Inclusion of temperature in the AUSMV scheme with simulation examples from Underbalanced and Mud Cap Drilling

Master's thesis in Petroleum engineeringTransient flow modelling is actively used to understand dynamic flow scenarios. In this thesis a modification and improvement has been performed for a transient flow model called AUSMV (Advective upstream splitting method). To get a understanding behind some of the simulation scenarios, a brief literary study has been performed on the dual gradient drilling concept and the dynamics behind underbalanced drilling The dual gradient drilling concept is built on the manipulation of pressure using two different fluid densities. The hydrostatic pressure curve produced using dual gradient makes it easier to handle low margins between pore and fracture pressure. Drilling can be performed faster, cheaper and give better completion solutions. The downside with the dual gradient concept is the need for rig modification, re-training of personnel and new well control procedures. The AUSMV scheme can be used to solve the transient drift flux model. This transient drift flux model uses a set of conservations and closure laws. The conservations of liquid and gas mass and the conservation of momentum. To close the system, four closure laws are added. Two of these closure laws are for the liquid and gas densities. These two closure laws were changed with new models which incorporate temperature effects. An existing Matlab code of the AUSMV scheme was used as basis for the modifications and simulations. Two different drilling cases were simulated. The first one was a underbalanced drilling scenario with three different configurations. The main purpose of this simulation was to include and test the temperature dependent density and viscosity models for liquid and gas. The simulation results were compared against results produced from the original code. First the density models were implemented and tested. Then the viscosity was modified to include temperature. Simulation showed that the largest effect of including temperature, was related to the density models and their impact on the hydrostatic pressure which was reduced. The second simulation was a controlled mudcap drilling scenario. The objective for this simulation was to control the mud level using a suction pump. A suction pump removed mud from the middle of the well during simulation and the hydrostatic column dropped along with the bottom hole pressure. A steady state was reached when the injection rate of mass was equal to the suction rate. By adjusting the mass rates, the mud column could be adjusted and the hydrostatic bottom hole pressure controlled. Several modification had to be done to the model in order to make this simulation possible. First a sink term had to be implemented in the liquid mass conservation equation. In addition, the floating mud level in the well causes challenges for the outlet boundary condition treatment and several approaches for handling this was investigated. Finally the effect of numerical diffusion on the mud level interface was demonstrated and by refining the grid, a more accurate description of the mud interface was obtained. However this type of refinement had a computational time cost