Water Wet Solids
Water wetting of solids in an oil-based mud begins with the failure of mud to flow through fine shale shaker screens. Continued problems may result from obstruction inside the drill string by water-wet solids buildup, causing high pump pressure and the eventual need to ream out the pipe. At latter stages, water wetting is associated with breaking of the oil mud's emulsion - causing settling of solids and disastrous failure (flipping) of the mud system.
There are several operational indicators and trends from the mud engineer's daily mud checks and a few special tests that can be used to identify water wetting of oil-mud solids before the problem becomes serious and irreversible.
Causes of Water-Wet Solids
Common causes of water wetting of solids can be anticipated or seen if the drilling supervisor and mud engineer are aware of potential problems and are communicating. Some common causes are:
- Influx of water or contamination by water-based mud into the oil mud.
- Insufficient oil-wetting chemical put into the oil mud over a period of time.
- Excessive downhole temperature and high drill solids consuming the oil-wetting chemicals.
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- Drilling highly water soluble salt formations, such as carnalite and bischofite - magnesium chloride salts.
- Adding freshly prepared, poorly treated oil mud into the pits and sending it down hole.
- Excessive additions of solid CaCl2 to the oil mud, exceeding aqueous phase saturation, thus allowing free CaCl2 crystals to exist in the mud.
Visual Indicators of Water-Wet Solids
Early visual indicators of water-wet solids are fairly subtle, but the intermediate and latter indicators are clear cut (such as gross settling and flipping of the mud). Visual indicators are:
- Dull, grainy appearance of the mud, rather than the normally mirror-like surface.
- Sticky, agglomerated solids blinding the shaker screens.
- Pump pressure increasing steadily from solids building up inside the drill string, usually the drill collars.
- Settling of agglomerated solids despite adequate gel strengths. Mud Test Indicators of Water-Wet Solids
Other indications of water-wet solids come from monitoring the mud testing that is routinely performed on oil-based muds. These indicators are:
- Rheology - Erratic flow properties resulting from solids agglomeration affecting Fann VG meter readings.
- Electrical Stability - Steadily decreasing ES voltages with time resulting from water film on solids causing a weakened emulsion.
- HTHP Filtration - High and increasing filtrate volumes with water in the filtrate.
- Mud Chemistry - Changes in salinity and decrease in lime as a result of: (1) drilling salts such as carnalite and bischofite; (2) excess treatments with calcium chloride; or (3) a water influx.
- Retort Analysis - Increasing water content as a result of a water flow or rain or water-mud contamination.
Special Tests and Indicators of Water-Wet Solids
These are special tests that the mud engineer can perform at the rig and observations he can make during testing which can indicate presence of water-wet solids.
- Sand Content Test - High percentage of sand-sized particles (agglomerated solids) measured by the Sand Content Test which screens the mud through a 200-mesh screen. The test for oil mud is performed as is the water-based mud test for Sand Content, except that oil is used to wash the solids on the screen rather than water. To show that these solids are actually water-wet and are agglomerated small particles, wash the oil-mud solids with a mixture of 50:50 xylene:isopropanol. If they disperse into finer size and pass through the 200-mesh screen, then they were agglomerated.
- Coating Test - Water-wet, oil-mud solids will adhere to a highly water-wet surface such as clean glass or clean metal. (That is why solids build up and plug the inside of drill collars.) The Coating Test is performed by filling a round jar about half full of the oil mud to be examined. The mud is stirred at moderate speed with a mixer for 30 minutes. Solids are slung against the glass and if they are water wet some will stick to the jar. The oil mud is poured out and the jar is left to drain. The jar is examined for a film. If no film exists (at the level of the stirrer blade) then there is no indication of water wetting problems. If there is a film, it can be judged by its thickness - the thicker and more opaque the film or coating, the more water wet are the solids. Also, the film will be difficult to wash or wipe off when solids are strongly water wet.
- Settling - When solids settle badly in the testing equipment (mud cup, rheometer cup and mud balance), yet the mud's gel strength seems adequate to suspend barite, that is an indication of solids being water wet and agglomerated.
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