Many formations are affected by the drilling fluid. The longer the fluid is in contact with the formations, the more they are affected.
Holes are difficult to clean when they:
- are overloaded with cuttings.
- have washouts or cave ins.
- are lined with sticky swelling formations or thick filter cakes.
Rapidly flowing water transports rocks easily.
However, when the flow rate drops, the stream will no longer carry the rocks.
Similarly, water transports cuttings up the hole when the up-hole velocity is high enough. When circulation stops, the cuttings settle quickly. Muds that are viscous or provide buoyancy are used to slow the settling.
Clays and viscosity
Clays and shales cut during drilling become finely divided and hydrated. If the drilling fluid is being recirculated, the colloidal particles formed from the clays and shales will increase the viscosity of the fluid, and this can make a mud.
Some clays, like pottery clays, do not expand when they are placed in water. Others, like bentonite are made up of stacks of platelets. When they get wet (i.e. become hydrated) they expand to many times their original size.
When clay platelets are dispersed through water, the increased solids content increases the viscosity of the fluid. While the mud is flowing, the platelets become lined up and move freely, thus reducing the viscosity. But when flow stops, the platelets become ‘disorganised’ and form a structure that traps the water. This structure will support the cuttings.
Viscosity and colloids
The viscosity of a clay mud can be changed by the addition of chemicals that change the way the clay platelets are arranged.
Adding lime (or cement) to a clay mud will thicken the mud by ‘flocculating’ the clay. The clay platelets then lock together at their edges.
Flocculated muds and muds that have become too viscous because of increased clay content may be thinned by the addition of thinners or dispersants.
Clay muds are described as being ‘versatile’ because of the way their viscosity (and other properties) can be adjusted by the mud engineer.
Drillers who must work without the assistance of a mud engineer often find that the more costly polymer muds work out to be cheaper in the long run.
The organic polymers used in drilling consist of very large ‘long chain’ molecules. It is these long molecules that increase the fluid viscosity when the polymer is mixed with water.
Other important characteristics of polymers include:
- They are 10–20 times more powerful than bentonite when introduced as viscosity forming agents.
- They restrict the hydration of clays so polymer muds require less treatment.
- They provide greater changes in viscosity between their flowing state and their still state. Pumping pressures are low.
- They can be weighted with salt to provide a low solids weighted fluid.
Clean mud depends on good mud properties, which include:
- Protecting the walls of the hole and the cuttings from breaking up and mixing into the mud.
- Rapid transport of cuttings to minimise contact time.
- Efficient separation of cuttings from the mud (i.e. using thin light mud and mud detergent).
Good mud cleaning is accomplished with:
- effective settling pits.
- clearance of cuttings and settled sand/silt.
- using desanders where necessary.
Factors that inhibit mud cleaning include:
- High viscosity.
- Using dispersion additives to break up shales and clays.
- Failure to monitor the sander content or density of the mud.
- Attempting to clean the pit while mud is circulating prevents effective settling.