Generally, barytes sag is linked to a rapid change in mud properties, for operational reasons: Increase mw because of kick, reduce mw because of diff sticking pb, thinning mud at TD for the cement job, etc.
- POBM systems are usually very thin and often run at low gels 5/20 and low "low end" rheology. This give very good drilling performance but makes the system sensitive to changes.
- When mud properties are specificaly engineered wells can be suspended for months without sag occuring.
- The procedure to change properties should be done by adding premix and not base oil or weighting agent directly. If practical, have available a volume of pre-treated base oil.
- Rig team when balancing against operational "priorities" should consider that a minimum rheology is required and sufficient time should be allocated to condition the system.
- Run a higher base fluid/water ratio (70/30 rather than 65/35). Increasing base fluid/water ratio and hence organophilic clay content reduces the tendency to sag. Use a concentration sheet for Carbogel in the mud.
- Be prepared to add clay as well as base fluid , when the centrifuge is being run to control low gravity solids and viscosity.
- Maintain mud suitable for cementing throughout the section and certainly within 1000ft of TD. For ERW, do not "thin" the mud at TD prior to running the casing.
- All muds show settlement, however oil based muds are worse than gel muds.
- The smaller the particle size of solids the slower the particles settle.
- Dilution prior to tripping out of the well increases the risk of sag.
- Tripping without rotation shows the effect more clearly. When the mud has gelled, baryte will not settle, it is at very low shear, running pipe that sag may developp.
- Rheology modifiers have little effect on the ability of a mud to suspend barytes.
- Maintain mud properties that are suitable for cementing throughout the section (gels less than 20/35lb/100ft2. If the mud goes out of spec more than 1000ft from section TD then treat the mud. If within 1000ft of TD, consult town to see if the cement job will be put at risk. If the cement job will be put at risk then be prepared to spend time circulating at TD.
- Send a sample of the field mud near TD to town to check for barytes sag. This test is impractical onsite.
- Sagging is more likely to occur when running slowly with DP. When the mud is gelled, no sag occurs.
- 90% of the sagging situation have occured after a treatment for kick or losses. Always have the base oil pretreated.
- Tandem pill should only used with care. Although they may help for hole cleaning. If they do not work than don't use them (some people keep on using them even if they bring nothing back).
- The sagging test are proven technique, successfully used to determine sag tendency in particular on new mud formulation. Often operations reports sag while laboratory investigation indicate that the mud is alright. Some possible reasons for this apparent contradiction are:
- If the problem is caused by an incorrect addition of chemicals, the problem should still be visible after a few days. However if the problem is linked to lack of shearing offshore, then the condition of the sample could improve during the time of transport. Although if the mud has not been sheared properly the water dropplets may not be "small enough".
- When the mud has sagged, it has reach another equilibrium, and no longer sag. We have heavy and light spots which become difficult to homogenise, but a local mud sample will not exhibit sagging.