Cement Contamination In Drilling Mud

Usually, Mud Engineers designee drilling mud to be able to carry out its basic functions. Thus, As the concentration of Cement contamination increases in a drilling mud system as there will be a detrimental effect on its performance. In general, we can define the contaminant as any material that causes undesirable changes in drilling fluid rheology and properties that will appear in mud lab tests.

Solids are by far the most prevalent contaminants. Excessive solids, whether commercial or from the formation, lead to high rheological properties and slow the drilling rate. While there are specific treatments for each contaminant, removing the contaminant from the system is not always possible. Pretreatment can be advantageous as long as it is not excessive and does not adversely affect mud properties. The instability in the drilling mud properties results from various contaminants in the mud system.

Generally, good drilling mud is simple and contains a minimum number of additives. This allows easier maintenance and control of properties even at elevated temperatures and pressure. Planning each change in the mud of the time is important.

How Does Cement affect Drilling Mud?

From a mud viewpoint, drilling cement introduces two main contaminants. The major contaminant is

• a calcium ion
• the second contaminant which compounds the problem is hydroxide ions.

Flocculating effects of increased calcium content, pH, and solids increase Don’t have a significant effect on Invert muds. However, grCementment will reduce base fluid-water ratios and, in turn, emulsion stability. Whenever possible, drill Cementment with seawater or expendable water base mud before displacing it to invert mud. Off course stuck in resentment is a big problem.

Water Based Mud, however, can experience severe complications if we are not considering precautions. Freshwater systems with high bentonite content or EZ-MUD systems are the most sensitive to cement contamination.

Generally, the rheological properties, filtration properties, and pH will dramatically increase as clay particles and polymers are flocculated by the calcium in combination with the high pH. EZ-MUD systems will liberate NH3 as the PHPA breaks down.   

Experiment 

There was an experiment on a small sample of mud and 50g, 100g &  150gCementent. Then mud tests were done after adding such volumesCementment. Below you can see the effect of the contamination on water base mud rheology.    

 At high temperatures (greater than 250 F), severely contaminated bentonite-based muds can solidify.

Drilling Cement Precautions

Proper planning and pretreatment will minimize problems associated with high flocculation, plugged flowlines, and cement-contaminated surface equipment. You can consider the following precautions if you are planning to drCementment, mainly when there is a risk that Cementment is green.

1. If possible, drill out as much of the Cementment with seawater if a ready supply is available.
2. Pretreat water base muds with sodium bicarbonate 0.25-0.50 lb/bbl (0.70 – .50 kg/m3).
3. Closely monitor mud returns at the shale shakers and immediately dump any green cement or badly contaminated Water Based Mud.
4. If you are planning to drill large cement sections and treatments are insufficient to counter the effects of Cementment, convert to a lime-based system that tolerates high cement levels such as POLYNOX.
    

Normal Treatments For Contaminated Drilling Fluids

Under normal conditions, if you displace Cement with a spacer and treated mud, the quantity of Cement to be drilled will be manageable. A pretreatment with bicarbonate at 0.75 ppb (2.1 kg/m3) for 20” casing or 0.25-0.5 lb/bbl (0.70 – 1.50 kg/m3) for 13” or 9” casing will be sufficient. SAPP in very low concentration can also deflocculate the mud to reduce flocculation from Cementment. Close monitoring of Mud returns is the key to adjusting further treatments with additional bicarbonate and water.

Conversion to a lime-based system

A type of water-base mud that is saturated with lime Ca(OH)₂, and has excess lime solids (not dissolve) in reserve. Lime muds are classified according to excess lime content: 

1. low-lime, 0.5 to 2 LBM/bbl, 
2. medium-lime, 2 to 4 LBM/bbl
3. High-lime, over 4 LBM/bbl.

All lime muds have a pH of 12, and the filtrate is saturated with lime. Fluid-loss additives include starch, HP-starch, carboxymethylcellulose (CMC) or polyanionic cellulose (PAC).   

Prehydrated bentonite can improve the fluid loss and rheology of lime mud. Maltodextrin in lime muds has been used as a clay deflocculant and shale stabilizer to increase calcium solubility. KCl in lime muds has been another innovation for the successful drilling of hydratable shales. The ability to carry very high mud alkalinity (as excess lime) to neutralize acid gases is one reason lime muds are used. H₂S zones can be drilled with more safety and copious amounts of CO₂ can be neutralized by a large excess of lime.  

How To convert to Lime based system

Should it be necessary to convert to a lime-based system, the conversion can be carried out while drillCementment. The first step is to reduce the solids and MBT below 17.5 lb/bbl (50 kg/m3) with heavy dilution, followed by a treatment of 2 ppb (5.7 kg/m3) caustic soda and 3 ppb (8.55 kg/m3) CARBONOX. The increased pH of the filtrate will suppress calcium solubility and retard the solidification of the fluid. During the breakover to a lime system, it is possible to experience a “viscosity hump”. Lime and caustic additions must be made to continue going over the hump. If you allow pH to drop, the conditions will worsen, and the mud will remain viscous. A lime mud can be checked for a full breakover by adding more lime. The mud is considered broken over if the fluid takes the lime without a viscosity increase.

Drilling Mud Contamination With Cement Summary

When drillCementment with or without WBM, it is critical to be fully aware of the potential problems. It is prudent to pretreat and prepare for the worst conditions than attempt to treat after the problem comes to the surface. Nothing is certain when preparing for drCementment because of uncontrollable variables such as channelingCementment, varying hardnesses and degrees of the interface. In many cases, economics will dictate the treatment. It may be more economical to discharge large quantities of contaminated mud Cementment than to treat and risk contaminating / recirculatCementment through the surface system. The key is preparation and planning for the worst case.