Bit balling usually occur while drilling shale. Clay adhesion is a function of the electrochemical attraction of clay to clay solids and clay to metal (surface tension). The reaction begins when clay solids become wet, and hydration/dispersion of the clay occurs. Adhesion magnitude is determined by the degree of clay hydration, the chemical properties of the clay, the chemical composition of the mud’s aqueous phase, and the proximity between reactive solids or the solids concentration. Massive concentrations of reactive solids can overwhelm most mud systems.
Undesirable Effect Of Balling Problem:
Because cuttings stick to the spaces between bit teeth, the penetration depth is reduced. Penetration Rate will not respond to rotary RPM increases or Weight On Bit (WOB); this may result in pulling a Drilling Bit before it is due to be replaced.
The nozzles are partially blocked; thus, flushing-fluid flow is reduced around the bit.
Individual cones may stop rotating during drilling with roller-cone bits, leading to excessive shear and bit-tooth wear.
Bit Balling Causes:
Balling can occur with any hydra-table clay. Clays particles can adhere to each other or metal surfaces, given the proper water and solids ratio. Therefore, reducing adhesion and/or balling can be achieved by controlling hydration and/or solids concentration. Bit balling is more of a problem when using Water Based Mud. When inverted emulsions are used, bit or bottom hole assembly (BHA) balling normally does not occur.
For bit and or Bottom Hole Assembly BHA balling to take place, two or more of these conditions must exist:
- A reactive clay formation must be present.
- Water must be available for the clays to become hydrated.
- Cuttings are compressed – causing adhesion.
- Sufficient concentrations of electrochemically attractive clays.
- Inadequate bit cleaning due to poor hydraulics.
- The electrochemical attraction of clay to metal.
Bit Balling Problem Prevention Procedures:
It is important to limit the concentration of cuttings in the annulus. When large volumes of dispersible solids or cuttings are generated into a specific volume of drilling mud, an infinite amount of surface area is created. If these cuttings are not quickly removed from the area of the bit, the electrochemical attraction of the clays for metal will cause these cuttings to adhere to the bit.
Control ROP vs. Flow Rate:
High concentrations of mud solids and drilled solids lead to bit balling. This is a function of mud composition and Penetration Rate vs. flow rate. Excessive penetration rates relative to flow rates can create a massive concentration of reactive solids in the annulus. Therefore, when drilling “clay-type” formations, the low gravity solids concentration in the mud should be maintained as low as possible (5% by volume or less). In addition, the cuttings’ concentration in the annulus should be limited to 4% by volume by coordinating the flow rate and ROP.