Spud Mud is used to start the drilling of a well and continues to be used while drilling the first few hundred feet of hole. Spud mud is usually an unweighted water-base mud, made up of water and natural solids from the formation being drilled. It may contain some commercial clay, added to increase viscosity and improve wall-cake building properties.
Mud used to drill a well from surface to a shallow depth. Guar gum or salt gel are commonly used offshore as spud mud. Onshore spud mud is usually a water-base mud containing bentonite clay that is flocculated with lime. In a large-diameter surface hole, a flocculated clay-based mud can remove large gravel cuttings encountered at shallow depths and is simple and inexpensive. –slb
Spud Mud with Drilling – 36″ Hole
| MUD SYSTEM | SEAWATER & SPUD MUD (SWEEPS) |
| Mud Density | 8.7 – 10.0 ppg |
| Funnel Viscosity | 100 + SEC |
Operations
The section was drilled using seawater and sweeps of hi-vis mud to clean the hole. Bentonite was pre-hydrated, and then diluted with seawater and treated to the correct properties. Kill mud was also prepared prior to spud, and displacement mud was made to leave in the hole prior to running the casing.
Hole cleaning seemed good. Sweeps were pumped about every 9 m. Together with reaming this seemed satisfactory.
The hole was left with 10 ppg/1.2 sg mud when pulling out.
A trial slurry of Mica, D124 and Hematite was mixed and ready for pumping ahead of cement.
No mud problems were encountered.
Spud Mud with Drilling 26″ Hole
| MUD SYSTEM | SEAWATER & SPUD MUD (SWEEPS) |
| Mud Density | 8.9 – 12.0 ppg |
| Rheology @ 120°F/49°C | |
| PV | 13 – 15 cps |
| YP | 24 – 32 lb/100 sqft |
| Gels | 11/15- 10/20 lb/100 sqft |
Operations
The drilling fluids used for this section were seawater and hi-vis bentonite sweeps. Returns were to seabed. Hole cleaning seemed good. Sweeps were pumped every single.
The stability of the hole was not fully satisfactory. Prior to the first wiper trip in the pilothole, 10 ppg/1.2 sg mud was spotted in the hole. Several tight spots were found. The bottom part of the hole was found tight on the way back in. After cleaning and reaming the hole, heavier mud, 12 ppg/1.44 sg was spotted in the hole.
The hole was not stable and logging could not be performed. A clean-out trip was performed, and 12 ppg/1.44 sg mud used again when tripping out. Logging went without problems.
When opening the hole to 26″, the bottom part was again found tight. Extensive reaming had to be done before the hole was stable. 12 ppg/1.44 sg mud was left in the hole.
Casing was run without problems.
The pumping of identification pills was performed to see if they could be spotted on the way out of the hole with the intention of using them to mark and identify cement returns at the seabed. Two of these were pumped; of which the first contained ceramic beads and the the second was made up of Mica. Neither was seen at the seabed by the ROV.
Conclusions and Recommendations
The main problem in this section was hole stability. The only way to control hole stability with mud in this situation was by increasing the density of the mud left in the hole.
Another alternative for future operations would be to drill this sections with returns. Then a weighted inhibitive mud could be used. This would require riser and sub-sea diverter.
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