Insufficient hole cleaning prior to cementing basically means that cuttings and solids remain in the hole by the time the cementing operation is being executed. This situation is of particular concern due to the higher carrying capacity of the cementing fluids (spacer and cement slurries). During the cementing operations all these solids and cuttings will be carried up in the annulus mixing with the spacer and lead cement slurry. Depending on the amount of solids, this effect will produce increased friction and eventually completely plugging or packing off the annular space preventing any further flow, potentially leaving a long column of cement slurries under-displaced inside the casing.
Preventing the condition at all, by proper drilling practices and hole cleaning techniques, is the best solution, but if an annular pack-off risk still exists before cementing, the following measures can be considered as mitigation:
o Do not be too optimistic about the number of centralizers. Centralizers in the washout areas would have a minor or insignificant contribution to improve the stand-off. They can also increase the likelihood of pack-off by mechanically weakening sensitive borehole areas usually near the washouts.
o If pipe reciprocation is considered, centralizers must be allowed to move freely at least half joint. This will also limit any abrasive action on sensitive areas of the borehole
– Circulation before cementing (casing on bottom)
o Simulate with software the expected circulating surface pressure at different rates. These values would provide a reference to identify when the hole is conditioned.
o Identify any erratic behavior of the pumping pressure; continue circulation until mostly removed while constantly monitoring the shakers. Determine extended circulation time to ensure hole is clean before cementing.
o Properties of the mud before cementing must be defined by balancing hole stability targets and cement placement. In case mud rheology could be lowered, hole stability permitting, do it only when hole cleaning is established.
– Placement of cementing fluids
o Proper and detailed pre-job planning is mandatory. This would ensure that the crew is fully aware of the planned actions and their role in the cementing operation.
o Do not pump any fluid lighter than the mud or with lower viscosity, like Newtonian fluids. These fluids would increase the settling of solids and further destabilize the hole.
o Do not pump scavenger slurries. These slurries are recommended in certain circumstance where additional abrasive action ahead of the cement slurry is seen as a benefit. However, if there is a risk of packing off the annulus this effect would be rather negative. Additionally, these cement slurries can be extremely unstable, particularly at lower rates.
o Ensure the spacer is stable, i.e. the solids are properly suspended and fluid rheology is as designed. Proper hydration of bentonite and any other polymer in the formula is of paramount importance.
o Proper density control of cement slurries is considerably critical within this context, and if not properly done, it can rapidly increase the chances of packing off the annulus. The cement slurry mixed below the designed density is likely to be unstable while the slurry mixed above would likely have shorter gel strength development and thickening time. Both effects would make the situation worse.
o Pumping rate as high as possible (within fracture gradient limit), ensuring annular velocity while cementing is always above the annular velocity estimated from hydraulic software for hole cleaning.
o Reduce the u-tube effect and static periods (dropping plugs) while cementing.
o The volume of the washouts should not be considered for cement slurry volume calculations, because the cement will not likely go inside.
o If possible, prepare proper plans to ensure how reciprocation will be done and maintained during the cement job. Casing reciprocation can also be used as a contingency in case signs of pack off are seen in combination with the following pumping strategy:
- Erratic behaviour, sudden fluctuation or unstable pumping pressure would likely indicate the possibility of a pack off.
- If additionally, the pressure trend is increasing at a faster pace than expected, the conditions for a pack off already exist.
- In this case, do not lower the rate and even increase it as much as possible, as at this moment the priority is to finalize the displacement of the cement out of the casing.