Though this type of drilling fluid is easy to describe, it is hard to define and even more difficult to find. In the field, a normal fluid generally means there is little effort expended to control the range of properties. As such, it is simple to make and control.
General rules include:
1. It is used where no unexpected conditions occur
2. The mud will stabilize, so its properties are in the range required to control hole conditions
3. The chief problem is viscosity control
Formations usually drilled with this type of mud are shales and sands. Since viscosity is the major problem, the amount and condition of the colloidal clay is important. To do this, two general types of treatment are used:
1. Water soluble polyphosphates
- (a) they reduce viscosity
- (b) can be used alone or with tannins
- (c) if filter cake and filtration control is required
- add colloidal clay to system
2. Caustic Soda and Tannins
- (a) they also reduce viscosity
- (b) used under more severe conditions than phosphate treatment
The upper portions of most wells can use “normal” muds
1. Care must be taken not to add chemicals which may hinder the making of special muds later on
2. Native clays used to make the mud are usually adequate
