There has quite recently been some discussion regarding the accuracy of gas kick slippage models used in transient flow models. This is not necessarily easy to give a correct answer on. However if pressure build up gradients for a migrating kick in a closed well is used for predicting the kick migration velocity, one can easily obtain a misinterpretation due to the fact that parts of the kick can become suspended in the mud. This was discussed in a paper presented two decades ago (Johnson et al., 1995). The purpose of the paper presented here is to show what kind of uncertainties that are involved in the transient flow modelling process and how that affects the pressure build up gradients.

In this paper, a transient numerical model that can be used for simulating two-phase flow will be presented. It is based on the Drift flux model and the one dimensional advection upstream splitting method hybrid scheme (AUSMV). When using transient models for predicting the kick migration time, there are two sources of errors. The first is obvious in the sense that the gas slip parameters are uncertain. However, a less known potential source of error can be numerical diffusion and discretization errors caused by the numerical solver itself. The paper will show how the AUSMV scheme can be made less diffusive by using the slopelimiter concept.

The paper will demonstrate that the effect of numerical diffusion can be quite substantial. This will be done by simulating pressure build up in a closed well varying both the slip parameters but also the degree of numerical diffusion. In addition, the simulated effect on the pressure build up when gas get trapped in the mud for lower gas concentrations will also be demonstrated.

This paper aims at showing that the uncertainty caused by numerical diffusion can be of the same order as uncertainty in the slip parameters. It will also confirm the results reported in Johnson et al. (1995) by the use of a transient low diffusion flow model.

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