An environmentally friendly casing cleaning treatment is bolstering efficiency on Statfjord and Sleipner fields.
A new oil-based soap (OBS) for cleaning casing reduces rig time and is environmentally safe, its makers claim. The method replaces a more expensive and time-consuming casing cleaning process that consisted of pumping base oil, followed by a sequence involving two or three cycles with water-based viscosified soap pills, followed again by pumping sea water.
The system, developed by BJ Services, is based on pumping only a small volume of base oil, followed by pumping a soap containing a specific water volume that is emulsified into a larger base oil pill. Thereafter, a water-based wash fluid, primarily for water wetting purposes, is pumped, followed by seawater.
Environmental benefits
During initial applications on the first two wells, the casing cleaning pill composition and volumes were optimized using a laboratory evaluation with a VG-viscometer prior to the operation. On Statfjord A-07A, the oil-based mud was first displaced by 5 cu m of base oil, followed by a 30-cu m emulsion pill. This emulsion pill consisted of 65% base oil. The remaining 35% was a mixture of water and casing-cleaning chemicals. Following the emulsion pill, a 10-cu m low-viscosity wash fluid pill was pumped. Finally, seawater was pumped until sufficient cleanliness was observed, whereupon the well was displaced to completion fluid. After pumping one hole volume with seawater, the effluent water exhibited a steady cleanliness of 96 NTU, and pumping of seawater was terminated. When the washing pipe was pulled out of the hole, it was completely clean.
In light of the wash pipe tool configuration, optimum pumping rate, wash pipe rotation and reciprocation were not practical. Experience on the initial Statfjord and Sleipner wells indicated that when a drilling operation is followed by a casing cleaning operation without delay, pumping of an efficient cleaning pill usually is sufficient to clean the well without rotating and reciprocating the wash pipe. By carrying out the pumping sequence using OBS, significantly less fluid volume is required than with the traditional approach.
No chemicals discharged
Because the OBS pill is sufficiently unstable, it is possible to separate most of the base oil from the effluent fluid, following a casing cleaning operation. This means the base oil may be recycled for future drilling and casing and cleaning operations. Therefore, waste production and related waste-handling costs are also dramatically reduced. And because the OBS cleaning process is completely contained, no discharges are made into the sea.
In field and laboratory tests, the new OBS method has proven to be superior to the traditional casing cleaning method. Casing cleaning operations carried out using OBS on behalf of Statoil have shown a 30% to 50% reduction in chemical consumption and rig time. As a result, Statoil has realized significant cost savings related to rig operation.
"The fact that the new emulsion-based casing cleaning soap method involves virtually no chemical discharge into the sea, allows for recycling of fluids for future use and reduces rig time is beneficial for the industry in Norway and throughout the world," said Alasdair Buchanan, BJ Services well services region manager for Europe and Africa.

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