Despite multitier safeguards in any drilling operation, blowouts occur. When such accidents take place, the total discharge of hydrocarbons becomes the focal point for all concerned, including the operator, the service provider, and the regulatory body. Rate estimation becomes a daunting task with scant information about the formation and fluids at the time of the accident. Given that new regulatory guidelines require such estimates for any offshore drilling, systematic investigation is imperative.
This study presents an analytical model coupling the flow in a reservoir/wellbore system of a gas well. The model considers flow in the tubing, annulus and riser, and the attendant heat transfer in this formulation. To gauge safety concerns, plume dispersion is modeled under various wind speed scenarios when ignition sources were not present. In the event of gas ignition, the energy of the explosion of the flammable gas is estimated with an empirical method in terms of trinitrotoluene (TNT) equivalency. The energy released by the jet fire associated with flammable gas is also evaluated.
Two published field cases provided insight into the model's applicability in actual situations and some perspective on solution quality. However, given the large number of unknowns in any setting, we adopted the statistical design of experiments to understand the impact of independent variables. These variables include formation permeability, connected reservoir pore-volume, and restrictions in flow path, among others. Overall, the model results are in agreement with the previous findings. Specifically, we observed that reservoir permeability controls the flow rate; hence, the cumulative production for a given flow cross-sectional area in the well. In addition, the reservoir pore-volume connected to the well has a large impact on the rate of production decline; therefore, the total discharge volume.