FUNCTIONS OF A DRILLING FLUIDDrilling engineers know that muds have many functions in the drilling operations. At any one time in the operation, one function may be more important than the other functions for that drilling interval, which is why a mud program is essential in well planning. Some publications may list ten to fifteen different functions of a drilling fluid. Many of these are variations of the same function. Filling a drill hole with a gas or fluid will inevitably generate a hydrostatic head or pressure as illustrated in Figure 1. This is calculated by the use of the following equation:
Hydrostatic Head or Mud Pressure (Pm) =
(Conversion constant) ‘ (Mud weight or density) ‘ (True vertical depth)
The concept of specifically controlling the density to regulate the downhole pressure was introduced in 1921. Barite was then used to increase the density. Downhole pressure needs to be controlled for two reasons.• The drilled rock must be supported and stabilized.
• The pressure of gases and fluids in the rock must be exceeded so they do not enter the wellbore.
The second reason is particularly important for safety. As the mud density supports the rock, excessive downhole pressure can also damage it by “fracturing” it in the manner that a hose pipe can be split by too high a pressure.
A key to a successful operation is the knowledge of the formation stresses, formation strength, and pore pressures, so that the correct mud weight and casing depths can be selected. Hopefully, the casing depths will isolate problem areas.
Excerpt from “Introduction and Basic Concepts of (Drilling) – Part1” by James William