The Karachaganak Field, which was discovered in 1979, is located in north-west Kazakhstan on the northern margin of the Pre-Caspian Basin. Production from this field contains 4 - 4..5% H2S. The Kashagan field, located offshore in the North West part of the Caspian Sea, averages 10-12% H2S concentration. An environmentally friendly H2S scavenger has been used in the Caspian area since 2006 in the invert emulsion drilling fluid systems. The operators had a standard practice of maintaining an excess of lime (2.5 kg/m3) as a supplementary sulphide removal additive and also raised concerns regarding the lack of a testing procedure to monitor concentration. This issue was regarded as being very important on the grounds of the significant risk that existed in the absence of more accurate method of monitoring scavenger concentration other than just mass balance calculations. The lack of ability to measure scavenger concentration more precisely in the invert emulsion fluid returned to the liquid mud plant (LMP), led to increased cost for the operator to maintain a safe concentration.

This paper discusses the design of a new method to be used in both OBM and WBM during field applications of the scavenger. The method is now used to monitor the excess scavenger in all the wells drilled in the Karachaganak field and to design subsequent treatments. The method has also contributed to improving the level of safety in the operation. The use of the proper scavenger concentration in combination with excess lime provides a primary and secondary barrier for protection against the influx of H2S

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