Surface sections can be drilled with a gypsum/polymer water based mud. Attention must be paid to preserving the inhibitive properties of the drilling fluid by maintaining appropriate polymer and gypsum concentrations in the mud. Particular attention must be given to solids control by optimising the operation of the solids control equipment in order to minimise mud maintenance requirements.

Product Functions

BARITE Weighting agent.

GYPSUM To provide a source of soluble calcium ions for shale inhibition.

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

FLR A polyanionic cellulose product which provides viscosity and fluid loss control, giving a shear thinning fluid with good PV/YP ratios. A fluid formulated with this product exhibits excellent resistance to contaminating ions and is temperature stable up to 175oC.

FLR XL Essentially a non-viscosifying form of PAC. Exhibits the same temperature stability and resistance to contaminating ions as FLR and has a secondary dispersing effect that is especially useful in weighted fluids.

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.

GLUTERALDEHYDE Environmentally acceptable biocide.

Initial System Formulation

Formulation of 1 m3 fluid:

MUD WEIGHT (sg)                                1.20                              1.40

WATER (m3)                                         0.92                              0.86

BARITE (kg/m3)                                    237                               502

GYPSUM (kg/m3)                                  14                                 14

IDVIS (kg/m3)                                       5.7                                5.7

FLR (kg/m3)                                          2.9                                2.9

FLR XL (kg/m3)                                    11                                 11

GLUTERALDEHYDE (kg/m3)                 0.7                                0.7

LIME (kg/m3)                                        2.0                                2.0

SODA ASH (kg/m3)                               2.9                                2.9

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 9.5 with additions of lime and soda ash.

Mix the FLR and FLR XL @ 10 - 15 mins per sack.

5. Add the gypsum.

6. Adjust rheology with IDVIS if required.

7. Add the barite 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.

Engineering Comments

1. The initial adjustment of sea water to pH 9.5 is done using lime and soda ash. Lime should be used for pH adjustment in the active system if the pH drops below the range of 9.5 - 10.0.

2. Yield point and API fluid loss must be closely monitored for any sign of drop in polymer concentrations. The concentration of polymers should be maintained at greater than 10 kg/m3 at all times. This must be judged according to dilution rates and reactivity of the formation, since polymers are removed with cuttings. In addition, it is important to observe the condition of the cuttings on the shale shakers. Cuttings should move quickly across the shaker screens. Sticky and balled up cuttings may indicate insufficient inhibition from either an inadequate gypsum concentration, polymer concentration or both.

3. All mud maintenance should be done by addition of premix. Barite additions should be  accompanied by additions of 10 kg PAC per MT barite, if mud weight is increased in the active system.

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

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

6. Only mud in good condition should be transferred from one section to another.

7. Maintain the Ca2+ ion concentration above 600 mg/l. Add further gypsum to the active if required.

8. Rheological properties should be closely monitored if the MBT approaches 60 kg/m3. MBT values of up to 80 kg/m3 are permissible if the rheological properties of the fluid are still in the desired range.

9. Maintain initial gels greater than 2 lb/100 ft2 with additions of IDVIS. Initial gels should be a minimum of 4 lb/100 ft2 before pulling out at TD, to avoid the possibility of barite sag.

10.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 9.5 - 10.0 with lime. 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.

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