Reservoir sections can be drilled with a low colloidal content sodium chloride/polymer mud system weighted with calcium carbonate. The recommended formulation is to minimise formation in a sandstone reservoir.

Product Functions

CACO3 Acid soluble weighting agent.

SODIUM CHLORIDE To improve shale inhibition.

IDVIS A pure grade xanthan gum, for providing yield point and progressive gel strengths.

FLOPLEX Fluid loss reducing polymer.

GLUTERALDEHYDE Environmentally acceptable biocide.

LIME pH control together with soda ash as well as to control soluble carbonate and bicarbonate concentrations in the fluid.

SODA ASH pH control together with lime as well as to control soluble calcium levels in the fluid.

Initial System Formulation

Formulation of 1 m3 fluid at 1.15 sg:

WATER             0.89 m3

CACO3                         191 kg/m3

NACL                            50 kg/m3

IDVIS                            2.9 kg/m3

FLOPLEX                     10 kg/m3

GLUTERALDEHYDE     0.7 kg/m3

LIME                             2.0 kg/m3

SODA ASH                   2.9 kg/m

Mixing Procedures

1. Clean all surface tanks and lines.

2. Fill tanks with the required volume of sea water and add the gluteraldehyde.

3. Adjust the pH to 9.0 with additions of lime and soda ash.

4. Add the sodium chloride.

Mix the FLOPLEX @ 10 - 15 mins per sack.

6. Adjust rheology with IDVIS if required.

7. Add the calcium carbonate to the required density.

Recommended Properties

*   MBT values between 60 and 80 kg/m3 can be accepted when all other mud parameters are good.

** In the circulating system, the pH should be maintained by additions of lime.

5.     Engineering Comments

1. A complete new low solids content drilling fluid containing no barite should be mixed for this section.

2. Use lime and soda ash for pH adjustment in the active system if the pH drops below the range 8.5 - 9.0.

3. All mud maintenance should be done by addition of premix.

4. It is essential to make full and effective use of available solids control equipment.

Note : Always drill cement with mud left over from the previous section.

6. Maintain initial gels greater than 2 lb/100 ft2 with additions of IDVIS.

7. Although this system is not normally susceptible to bacterial degradation, mud stored in surface pits   for extended periods of time should be treated with approximately 1 kg/m3 gluteraldehyde and the pH should be maintained between 8.5 - 9.0 with lime and soda ash. Maintenance additions of          gluteraldehyde are necessary since it is an active chemical which will lose its effect over time. All    mud stored over time should be monitored for odour, drop in pH and rheology.

8. Total LGS content of the mud should be maintained below 6% by diluting with new mud as required.