|Publisher||Society of Petroleum Engineers||Language||English|
|Content Type||Conference Paper|
|Title||USE OF CHEMICAL SALT PRECIPITATION INHIBITORS TO MAINTAIN SUPERSATURATED SALT MUDS FOR DRILLING SALT FORMATIONS|
|Authors||Earl, S.L., Shell Oil Company; Nahm, J.J., Shell Development Company|
SPE Annual Technical Conference and Exhibition, 4-7 October 1981, San Antonio, Texas
|Copyright||Copyright 1981, Society of Petroleum Engineers of AIME|
Five commercial salt inhibitors were laboratory tested in NaCl solutions to evaluate their effectiveness in maintaining salt (NaCl) saturations over the temperature ranges encountered from the bottom of the hole to the surface when drilling through salt formations. The effect of salt inhibitors was also studied in NaCl solutions with high Ca++ and Mg++ contamination. Three of the inhibitors were found to be effective in maintaining a supersaturation condition while two were not. Effectiveness was shown to be significantly reduced in the presence of high hardness.
The test results were successfully applied to a drilling program in the Williston Basin to eliminate salt zone washouts with the use of a supersaturated salt mud. Comparison of caliper logs and cement bond logs from previously drilled wells to those of the test wells shows significant reduction in hole enlargement through the salts and improvement in the quality of the cement bond.
During the early drilling days in the Williston Basin area, there were numerous case histories of hole enlargement in salt intervals and subsequent casing failures. These failures occurred due to underdesigned casing strings, salt movement, and poor cement jobs which did not uniformly distribute the loads from the salts moving in around the casing. Corrosion associated with uneven cement coverage was also mentioned as a secondary cause. Attempts were made to solve this problem through casing design change and the use of saturated salt water emulsion muds which would reduce hole washout and allow for better cement jobs.
In the development of the Cedar Creek Anticline and other fields in the Williston Basin, it was discovered that wells drilled with a salt saturated or partially saturated mud (150,000 mg/l Cl- or greater) were successful in reducing hole washouts in the salt formations. A 1.0 psi/ft. collapse load was used for casing design opposite the salts. A similar drilling program was applied to the development of the Mondak Field with no apparent success.
Several salt zones are encountered when drilling wells in the Greater Mondak Area. The Charles salt is the most massive, occurring at a depth of 8000-9000 feet with a net interval of 340-400 feet of salt in a gross interval of 550-700 feet. Typical caliper logs through these salt zones exceed 16 inches. A normal gauge hole is 8-8 1/2 inches for 7 7/8 inch bit. A comparison of salt saturation and temperature indicated that the chloride level should be maintained at 200,000 mg/l for Mondak wells to insure saturation at bottomhole temperature. This chloride requirement for salt saturation increases as the hardness of the makeup water increases.
The effect of temperature on salt solubility makes it difficult to maintain such a supersaturated system. As the mud is circulated, its temperature changes with depth and the required saturation increases. The produced brines used for mud makeup are occasionally saturated with sodium chloride, but can also contain large amounts of CaCl. and MgCl, and other inorganic salts. For example, the brines produced in the Williston Basin area contain up to produced in the Williston Basin area contain up to 30,000 mg/l calcium ion and 7,500 mg/l magnesium ion. As the temperature of the mud prepared with this type of brine is reduced, the dissolved salts of the more concentrated brines tend to precipitate out in the form of crystals. This results in an undersaturated system based on bottomhole temperatures. This mud is pumped downhole, dissolves more formation salt and the process repeats itself resulting in an enlarged hole. The precipitated salts are also abrasive and a nuisance to drilling operations.
During the late 1960's, a chemical salt recrystallization inhibitor, a liquid formulation of certain amides, was introduced to inhibit precipitation of sodium chloride from saturated brine muds and precipitation of sodium chloride from saturated brine muds and produced oil field brines. produced oil field brines.
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