[Pie]

Originally Posted by Griff

I wonder what "heavy mud" is?

Drilling mud is usually a clay and water mixture. A common drilling mud is made of bentonite clay and is called gel. A heavier drilling mud can be made by adding barite (BaSO,). Various chemicals are also used in different situations. The drilling mud liquid is usually water (freshwater based or salt-water-based) but is sometimes oil-based. Drilling muds are described by their weight. Water weighs 8.3 pounds per gallon. Average bentonite drilling mud weighs from 9 to 10 pounds per gallon. Heavy drilling mud weighs from 15 to 20 pounds per gallon. The heavier the drilling mud, the greater the pressure it exerts on the bottom of the well.

Circulating drilling mud serves several purposes. The mud removes cuttings from the bottom of the well. As the mud flows across the bit, it cleans cuttings from the teeth. The drilling mud cools the bit from heat generated by the friction of drilling. In very soft sediments, such as in a coastal plain, the jetting action of the drilling mud squirting out of the bit on the bottom of the well helps cut the well. The drilling mud also controls pressures in the well and prevents blowouts. At the bottom of the well, there are two fluid pressures. Pressure on fluids in the rock tries to cause the fluids to flow into the well. Pressure exerted by the weight of the drilling mud tries to force the drilling mud into the surrounding rocks. If the pressure on the fluid in the subsurface rock is greater than the pressure of the drilling mud, the water, gas, or oil will flow out of the rock into the well. This often causes the sides of the well to cave or stuff in, trapping the equipment. In extreme cases, it causes a blowout. In order to control subsurface fluid pressure, the weight of the drilling mud is adjusted to exert a greater pressure on the bottom of the well. This is called overbalance, and the drilling mud is then forced into the surrounding rocks. The rocks act as a filter, and the solid mud particles cake to the sides of the well as the fluids enter the rock. This filter or mud cake is very hard. Once the filter cake has formed, the sides of the well are stabilized and subsurface fluids cannot enter the well.

From here. Interesting stuff!