Bit Balling | Definition, Causes, Prevention, Treatment

 BIT BALLING 

when clay and shale rocks are being drilled they may absorbe the water and may swell and get deposited at bit nozels or around the bit. this may lead to reduction in rop and rotation of bit  this phenomena known as bit balling. 

About 60% of oil well footage is drilled in clays and shales. These rocks are neither hard nor abrasive and thus should be easy to drill. Problems arise, however, because of the soft and sticky nature of the cuttings produced. These cuttings absorb water from the drilling fluid, swell, and stick to the Drilling Bit.

 Balling occurs when clay based drilled solids adhere together and cling to the metal surfaces of the bit and pipe 

• Bit balling usually occurs 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, 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.

Effect Of Balling Problem

• 1. 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.
  2. The nozzles are partially blocked; thus, flushing-fluid flow is reduced around the bit.
  3. During drilling with roller-cone bits, individual cones may stop rotating, leading to excessive shear and bit-tooth wear. 

Causes

Balling can occur with any hydra-table clay. Clays particles can adhere to each other or metal surfaces, given the right water and solids ratio. Therefore, reduction of 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 invert emulsions used, bare it 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.
• 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. The following procedures can aid in cuttings removal.

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. This may require controlling instantaneous rates of penetration.

Bit Type and Hydraulics

Fluid dynamics such as velocity and turbulence are critical for cleaning the drilling bit and preventing balling. Create high velocity and a high degree of turbulence. Flow rates alone are not the key. Fluid viscosity and/or turbulence at the bit are functions of fluid composition and velocity. Solids surface area is the limiting factor for a drilling fluid to shear thin. Therefore, optimizing solids concentration is critical for effective fluid dynamics at the bit. Hydraulic horsepower at the bit must be optimized. Bit design can contribute to bit balling. Anti-Balling (AB) coated bits are recommended.

Hole Wiping

Frequent short trips (check also tripping pipe procedures) in directional wells are very beneficial for reducing the buildup of cuttings beds. The cuttings bed is disturbed by the bit so it can be removed by annular flow, after circulation is resumed. This technique will also help reduce Packoff and gumbo attacks.

Balling Reduction by Mud Composition

Solids adhesion can be reduced by neutralizing the attractive charges on clays by ionic satisfaction, i.e., sodium, calcium, potassium, cationic and anionic polymers, and surface active agents (surfactants). Balling severity is reduced by limiting the “specific surface area” of reactive solids within the fluid. This process is partially accomplished by preventing hydration and dispersion of drilled solids with inhibitive drilling fluids. Among the basic fluids for consideration are those that contain chloride, calcium, potassium, cationic additives, surfactants, oil, esters, formates, silicates, glycols, and the multiple combinations of these basic ingredients. Effective mud systems include:

• INVERMUL
• PETROFREE
• CAT-I
• EZ-MUD
• CLAYSEAL
• BARASILC
• GEM
• POLYNOX

PH control is an important consideration since the hydroxyl ion is dispersive. First, hydroxyl ions promote hydrogen bonding of water molecules to the steel surfaces. Second when the hydroxyl ion is hydrated, its large volume of associated water forces clay platelets and layers apart. This dispersive action increases as the pH is increased. PH ranges should be adjusted to coincide with the inhibitive nature of the mud system being used.

Minimizing the clay concentration by solids removal equipment and dilution of reactive solids also reduces the “specific surface area” available for adhesion and balling. Commercial bentonite can aggravate the problem, it should be added very cautiously. When balling is a potential problem, low gravity solids should be maintained at 5% or less by volume and the equivalent bentonite concentration should be 20 lbs/bbl (57 kg/m3) or less, determined by the methylene blue test. Encapsulate cuttings with EZ MUD to prevent dispersion and mechanical degradation. Coating solids with EZ MUD will have two beneficial effects. It binds a solid to prevent dispersion and, it provides lubricious film that allows solids to slide past one another thus preventing mechanical disintegration. Adding DRILL-N-SLIDE will reduce electrochemical attraction of clay to metal.