Losses are described as seepage, partial, or total, depending on how much still comes back. Causes include drilling with the equivalent circulating density above the fracture gradient, crossing a naturally fractured zone, or entering a depleted sand whose pore pressure sits below the mud weight.
Total losses drop the fluid level in the annulus. The hydrostatic pressure at a deeper zone can then fall below its pore pressure, and a kick follows the losses. Curing losses with cement or with lost-circulation material has to be weighed against the risk of sticking the pipe or of damaging a zone that was meant to produce.