Environmental constraints have led to the development of an invert mud system which has excellent toxicity, health and safety aspects, and is biodegradable. This system is particularly suitable for North Sea use where discharges particularly suitable for North Sea use where discharges from drilling with mineral oil-base drilling fluids are limited. Based on a synthetic material as the continuous phase, this mud system gives the same high performance as that of mineral oil-base systems. This paper describes this novel mud system and focuses on the impact of its use in environmentally sensitive areas.

The ratio of synthetic liquid continuous phase and aqueous phase (S/W ratio) can vary from 90/10 to 50/50 with mud phase (S/W ratio) can vary from 90/10 to 50/50 with mud weights ranging from unweighted to 18.0 ppg. The rheology parameters can be altered with rheology modifiers to give increased low-shear-rate viscosities for enhanced hole cleaning capabilities. Huxley-Bertram HTHP viscometer data shows good stability of the system under downhole conditions. Additional work indicates this system has reduced gas solubility, better lubricity, and lower retention on cuttings when compared to mineral oilbase muds.

Favorable toxicity data, bioaccumulation studies, and biodegradation work have been compiled for both the synthetic material and mud system. U.K. toxicity tests give highly acceptable values. These results, along with the bioaccumulation data, have led the U.K. Department of Energy (DEn) to assign the system a Category O classification under its Chemical Notification Scheme. Likewise, the Norwegian government agency (SFT) has agreed to permit this system to be used on a field trial basis. Currently, the U.S. Environmental Protection Agency (EPA) is considering its use, and subsequent discharge of cuttings, for Gulf of Mexico drilling.


Environmental concerns have been an impetus for change associated with the use of oil-base muds (OBM). These concerns and forces for change are nowhere better represented than in the North Sea area. Each of the major oil producing countries bordering the North Sea exert their own individual regulatory influence on the nature and use of OBM.

Some countries have set restrictions on the amount of oil discharged into the sea. Typically, this quantity is regulated by monitoring the total oil retained by the drill cuttings. Depending on the sector of the North Sea, values for cuttings-oil-retention (COR) associated with the discharged drill cuttings may vary from as high as 150 g/kg dry retorted solids to as low as 10 g/kg. In other areas, a zero-discharge limitation may become the norm.

Initially, OBMs in the North Sea were chosen in order to achieve the desired drilling performance.

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