Magnesium Oil Mud Provides Gauge Hole in Bischofite / Carnallite Formations
- T.S. Carter (Conoco Inc.) | J.E. Smith (Conoco Inc.)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling Engineering
- Publication Date
- April 1986
- Document Type
- Journal Paper
- 115 - 121
- 1986. Society of Petroleum Engineers
- 3 Production and Well Operations, 1.6 Drilling Operations, 4.1.2 Separation and Treating, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 1.6.10 Running and Setting Casing, 2.7.1 Completion Fluids, 1.6.1 Drilling Operation Management, 4.2 Pipelines, Flowlines and Risers, 1.12.6 Drilling Data Management and Standards, 1.10 Drilling Equipment, 6.5.4 Naturally Occurring Radioactive Materials, 1.11.4 Solids Control, 1.14 Casing and Cementing, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 1.5 Drill Bits, 1.1.6 Hole Openers & Under-reamers, 1.14.1 Casing Design, 1.11 Drilling Fluids and Materials, 4.3.1 Hydrates
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Summary. Laboratory tests conducted to investigate the possibility of using oil muds through the Zechstein formation of the Viking field showed that diesel-oil muds formulated with sodium- and/or calcium-type emulsifiers could break down if the wrong treatment was used to counteract contamination from the solubility of magnesium minerals. However, oil-mud samples prepared with magnesium compounds offered good mud stability and did not dissolve the contaminant. Therefore, this formulation was selected for a field test on a Viking well; this paper reviews the results. Drilling data from previous wells in the area are compared with values measured on the test well. Results indicate that the magnesium oil muds can provide a gauge hole in the bischofite/carnallite zones that occur within the Zechstein formation.
The Viking field shown in Fig. 1 is located in the U.K. sector of the North Sea and produces gas from the Rotliegendes sandstone on seven platforms installed in Blocks 49/12, 49/16, and 49/17. From 1977 to 1982, six wells failed because of mechanical restriction of the tubulars. In each case the point of collapse occurred within the evaporite sequence of the Zechstein formation.
Many wells drilled in the Viking field penetrate a zone in the Zechstein formation with a high content of potassium and magnesium salts. Drilling a gauge hole through this interval has been difficult. The hole enlargement is caused by the high solubility of the sylvite (KCl), bischofite (MgCl2-6H2O), and carnallite (KMgCl3-6H2O) minerals present in the formation. Mixed-salt muds that were previously used still had washouts, and their 11.0-lbm/gal [1318-kg/m3] density caused lost circulation and stuck pipe problems.
An engineering study on the cause and potential solution to this problem was initiated in 1981 before a well replacement program that started drilling operations in 1982. This paper is concerned with the selection and performance of the drilling fluid programmed for these wells.
Theory on the Cause of Well Failure
The following sequence of events has been identified as a major factor that contributes to the loss of production of these Viking wells during the drilling phase.
1. Hole washout occurs in the Zechstein-complex salt zones while water-based muds are used to drill in the intermediate casing interval. 2. Casing is set through the Zechstein formations but cannot be centralized because of the irregular hole geometry. 3. Cement channels through the mud in these washed-out zones during displacement and offers little or no bonding to the formation.
These events have been associated with a loss of production during the production phase.
1. High overburden pressures cause these salt formations to deform with time and to contact the nonuniform cement sheath or the exterior wall of the casing. 2. The resultant point loading exceeds the maximum design criteria, and tubulars fail by a collapse and/or shear mechanism.
With this analogy, the only variables that could be changed to prevent future well failure are mud type, casing design, and cementing techniques. The last two variables were investigated thoroughly and design improvements were recommended, but unless the borehole is drilled in gauge with a uniform shape, these improvements would be of little value. Therefore, the first objective was to select a fluid that would minimize washout of the Zechstein evaporite intervals.
Unlike most reactive formations, the Zechstein evaporite series contains several different chemical compounds that are all water-soluble to some degree under downhole temperature conditions. The design, formulation, and maintenance of a drilling fluid that is stable and resistant to this type of contamination were unique challenges. It is generally accepted that the two fluid designs that offer the best technical answer to this problem are a mixed saltwater-based system and an invert-oil-emulsion mud with a high-salinity water phase. Because a mixed-salt system was used in five of the Viking failures, it was reviewed first.
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