BAKER HUGHES - Drilling Fluids Reference Manual

RESERVOIR APPLICATION FLUIDS
CARBO-CORE
Native State Drilling and Coring Fluids
The CARBO-CORE system is a no-water-added oil-base fluid system used as a native state coring
and drilling fluid. It is used whenever potential invasion of water or oil filtrate with surfactants,
into producing reservoirs must be avoided. The CARBO-CORE system is temperature stable,
even after contamination with water. Should contamination be severe, it can be converted to a
CARBO-FAST or CARBO-TEC ® system.
A drilling fluid that minimizes changes in water/formation wettability and water saturation in a
producing reservoir will result in maximum permeability and oil production. Invasion of water
filtrate into producing reservoirs can alter connate water saturation. Invasion of oil filtrate,
containing strong surfactants, can possibly alter the wettability characteristics of the formation. A
naturally water-wet sand may become oil-wet if the filtrate contains strong surfactants.
The alteration of water saturation and/or wettability characteristics can result in errors in predicting
and evaluating reservoir performance based on core data. In some cases these actions may also
decrease production after the well is completed.
No-water-added oil-base coring fluids have been used previously to retrieve unaltered cores for
laboratory analysis. These cores are used to better define water saturation, connate water salinities,
and oil water contacts. Cores that are recovered are also used to determine reservoir permeability
and porosity as well as for laboratory flow tests to better define the reservoir characteristics. This
technology is now present as the drilling fluid, CARBO-CORE.
Lease crude has been used, and is preferable as a drilling or coring fluid when unaltered cores are
required. However, density requirements, or the lease crude characteristics, may make its use
impracticable.
CARBO-CORE is prepared with minimum surfactants and a near zero water content, using
mineral or diesel oil.
Components
Base Oils
CARBO-CORE fluids may be prepared with lease crude, No. 1 diesel, No. 2 diesel, mineral oil, or
a combination of oils to meet individual requirements. If lease crude is used, it must be free of
emulsion breakers and filming amine corrosion inhibitors, which can alter wettability
characteristics. Lease crude must also have an aniline and flash point above the desired minimum.
High viscosity lease crude may be blended with mineral oil or diesel to the desired viscosity range.
No. 1 diesel oil or mineral oil is more desirable than No. 2 diesel, since No. 2 diesel may contain
additives that may cause formation contamination. Pilot testing is recommended prior to the use of
various oils due to their solubility and viscosity differences.
Water Phase
CARBO-CORE systems are prepared without water. However, 2% to 5% water may be
incorporated during displacement of a water-base fluid. However, if the water content is in excess
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 6-39