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Cement Contamination Drilling Mud Effects & Precautions

Introduction To Cement Contamination Drilling Mud

Usually, Mud Engineers designee drilling mud to be able to carry out its basic functions. Thus, As concentration of Cement contamination increases in a drilling mud system as there will be detrimental effect on its performance. In general, we can define the contaminant as it is any material which causes undesirable changes in drilling fluid rheology and properties which 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, it is not always possible to remove the contaminant from the system. 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 is as a result of various contaminants incorporated in the mud system.

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

How Cement affect Drilling Mud

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

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

flocculating effects of increased calcium content, pH and solids increases Don’t have a big effect on Invert muds. However green cement will reduce base fluid – water ratios and in turn emulsion stability. Whenever possible, drill out cement with seawater or expendable water base mud prior to displacing to invert muds. Off course stuck in green cement is a big problem.

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

Generally, the rheological properties, filtration properties and pH will show a dramatic increase as clay particals 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 were an experiment on a small sample of mud and 50g, 100g &  150g of cement . Then mud tests was done after adding such volumes of cement. Below you can see the effect of the contamination on water base mud rheology .    

Cement Contamination Drilling Mud Experiement
Yield with cement contamination
drilling mud contamination with cement
gel strength with cement

 At high temperature (greater than 250 F) severely contaminated bentonite based muds can solidify

Drilling Cement Precautions

Proper planning and pretreatment will serve to minimize problems associated with high flocculation, plugged flowlines and cement contaminated surface equipment. You can take the following precautions into your consideration if you are planning to drill cement, particularly when there is a risk that the cement is green.

  1. If it is possible, drill out as much of the cement 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 not sufficient to counter the effects of the cement, 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 be used to effectively deflocculate the mud to reduce flocculation from the cement. Close monitoring of Mud returns is the key to adjust further treatments with additional bicarbonate and water.

Conversion to a lime based system

A type of water-base mud that is saturating 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 pH in the range 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 a lime mud. A maltodextrin in lime muds has been used as a clay deflocculant, a shale stabilizer and to increase calcium solubility. KCl in lime muds has been another innovation for 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 drilling cement. 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 the you allow pH to drop, the conditions will become worse and the mud will remain viscous. A lime mud can be checked for a full breakover by adding more lime. If the fluid takes the lime without a viscosity increase, the mud is considered broken over.

Drilling Mud Contamination With Cement Summary

When drilling cement 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 surface. Nothing is certain when preparing to drill cement because of uncontrollable variables such as channeling of cement, varying hardnesses and varying degrees of interface. In many cases, economics will dictate the treatment. It may be more economical to discharge large quantities of contaminated mud and cement than to treat and risk contaminating / recirculating cement through the surface system The key is preparation and planning for the worst case.

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