A New Salt-Based Mud System for the North Sea
Authors: Achterberg A. 
Published: December 2007

This paper was prepared for the Second Annual European Meeting of the Society of Petroleum Engineers of AIME, to be held in London, England, April 2–3, 1973. Permission to copy is restricted to an abstract of not more than 300 words. Illustrations may not be copied. The abstract should contain conspicuous acknowledgment of where and by whom the paper is presented. Publication elsewhere after publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF PETROLEUM ENGINEERS JOURNAL is usually granted upon request to the Editor PETROLEUM ENGINEERS JOURNAL is usually granted upon request to the Editor of the appropriate journal provided agreement to give proper credit is made.
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Abstract
This paper describes a new type of saturated saltwater-based mud system ("magnesium mud") that has been used successfully by NAM in over 30 onshore wells and in four offshore North Sea wells. This magnesium mud is based on sodium and magnesium chloride solutions with some other inorganic magnesium compounds and starch products. The mud was specifically designed to improve the safety of rig personnel and for environmental reasons.
To date, our experience indicates that the magnesium mud has a specific application in drilling through salt sections that would otherwise be subject to severe washouts resulting in hole problems, relatively more difficult mud property control, and increased cementing problems and perhaps casing failures. problems and perhaps casing failures. Advantages may also be expected from its shale inhibition characteristics.
Results of laboratory work and field testing are described and compared with the results obtained with the salt-base mud used previously. Examples are given on lithology and hole calipers through bischofite, carnallite and halite sections together with the corresponding mud properties, limitations and costs.
The data obtained so far suggest this mud system has a promising application in both onshore and offshore drilling, especially in the North Sea environment.

Introduction
Drilling activities in the North Sea and onshore in the northern part of The Netherlands have often been hampered by the presence of potassium, calcium and magnesium salts in the potassium, calcium and magnesium salts in the rock-salt formation called the Zechstein. Excessive washouts and detrimental effects to the drilling fluid caused by magnesium salts were often experienced. Therefore, during the last 10 years of NAM's drilling operations, the massive Zechstein salts were penetrated using a sodium chloride-saturated mud with a very high caustic soda concentration (Pf-values of 40 to 60) and a rather high concentration (some 2 to 3 percent) of chromium compounds. Although the application of this mud type was fairly successful in most cases, difficulties were still experienced in formations containing unstable and thick magnesium and potassium salts. Another disadvantage was the safety aspect for rig personnel when working with a fluid of a high caustic soda content (4 to 5 percent). In addition, difficulties could be expected with the disposal of waste mud due to the presence of chromium compounds and of its high pH. In order to combat these problems the magnesium mud was introduced in NAM's drilling operations by the end of 1971.