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There are a number of ways in which whole drilling fluid or cement slurry can leave the wellbore and give rise to partial returns or lost circulation. For some of these ways, the conditions for lost circulation are simple, for others, less so.

The figures in this chapter illustrate the loss of drilling fluid; similar pictures could be drawn for cement slurry.

If the drilling fluid or cement has poor fluid loss characteristics, the whole fluid, including its particle content, can invade the formation. The rate of invasion will be higher when the formation is very permeable and/or the overbalance is high; for example, shallow unconsolidated beds or depleted reservoirs. Flow into intact formations, as shown in Fig. 3.1, can occur when wellbore pressure is higher than formation pressure, and the pore throats are so large that the fluid cannot form a stable low-permeability filtercake. If it becomes an operational problem, solutions could include reduction of bottomhole pressure, improvement of the fluid loss characteristics of the fluid, or reduction of the pumping rate to reduce erosion of any filtercake.

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