BAKER HUGHES - Drilling Fluids Reference Manual

Baker Hughes Drilling Fluids
reverse mass action effects, i.e., sodium will displace calcium from clays if a sufficient
concentration is present. Therefore, Base Exchange depends on the type and concentration of all
cations present as well as the surrounding environment.
High concentrations of calcium are added to a drilling fluid in an effort to convert sodium clays to
calcium clays and also to create a calcium environment for incorporated cuttings to minimize
their hydrated volume through Base Exchange. Consequently, the relative viscosity effect will be
less. This method of controlling the hydrated volume of formation clays is often called
inhibition.
There are currently two main types of calcium treated fluids available.
Lime, Ca(OH) 2
Gypsum, CaSO 4 • 2H 2 0
Lime Muds
Lime fluids have been used since the early 1940s and were the first of the calcium-base fluids.
Primary application of lime fluids has been in areas where inhibition of reactive clays was desired
and to improve the fluid systems' tolerance for salt, anhydrite, and drilled solids. Due to their
tendency to solidify at elevated temperatures, particularly in fluids with high solids
concentrations, these fluids are seldom used in wells having downhole temperatures in excess of
275° to 300°F.
Classification of Lime Muds
1. High-Lime Fluid
• 5 to 10 ml Pf
• 5 to 15 lbm/bbl excess lime
• 80 to 120 mg/L filtrate calcium
2. Low-Lime Fluid
• 0.8 to 2.0 ml P f
• 1.0 to 2.5 lbm/bbl excess lime
• 75 to 200 mg/L filtrate calcium
3. Modified-Lime Fluid
• 2 to5 ml P f
• 2 to 5 lbm/bbl excess lime
• 80 to 200 mg/L filtrate calcium
Preparation of Lime Fluids
1. The degree of viscosity change during and after conversion to a lime fluid depends upon
the total solids concentration of the fluid system. Therefore, dilution with water is
necessary prior to conversion. Fine screen shakers and centrifuges are recommended for
weighted fluids, and shakers and hydrocyclones (desilters and desanders) are desirable for
low-weight fluids.
Baker Hughes Drilling Fluids
Reference Manual
Revised 2006 3-39