Laboratory tests showed that tetra-potassium pyrophosphate (TKPP or K4P207) gave levels of shale inhibition well beyond that obtained with equivalent concentrations of potassium chloride. A detailed evaluation indicated that stable mud formulations could be prepared which were considerably more inhibitive than other mud systems currently available. Inhibition even approached the level of that observed with oil based mud with some shale types.

Two North Sea field trials were selected to evaluate the field performance of these phosphate muds. In both cases there were extensive formations of very reactive Tertiary shale which were known to be unstable when drilled using conventional water based muds. Each location also provided good offset data from similar wells drilled with both oil based and conventional water based fluids. This allowed the performance and economics of the phosphate muds to be thoroughly assessed.

In both field trials the highly inhibitive nature of the new systems was confirmed when the shale sections remained open and stable for extended periods. However, in both wells the overall performance of the system was disappointing because of severe problems caused by very firm cuttings adhering to the stabilisers. This "accretion" resulted in slow, tight trips to surface, gave problems with blocked flowlines and a rapid buildup of drill solids, which consequentially required higher than planned mud dilution.

The cause of the accretion problem has been investigated and explained. Work is now underway to find a practical solution.

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