Drilling & Completion Fluids

Drilling mud has a number of very important functions in maintaining the borehole in sound and proper condition. Its most important aspects are:

1. Transportation of drill cutting to the surface
2. Drive, lubrication and cooling of the drilling bit
3. Maintenance of bore hole formation by forming a mud sheath or cake on the drilled hole 
4. Sealing off the bore hole against ingress of oil, gas, water, etc. and alternatively egress of mud to the formation (loss of circulation).
5. Mud pumps should be shut down to maintain in suspension drill cuttings, silt, and e.t.c. (Thixotropy).
6. Supply of information regarding rock formation.

To meet the above purposes, the drilling mud must possess definite properties, and in particular must harmonize with the rock formation being drilled. The characteristics of the mud should change little during use.

Form this it follows that the mud must be so processed that it remains stable in composition and character. Drilling mud can be affected by thermal and chemical conditions, in the latter case where drilling takes place in a salt bearing information. The mud is processed to combat such conditions. However, the primary mud processing consideration is the removal of the drill excess cuttings, salt, etc. which are brought to the surface by the pumped mud stream.

Generally speaking mud can be classified into oil and water based mud, and further into low or normal weight (loaded) mud. As mud contains a multiplicity of additives to give it certain characteristic, e.g. weight, viscosity, thixotropy, loss of water, the cuttings can only be removed by exact mud, the barites (barium sulphate) being the mass of the solids, must be recovered and the mud reused, while with the low weight mud the cuttings represent the solid matter content and as such must be removed.