Wednesday, 12 February 2020

2 SIMPLE TIPS TO CONSIDER WHILE DRILLING ANHYDRITE / GYPSUM FORMATIONS

Gypsum and Anhydrite drilling operations guidelines

Anhydrite/Gypsum Drilling Problems

Introduction

Calcium sulfate occurs in nature as Gypsum (CaSO4 l H2O) or Anhydrite (CaSO4 ). It is found in thick sections, stringers, in make-up water, embedded in silts as in evaporite formations and sometimes in the caprock of a salt dome.

Causes of Anhydrite/Gypsum Contamination

Calcium sulfate causes aggregation and flocculation of a fresh water mud, resulting in thickening. The calcium sulfate causes an increase in apparent viscosity, yield point, gel strength and filtrate. The partially soluble calcium sulfate increases the hardness and sulfate content of the filtrate. If a calcium base mud is in use, the calcium sulfate contamination has little or no effect on the mud properties.

Preventing and Curing Anhydrite/Gypsum Contamination

1 A common method of drilling anhydrite or gypsum formations is to pre-treat the mud with thinners that works effectively in the presence of calcium and sulfates and alkali’s. The contamination of the mud by the drilled calcium sulfate is nullified. If it is desired to maintain a fresh water mud after calcium sulfate contamination has occurred, it is necessary to treat out the ions that cause aggregation and flocculation. This may be done by adding soda ash (Na2CO3) or Barium carbonate (BaCO3).

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
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