Santos Basin, Brazil
CHALLENGE:
In the deepwater environment of the Santos Basin, a salt formation lies between the carbonate/anhydrite caprock and the production formation. The salt formation is often drilled riserless using a pump and dump strategy where high mud weight fluid is cut back with seawater. Offset data shows that drilling the salt with this undersaturated fluid can be problematic. Salt from the formation leaches into the drilling fluid causing hole enlargement, which in turn reduces the quality of cement jobs. In addition, rates of penetration (ROP) while drilling salt are often slow, which augments the hole enlargement. An operator in the region sought a solution to these challenges.
SOLUTION:
Rather than cut back high-density fluid, the operator chose to cut back supersaturated brine. A limiting factor, as with all pump and dump operations, is the rig’s capacity to store the large fluid volumes needed when drilling without a riser. As there were no shallow hazards, the operator chose to drill the caprock and the first 50 meters (164 feet) of salt with seawater, thereby reducing the fluid volumes needed on the rig.
The next 150 meters (492 feet) of salt were using cut-back operations in which the supersaturated fluid was diluted with seawater. To increase ROP while limiting the washout, 100 meters (328 feet) were drilled with a fluid diluted back to slightly undersaturated. The next 50 meters (164 feet) were drilled with a saturated fluid to maintain the integrity of the hole and the quality of the cement job. After cementing casing and installing the riser, the rest of the salt section and the production zone were drilled with Newpark’s synthetic-based drilling fluid.
RESULTS:
Newpark and the operator extensively planned the pump and dump operation to maximize the success of the operation with the limited storage space available. A flow rate of 1,000 gal/min was chosen to maintain hole cleaning while minimizing fluid volume requirements. Looking at the fluid requirements per activity, the volume of water-based fluids required for throughout the salt section was determined to be 56,138 barrels of various combinations of seawater and supersaturated brine. A supersaturation of 100 pounds of NaCl per barrel of saturated water was calculated to create 26,167 barrels of 11 lb/gal supersaturated brine. The supersaturated brine formulation included xanthum gum viscosifier to suspend NaCl and caustic soda, soda ash and a corrosion inhibitor to minimize corrosive properties of divalent brines. Pit management diagrams were drawn to describe the location of all the fluid types on the rig and vessels. The storage included 3,000 barrels in active pits, 4,500 barrels in reserve pits, 15,400 barrels on two supply vessels and a small fast supply vessel. Cementing brine was to be held in separate tank. Two wells were planned.
The supersaturated fluid was blended at the shorebase and delivered to the rigs several weeks before the cut-back operations began. On the first well, salt-sag lowered the salinity as indicated by a 0.1 to 0.2 lb/gal drop in mud weight. To ensure the fluid pumped was of the correct salinity, chloride measurements and dilution calculations were made throughout the operation. On the second well, the xanthum gum concentration was increased to combat sag. With confidence in the salinity of the supersaturated fluid, the second well used a blending chart for quick reference on ratios of supersaturated fluid to seawater. The desired improvement in rates of penetration was not seen.
After running the casing and riser, the pipe was cemented in place. Using, Newpark’s synthetic-based fluid, the cement and shoe were drilled out and a formation integrity test (FIT) was performed to a 12.5 ppg equivalent mud weight, verifying the integrity of the cement job.
|
Interval (MD) |
Hole Size |
Casing (MD) |
Fluid(s) Used |
Volume (bbl) |
|
2,246m – 2,344m (7,369 ft – 7,690 ft) |
42” |
36” @ 2,344m (7,690 ft) |
Seawater |
1,364 |
|
2,344m – 3,181m (7,690 ft – 10,436 ft) |
28” |
22” @ 3,313m (10,869 ft)
|
Seawater |
8,840 |
|
3,181m – 3,281m (10,436ft – 10,764 ft) |
9.8 lb/gal (1.17 sg) NaCl mud |
20,408 |
||
|
3,281m – 3,313m (10,764ft – 10,213 ft) |
10.0 ppg (1.20 sg) NaCl mud |
10,204 |
||
|
Pre-Cement and Cement Operations
|
28”
|
Circulate Hole Clean |
4,059 |
|
|
12.0 lb/gal (1.44 sg) PAD mud |
5,100 |
|||
|
Sat. Mud for Cement |
2,000 |
