Anhydrite and gypsum are calcium sulfate compounds (CaSO4) which are sometimes encountered while drilling. Gyp is calcium sulfate with water of crystallization (CaSO4 2H2O), while anhydrite is the anhydrous (waterless) form of calcium sulfate (CaSO4). These may occur as thin stringers or massive beds.
Calcium sulfate contamination is similar to cement contamination because both liberate calcium ions, which in turn cause flocculation. Unlike cement, calcium sulfate does not cause a pH increase since it supplies a sulfate ion in lieu of an hydroxyl ion. The sulfate ion contributes to flocculation of clay solids, although its effect is small compared to calcium ions.
In lightly-treated muds, small amounts of CaSO4 increase the rheological properties. The severity of flocculation depends to a great degree on clay content. When CaSO4 causes the calcium concentration to increase above 200 mg/L, viscosity may fluctuate drastically and fluid loss may become difficult to control. As calcium ion concentrations further increase, a base exchange occurs in which sodium montmorillonite becomes calcium montmorillonite. Flow properties tend to decrease, and fluid loss becomes very difficult to control.
Treatment - There are several methods for handling CaSO4 contamination. The drilling fluid can be maintained as a low-calcium fluid by chemically precipitating calcium from solution or it can be converted to a gyp system. For smaller amounts of CaSO4 contamination, chemical removal of the calcium ion is best achieved by adding soda ash. Excess CaSO4 can be estimated from the filtrate versenate (VF), whole mud versenate (VT), and volume fraction of water from retort analysis (FW) by the following equation:
Approximately 0.093 lb/bbl of soda ash is required to precipitate 100 mg/L of Ca ion. Caution should be exercised to avoid overtreatment. When Ca++ concentration is reduced to 100-150 mg/L, treatment should be suspended.
The reaction between gypsum or anhydrite with soda ash is:
Soluble sodium sulfate (Na+ + SO4 ions) is formed from this reaction and could cause flocculation problems after prolonged treatments. For this reason, it is generally necessary to convert to a calcium-based fluid when massive anhydrite is to be drilled with a freshwater system.
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