When a well, in "safe" overbalance in a reservoir zone, is left undisturbed for a period of time with oil based mud (OBM), methane will diffuse into the OBM. The rate of methane diffusion is a function of temperature, pressure, the reservoir and OBM characteristics, and the condition of the near well region. "The condition of the near well region" includes the extent of the invasion zone of the drilling mud, the mud cake, and the porosity of the formation. The reservoir type will also influence the amount of the diffusion; a pure gas reservoir will cause higher diffusion than an oil reservoir.

For a given drilling history, OBM description and reservoir description, it is possible, by using a diffusion solver, to compute the amount of methane invading the OBM as a function of time.

In long horizontal reservoirs, this could add up to a large amount of methane. This paper will address the dynamics of circulating the methane contaminated OBM. Although there could be a large amount of methane, the methane is distributed over a large volume of OBM with, in some cases, a small concentration at any given position. The problem that occurs is that the methane is of such low concentration that it is nearly "invisible" until the methane starts boiling out of the OBM in the riser, casing a possible domino effect after the BOP. This could dramatically reduce the bottom hole pressure during circulation.

This paper will discuss the elements of computing methane diffusion rates in a well drilled with OBM, and the dynamics of circulating the contaminated mud out of the well. A dynamic well simulator containing a chemical equation of state solver will be used on an example well to illustrate the possible problems and solutions when circulating OBM with diffused methane. By using proper physics and chemistry simulation tools it is possible to predict and avert dangerous situations.

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