Oil Drilling Starch Starch
is used for reducing fluid loss in a variety of water based drilling fluids
and has beneficial secondary effects on mud rheology. In drilling wells,
a liquid (mud) is pumped into the hole to clean and cool the drill bit and
to flush to the surface the drill bit cuttings and suspending the drill cuttings while drilling is paused. The most important physical
characteristics of the drilling fluid is the viscosity and the water holding/retaining
characteristics.
[<< Danish oil rig in the Halfdan field, Danish sector, North Sea. Denmark is self sufficient in oil from the North Sea.]
Starch is traditionally supplied as cold water soluble - either made by a semi moist/dry thermo-mechanical gelatinisation on screw extruders or by suspension reactions and subsequent drying on drum dryers. Wet reactions carried out in starch suspensions are easy to control and the resulting precisely engineered starches are of high, uniform quality with wider applications. Potato starch retain fluids better than other starches due to the large size of its molecules.
The quality standards are laid down in "Drilling fluid materials – Specifications and tests (ISO 13500)". API Spec 13A is a modified adoption of ISO 13500 and includes the addition of an API Monogram Annex. These international standards covers physical properties and test procedures for materials manufactured for use in oil-and gas-well drilling fluids such as bentonite and other inorganic additives as well as organic thickeners as carboxymethylcellulose, xanthan gum, guar gum, and starch.
Drilling fluids represent a harsh environment for organic constituents. Their polymer nature tends to break down rapidly by shear, heat and microbial attack. Protection against the physical exposure - shear and heat - is obtained by cross-linking the starch. Cross links add internal strength to the starch molecule. Very heat stable drilling starch are made that way and will keep fluid loss in check at working temperatures up to 125°C - 150°C.
Cross-linking adds physical stability and resistance to the starch, but the functional properties are made by further modification. Pre-gelatinized cross linked carboxymethyl starch ether has all the required features. It is pre-gelatinized meaning it is cold water soluble. It is cross-linked meaning it is resistant to physical stress in the oil well and it is further modified as carboxy methyl ether preventing bacterial break down in areas. It synergises with xanthan gum giving fine low shear rheology and enables lower gum quantities. It may also replace cellulosic polymers.
Protection against microbial break down of the starch in neutral and non-saline fluids is obtained by the addition of biocides like tetrahydro-3,5-dimethyl-2H-1,3,5-thiadiazine-2-thione (Dazomet). Where use of biocides is a concern starch is made resistant to microbial attack by chemical modification. Microbes break down starch by excreting amylolytic enzymes hydrolyzing the starch molecules bit by bit to starch sugars. Alfa-amylase needs starch chains minimum five glucose units long to align and attach to the starch molecule and do the cutting job. An ether group is a hindrance for proper alignment of the enzyme and highly substituted carboxymethyl starches are resistant to amylolytic degradation. It is slightly anionic and can be affected by hardness in the mud. Also highly substituted hydroxypropyl and hydroxyethyl starch are resistant to microbial attack and being nonionic, they are only slightly affected by salinity and hardness. Even highly fermentation stable all starches will, however, completely biodegrade after an extended time period.
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