BAKER HUGHES - Drilling Fluids Reference Manual

RESERVOIR APPLICATION FLUIDS
solutions and high temperatures and pressures. They are ubiquitous environmental organisms that
are usually introduced through untreated drilling, completion and workover fluids.
Formation Protection
To ensure the best possible prospect for reducing or eliminating formation damage during drilling,
completion and workover activity, a compilation should be made of all the information available on
the well and the producing formation. Geological information may be obtained from logs,
production formation cores, and mineralogy (with good samples, independent mineralogical
analysis is possible). A well history, including depth, bottomhole temperature, type of drilling fluid
and/or the nature of any fluids previously in contact with the producing formation, and composition
of produced fluids, especially gases, will help immeasurably with planning to prevent formation
damage. If practical, a laboratory evaluation should be made of a formation core to determine
permeability and fluid compatibility and to ascertain potential detrimental interactions. When all
the examinations and analyses are complete, the optimum fluid(s), additives, and procedures
designed to minimize formation damage can be recommended.
Even without the extensive analysis recommended, it is generally prudent to follow some generally
accepted practices to prevent formation damage during drilling, completion and workover. First,
filtered brine usually reduces the chance of both introducing foreign particles into the formation
and causing water sensitive clays to swell. Second, use of an empirically selected surfactant helps
reduce the chance of emulsion formation, wettability change, and even fines migration. Third,
selection of a chemically compatible drill-in/completion/workover fluid reduces the probability of
insoluble salt precipitation.
Of all the possible mechanisms to prevent formation damage during drilling, completion or
workover, the most significant is the prevention of fluid loss. This sometimes is accomplished by
increasing the viscosity of the working fluid. Because flow rate in porous media is inversely
proportional to viscosity, increasing the viscosity of the working fluid at bottomhole conditions can
significantly reduce the loss of fluid to the formation. The most common practice from preventing
loss of fluid to the formation is to use a combination of increased viscosity and salt or calcium
carbonate based particles to bridge the pore spaces on the formation face. Both these materials are
acid soluble and therefore can be removed by acid treatment if they are not otherwise removed by
washing or production of the well.
In a study designed to find and use the best practice for preventing formation damage during
workover operations in Prudhoe Bay, Dyke and Crockett reached the following conclusions:
1. The primary cause of formation damage during workovers was the invasion of fine solids
and debris from downhole operations, such as near perforation, milling and scraping.
Stopping fluid losses prevents the entry of these fines into the formation.
2. Loss control material (LCM) pills, when properly formulated, usually clean up easily from
perforations and even more readily from open hole formation face, leaving no formation
damage. Badly formulated pills cause severe damage.
3. The loss of clean, i.e. filtered fluids during completion/workover operations has little effect
on future productivity when the formation has relatively high permeability.
4. Tight formations that inherently take small amounts of fluid are most likely to be damaged
by the loss of fluid. Fluid loss is therefore more important in these wells.
5. In non-fractured wells, viscous pills are capable of reducing, but not stopping, fluid loss.
To be effective in stopping fluid loss, graded particulates must be added to the viscous pill
to seal the formation.
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 6-20