From the magazine, Microtunnelling, Tunnelling

Drilplex comes to the rescue for a demanding under-river bore

The situation

Following floods that occurred four years ago, the Kempsey Shire Council had repeatedly attempted to bore beneath the Macleay River to replace a 375mm water main. Three HDD contractors had failed to complete the task, primarily because of a bed of cobble, between 20 and 30 m thick, lying beneath the river.

Arogen was contracted to re-try the bore and recommended rigging up away from the river and drilling underneath the cobble zone through unconsolidated rock. But doing so would add to the difficulties and Council suggested that the process did not appear likely to succeed. Nonetheless, Arogen was selected and contacted ADG Global Supply, Australian distributor of the US-based M-I SWACO brand of drilling fluids. ADG representatives and M-I SWACO mud engineers assisted Arogen with engineering the drilling fluid program. Among the challenges the system would face was boring through 80 m of black clay and then sealing the ensuing 40 m of porous fractured shale.

The solution

Upon reviewing the geological report, the mud engineer recommended Arogen employ its Drilplex mixed-metal oxide (MMO) water-based drilling fluid system. The high-performance fluid would incorporate Floplex fluid-loss control agent to prevent losses in the pilot bore.

The results

The selection of the system proved to be the ideal choice for the under-river bore. The pilot bore was completed with 100 per cent cuttings and fluid returned some
755 m to the rig.

The system delivered a clean bore hole after the ream and the water main was hooked up immediately. Arogen estimated the Drilplex/Floplex tandem saved three to four weeks rig time by not having to clean-up the bore hole.

As the pipe was pulled from the bore, minimal pipe weight was
observed throughout the pullback and thrust pressure did not exceed 1,000 psi. The operator was extremely pleased the bore was clean after the final reaming pass with no extra clean-up required.

The details

The geological report indicated the bore would penetrate a 500 m basal zone of extremely hard quartz seams, followed by 120 m of porous and fractured shales. The lead-out would cross 20 m of clay for a total bore length of 760 m.

The mud engineer formulated the Drilplex system very lean to facilitate sealing of the 80 m lead-in zone comprising black clay and the 40 m of fractured shale that followed.

The fluid program and properties engineered for the pilot bore comprised:

Soda Ash for PH: 10
Maxgel: 14 bags
Drilplex: 2 bags (11.7 kg)
Floplex: 1.5 bags
Rodease: 1 bucket
Viscosity: 50 seconds
PH: 10
Mud Wt: 1.06 SG (8.9 lb/gal)

The fluid program was designed to accommodate a 10,000 litre tank.

Floplex was added to the system until the fluid loss was eliminated. As the pilot bore proceeded into the hard basal, the volume of Floplex was reduced and incorporated only as necessary to stop or minimise any fluid losses. The relatively low rig pump flow rate of 140 gallons per minute in combination with the depth of 45 m beneath the river and a negative 30 degree pitch, may have contributed to the fluid loss concerns. At completion, the Floplex had to be displaced from the system in order to obtain a thin and permeable filter cake for optimal cuttings removal.

For the clay sections, the Drilplex system was displaced with a PHPA fluid and after those intervals were reamed, Drilplex was re-applied for the shale and rock hole opening. Owing to the size of the rig and the hardness of the rock, this reaming process had to be carried out in small up sizes.

The first 305 mm ream required three weeks to complete. After the initial ream was completed, the hole was staged to 356 mm, 406 mm, 457 mm and the final 508 mm ream. For the final three reaming passes, a 350 gpm mud pump was installed in line to enable the Drilplex system to clean the hole without the need of extra cleaning passes. All the returns were transferred into a vacuum truck unit and removed from the location. The water main was installed successfully immediately after the final reaming pass.

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