Anhydrite/Gypsum Drilling Problems
Introduction
Causes of Anhydrite/Gypsum Contamination
Preventing and Curing Anhydrite/Gypsum Contamination
Na2CO3 + CaSO4 = CaCO3 + Na2SO4 (Precipitate)
The calcium is precipitated as insoluble calcium carbonate (limestone). A general rule is to add 0.02 lb/bbl of soda ash for every epm of hardness. After adding the soda ash, a thinner is usually added to reduce the viscosity and gel strength. A difficulty is encountered if large amounts of soda ash are added. The soluble sodium sulfate tends to build up and cause “ash gels” which are indicated by high progressive gel strength.
2 Another method to treat out calcium sulfate contamination is to treat the thickening and filtration increase that has occurred and let the system become an aggregated-deflocculated one. This can be done by using a thinner, adjusting the pH and using a fluid loss controller. If a high pH is maintained, this too may result in “ash gels” due to the formation of sodium sulfate. If high sodium sulfate (Na2SO4 ) occurs, it will require water dilution and lime additions for alkalinity.
Contaminant
|
Contaminant Compound/ ion
|
Contaminant
Source
|
Method of
Measurement
|
Possible Effect on Mud
|
Course of Action
|
Anhydrite/
|
CaSO4
|
Formation
|
Ca+2
|
High Yield
Point
|
Treat with Soda Ash
|
Gypsum
|
CaSO4 + H 2O
|
Commercial
|
titration
|
High Fluid
Loss
|
Ca+2(mg/L) x 0.00093
= lb/bbl Na 2CO3
|
Gypsum
|
Ca+2
|
Gypsum
|
High gels
|
or
|
|
Thick filter cake
|
Ca+2 (epm) x 0.0188 =
lb/bbl Na 2CO3
|
||||
Ca+2increase
|
|||||
pH decrease
|
Break over to a gypsum mud
|
Materials and Systems
Mud systems to use if thick sections of Anhydrite/gypsum are expected:
- POLYNOX
- Gypsum mud
Chemicals to treat out calcium sulfate contamination:
- Soda Ash (CaCO3 )
- Barium Carbonate (BaCO3 ) Caustic products
Products to condition mud after Calcium has been removed:
- Q-BROXIN
- LIGNOX
- DEXTRID
- PAC