Gas-Kick Simulation in Oil-Based Drilling Fluids with Nonequilibrium Gas-Dissolution and -Evolution Effects

Summary The nonequilibrium dissolution and evolution characteristics of gas in oil-based drilling fluids (OBDFs) greatly affect the ratio of free gas to dissolved gas in the wellbore, thus influencing the prediction accuracy of the wellbore-pressure and surface responses. Previous equilibrium-state models can result in the incorrect estimation of the multiphase-flow parameters during a gas kick in OBDFs. Therefore, a nonequilibrium gas/liquid two-phase-flow model is developed for simulations of gas kicks in OBDFs. Nonequilibrium gas-kick behaviors in OBDFs are investigated using the proposed model, and it is concluded that there is a unique gas-dissolving stage in comparison to the equilibrium gas-kick conditions. In this stage, the pit gain decreases to a large extent, and this phenomenon can be misinterpreted by the drilling crew as a loss of circulation or a decrease in the gas-kick intensity. The drilling-fluid-outflow rate is not a reliable gas-kick indicator because of the lower increment in the drilling-fluid-outflow rate under both nonequilibrium and equilibrium gas-dissolution conditions. Neglecting the gas-evolution rate in OBDFs could lead to overestimations of the maximum pit gain and the drilling-fluid-outflow rate. More gas moves from the wellbore in the form of dissolved gas under noninstantaneous gas-evolution conditions. The results of this study provide a theoretical basis for the safe and efficient treatment of gas kicks in OBDFs.

Analysis of the Pressure Build Up Taking into Account Nonequilibrium Gas Dissolution

The article discusses practical issues of modeling the reverse dissolution of gas released from oil, taking into account the nonequilibrium phase transition. An approximate method has been developed for taking into account the nonequilibrium gas dissolution process based on standard reservoir flow simulators without the corresponding option. The developed methodology was tested on the example of pressure build up (PBU) taken on a real field. The possibility of determining the characteristic relaxation time from the PBU is demonstrated in the framework of relaxation models for nonequilibrium phase transitions. The effect of nonequilibrium phase transitions on the dynamics of pressure recovery is determined. Well testing design is proposed that makes it possible to detect nonequilibrium gas dissolution and determine the necessary parameters for its consideration in flow simulations.