BAKER HUGHES - Drilling Fluids Reference Manual

Water Based Drilling Fluids
Treatment and Control Techniques
Prior to Cementing
1. Reduce low-gravity solids to a minimum level. This can be accomplished by utilizing
solids control equipment and dilution/displacement with water.
2. Use pump rates sufficient to achieve turbulent flow of the drilling fluid.
3. Pretreatment of the system with a small concentration of sodium bicarbonate or phosphate
This will help in two ways:
• Removes calcium as a precipitate with sodium bicarbonate or by sequestering the
divalent cation with phosphate.
• Reduction of the pH while drilling the cement
Drilling Cement
1. Avoid adding NEW-DRILL prior to, or while drilling cement.
2. If cement is hard, small treatments of sodium bicarbonate may be made to precipitate
calcium, reduce pH, and reduce viscosity.
3. If the cement is “green” or soft, the method of treatment will be the same although greater
increases in rheology may occur. This will especially be true if the solids level has not
been lowered enough before drilling cement.
4. Operate the solids control equipment to remove cement particles.
5. Various acids can be used to control pH more rapidly after drilling cement (Citric, Acetic,
hydrochloric, or phosphoric are the most commonly used.)
Two problems can result from the use of sodium bicarbonate.
1. Overtreatment can initially cause the fluid to thin severely.
2. Overtreatment can cause a carbonate problem which can result in
fluid instability and high gel strengths.
Carbonates
Source
Large accumulations of soluble carbonates or bicarbonates can affect rheological properties in
much the same way as salt or calcium. Carbonates may come from aeration of the fluid, CO 2
from the formation, thermal degradation of organic fluid additives, and over treatment with soda
ash or sodium bicarbonate.
Effects on Fluid Properties
Carbonates adversely affect rheological properties in several ways. First, carbonate ions cause
reactive clays to flocculate, thereby increasing yield point and gel strengths. Secondly, organic
deflocculants do not function properly because of the chemical imbalances created when
carbonates react with the hydroxyl ions in solution. Thirdly, in a NEW-DRILL ® system, there is
some evidence to indicate an excessive concentration of carbonates in the fluid system will cause
the NEW-DRILL ® polymer to lose some of its encapsulating ability. The concentration of
carbonates that will create a problem is greatly dependent on the concentration of reactive solids
in the fluid. In the NEW-DRILL ® system, reactive solids are typically maintained at very low
Reference Manual
Baker Hughes Drilling Fluids
3-58 Revised 2006