The "mud " is a name used for a combination of chemicals.. It is pumped through, the bit and then up to the surface through the annular space between the walls of the well and the drill-string. The mud lubricates and cools the bit and flushes out the chippings created while drilling and brings them to the surface where they are filtered out of the mud so that it can be used again. The chippings provide valuable information about the formation penetrated. Traces of oil and gas brought up can give the first hint of a discovery. In addition the mud lines the walls of the hole preventing it from caving in. The weight of the mud balances the formation pressure encountered during drilling.
- 39 -
I. Types of drilling fluids: Classification of drilling muds by generic name results in two mud systems, i.e. water-base and oil-base. Water-base muds are used mush more extensively than the oil-base types, as the latter are primarily restricted to special purpose drilling. Water-base muds consist of a large number of separate systems since the :only requisite to fall in this class is the use of water for the base vehicle.
Drilling fluid types in use to-day are classified into the following groups; 1. Fresh Water Muds.
2. Salt Water Muds.
3. Calcium Treated Muds.
4. Oil Emulsion Muds.
5. Sodium Silicate Mud.
Of all these mud systems the fresh water type is the basic, most universally used mud system. This is the natural result of the availability and normally satisfactory functioning of water as the fluid vehicle. The remaining mud systems have been developed to overcome drilling conditions which fresh water muds have difficulty in handling or for which they are totally unfitted. These substitute systems are specially compounded and usually are more costly to build and maintain than fresh water type.
II. Function and properties of drilling fluids.
The properties of the drilling fluid should be such as to promote safe and speedy drilling and completion of the well with the maximum productive capacity.
Cooling and lubricating the bit and drill pipe are absolutely necessary as considerable frictional resistance is encountered by the bit in drilling the formation and by the drill pipe in rotating against the side of the hole. If no fluid were present, the bit would soon be burned and dulled to a useless condition and the drill pipe would be severely abraded. The presence of liquid mud reduce the friction factor of the pipe and bit for the hole and dissipates any heat so generated.
Removing cuttings from the hole is also a basic and vital work of the drilling fluid. This removal process is linked both to the mud properties and to the mud velocity, the mud velocity depending upon the size and speed of operating pump. The pumps contribute but one property, velocity. The rate at which a cutting will settle in a quiescent mud column depends upon its size and shape, the difference in density between the cutting and the mud and the viscosity of the mud.
The wall-building properties of the drilling fluid are often of considerable importance. The thickness of the wall cake and the quantity of fluid lost to the formation may exercise considerable influence over the drilling of the well. If a thick cake is formed over the face of the producing formation it may not become properly cleaned off during the well.
(Drilling Engineering Handbook).
- 40 –
V. Read and remember the following words and expressions: