Managed pressure drilling (MPD) techniques such as; constant bottomhole pressure (CBHP) and pressurized mud cap drilling (PMCD) have made it possible to drill challenging prospects that with conventional techniques are considered undrillable. Drilling in naturally fractured carbonates, highly permeable unconsolidated sands and/or cavernous/vugular formation, often leads to loss of circulation. The mud losses in these kinds of formations are however not normally caused by hydraulically induced fracturing due to excessive pressure. When these extreme loss zones are intersected, the more heavy drilling mud will fall to the bottom and exchange or swop places with the less dense formation fluids.

In some areas like the Gulf of Mexico, deep water outside Brazil and in the arctic climate of the Barents Sea in northern Norway, we can see these kinds of challenging formations in combination with deep and cold seawater. The drilling riser will act as a giant heat exchanger that will cool down the mud that is circulated. The objectives and scope of this paper have been to see what happens to the total active volume if relative cold mud swops place and starts to mix with a relative warm hydrocarbon gas deep down in the loss zone and/or in the wellbore annulus.

PVT simulations and a study found that a net volume reduction will take place when the fluids are mixed, due to higher compressibility and lower specific heat capacity of the relative warm hydrocarbon gas compared to the mud. Equally important, the study also showed that there can be a risk of gas hydrates forming. If hydrates form it will result in an additional reduction of the volume. These volume reductions can be observed as loss of circulation or abnormal reduction in the active pit volume. Hydrocarbon gas influx caused by gravity-induced swop-out will be partly or completely concealed by the volume reduction taking place at the same time. Kick detection based on volume control (flow-in vs. flow-out) may therefore not work when drilling in naturally fractured or highly permeable formations, where influx caused by gravity-induced swop-out takes place.

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