BAKER HUGHES - Drilling Fluids Reference Manual

RESERVOIR APPLICATION FLUIDS
The objective of the clean out operation is to remove all kinds of debris from the well. An absolute
definition of debris is hard to give, but can generally be described under three categories:
Solids generated during the well construction process typified by:
• barite from settled mud
• cuttings from cement and formation because of poor hole cleaning
• steel shavings from milling operations
• scale and rust from poorly conditioned tubulars
Junk introduced to the well e.g.:
• solids from surface equipment due to poor surface clean-out
• foreign materials from BOP and seal stacks
• cement plugs and float equipment after drill out
• perforation debris
• dropped objects from drill floor
Gunk formed from the fluids used in the well:
• pipe dope
• synthetic mud at low temperature
• gelled oil based mud from mixing with water
The first step in the well clean up process is to clean the casing (and possibly the riser). To help
evaluate the relative effectiveness of methods to clean a wellbore, a short description of the
displacement objective is in order.
The following figures illustrate the likely condition of the wellbore after drilling. (The figure and
the following description, is primarily for open hole, but the description will also be fully applicable
for cased holes). Not only is this section of the hole filled with drilling mud (typically one that has
been conditioned prior to pulling out of the hole with the drill bit), but there is also a bed of drill
solids on the low side of a horizontal wellbore. In addition, there is a static filter cake (dehydrated
fluid) on top of the dynamic cake. This static cake is made up of polymer residue and trapped drill
solids. To remove this material, the drilling fluid must be displaced from the hole with clear brine
or seawater and then, the filter cake must be scoured. Displacement of the drilling fluid is best
accomplished through the use of a viscous “push pill” followed by clear fluid. The purpose of the
viscous push pill is to remove as much mud as possible in a “piston-like” process. To achieve this,
the pill should have a yield point 1 to 2 times higher than the mud being displaced. This is typically
achievable with a 1.5-ppb xanthan gum system. To further ensure a piston-like displacement, the
density should be ~0.1 to 0.2 ppg heavier than the drill-in fluid.
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 6-82