by simulating the possible worst-case discharge (WCD) scenarios under field conditions.

The exactness of a WCD prediction tool is strongly related to the accuracy of models used to analyze the two-phase flow and the formation of sonic flow condition. The WCD tool developed in this study utilizes the nodal analysis technique to precisely predict the wellhead conditions post blowout incident. The tool has two main parts: i) reservoir portion, and ii) wellbore portion. The tool integrates various reservoir, PVT and production models to estimate the deliverability of a reservoir. Different forms of hydrocarbon production (oil, dry gas, and condensate) from multiple reservoir layers are considered in tool development. To forecast pressure loss along the wellbore, well-established models have been selected to make predictions for different flow patterns (bubble, low-velocity slug, high-velocity slug, and annular flows). In addition, various two-phase flow models have been modified and adapted to suit the Worst-Case Discharge (WCD) predictions. Furthermore, based on experimental observations, a new flow pattern map has been established and implemented in the model.

One of the findings of this study is that the WCD rate is not only dependent on conditions of the wellbore section but it is also influenced by the fluid properties and reservoir characteristics. Therefore, the tool accounts for reservoir type, the number of producing layers, formation type (consolidated and unconsolidated), fluid types (oil, gas water, and gas condensate), and the pay zone thickness. In addition, the tool allows the user to specify fluid and reservoir properties for each layer including permeability, water, oil and gas saturation, API gravity of oil, salt concentration, bubble point and reservoir pressure, irreducible water saturation, critical oil and gas saturation, and gas specific gravity. With respect to the wellbore section, the tool provides flexible options for the user to design the desired wellbore configuration. These options comprise of the depths of cased and open-hole sections, casing and hole diameter, the roughness of casing and open-hole section and the inclination angle of the wellbore. The tool provides good WCD rate predictions for wells with an inclination up to 45° from the vertical.

This article presents a novel WCD tool developed based on modified mechanistic two-phase flow models, which are validated for wide ranges of gas and liquid superficial velocities. As outputs, the tool predicts the wellhead parameters post blowout incident such as WCD rate, gas and water rates, the occurrence of the sonic condition and surface pressure. In addition, it provides an inflow performance relationship (IPR curves) for each reservoir layer. Predicting the wellhead pressure at the moment of the blowout helps to control the well by determining the density of the killing fluid.