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 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. 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.
Sweeps
Depending on hole deviation, high-viscosity and low-viscosity sweeps can effectively remove cuttings from the wellbore. The turbulence of the low-viscosity sweep stirs the cuttings bed, and the high-viscosity fluid carries the solids to the surface. Use BARAZAN PLUS and N-VIS (instead of commercial bentonite) to increase viscosity and avoid increasing the clay content of the mud system for better hole cleaning.
Bit Type and Hydraulics
Fluid dynamics such as velocity and turbulence are critical for cleaning the drilling bit and preventing balling. Create high speed and a high degree of turbulence. Flow rates alone are not the key. Fluid viscosity and turbulence at the bit are functions of liquid composition and velocity. Solids surface area is the limiting factor for 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 Pack off 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 liquids for consideration are those that contain chloride, calcium, potassium, cationic additives, surfactants, oil, esters, formates, silicates, glycols, and multiple combinations of these essential ingredients. Effective mud systems include:
- INVERMUL
- PETROFREE
- CAT-I
- EZ-MUD
- CLAYSEAL
- BARASILC
- GEM
- POLYNOX
PH control is an essential consideration since the hydroxyl ion is dispersive. First, hydroxyl ions promote the 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 are increased. PH ranges should be adjusted to coincide with the mud system’s inhibitive nature.
Minimizing the clay concentration by solids removal equipment and diluting reactive solids also reduces the “specific surface area” available for adhesion and balling. Commercial bentonite can aggravate the problem, so it should be added cautiously. Low-gravity solids should be maintained at 5% or less by volume when balling is a potential problem. 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 avoid dispersion and provides a lubricious film that allows solids to slide past one another, thus preventing mechanical disintegration. Adding DRILL-N-SLIDE will reduce the electrochemical attraction of clay to metal.
Treatments Associated with Cleaning Balled Bits and Assemblies
These pills can be spotted or circulated through the drilling bit and annulus to help eliminate balling problems. Hydrostatic pressures must be maintained when utilizing these pills. The appropriate pill will depend on the mud type used, materials available on the rig, formation sensitivity, and safety/environmental concerns.
Caustic Pill
A caustic pill can be spotted or circulated through the bit. Caustic can be mixed in freshwater or seawater to accelerate the hydration and dispersion of reactive clay. When pumping water, more significant turbulence and a jetting action is formed in the balled area. CONDET Pill (Detergent) This pill comprises whole (active) mud with 3 – 20% CONDET. This also can be done with fresh water and circulated through the bit. CONDET performs by reducing surface tension, increasing lubricity, and reducing the sticking tendency of the clay. If using whole mud, mud weights can be maintained.
Note: Detergents may affect several aspects of a drilling fluid system, i.e., foaming and environmental concerns.
WALL-NUT Pill
This pill is made up of whole (active) mud. WALL-NUT comes in three available sizes; fine, medium, and coarse. WALL-NUT can be mixed from 5 to 60 lbs/bbl (14 to 171 kg/m3) depending on the mud type and weight. This pill is pumped down and through the bit with high pump, rates to physically erode the ball of clay adhering to the bit or drill string.
SAPP or Q-BROXIN Pill (Dispersant)
A highly concentrated dispersive pill can be mixed in water or whole mud. This pill is designed to disperse balled-up bits and assemblies. High pH ranges can also aid in dispersing clays. Q-BROXIN may be used up to 20 lbs/bbl (57 kg/m3). SAPP may be added from 1 to 3 lbs/bbl (2.85 to 8.5 kg/m3). Do not use SAPP in high Calcium environments.
Note: These pills are highly dispersive and can cause wellbore washout. Surfactant Pill
Highly concentrated blends of surface active agents can be added directly to the suction pit, dumped down the drill pipe on connections, or sprayed directly on the bottom hole assembly. These blends will lower the surface tension of the water and help neutralize the surface charges of the clays, minimizing hydratable clay adhesiveness. EZ-MUD/CLAYSEAL Slugging the pipe on connections with neat EZ-MUD or CLAYSEAL