Stability and Corrosivity of Packer Fluids

Abstract

ABSTRACT For over twenty years, the industry has been concerned with the problems ofgelation and suspension of solids in packer fluids. In recent years, considerable attention has been given to corrosion aspects. This paper discusses the types of water-base packer fluids used in the past and the difficulties encountered. It presents laboratory data comparing suspension agents for oil muds and cites field examples of oil muds used successfully to suspend weighting material and to protect against corrosion. Corrosion protection when using liquids as packer fluids is also discussed. INTRODUCTION WHAT IS REQUIRED of a fluid to be left in the casing tubing annulus of awell? This question has been given careful consideration from time to time overthe past twenty years, and yet is still a matter of controversy. There arelogical explanations for why the packer mud problem has in the past becomeacute, has appeared to be solved and then has reappeared in somewhat differentform. To understand the situation, one should keep two things inmind:Operations are simplified if the fluid used for drilling the well can beleft as the packer fluid.Performance of a packer fluid involves a consideration of hydraulicproperties (density, rheology' gelation) as well as corrosioncharacteristics. A brief review can illustrate how: changes made to provide a better drilling fluid have sometimes created packer fluid problems; how packer fluids designedfor better hydraulic properties may have made corrosion problems more severe; and how packer fluids designed for corrosion protection have at times resultedin hydraulic problems. High-pH Muds During the 1940's, difficulties were being experienced in controlling the flow properties of the weighted muds required for drilling abnormally pressuredformations. A salt-water flow or severe cement contamination would thicken themud excessively, cause stuck pipe and lost circulation, and result in the holebeing lost. Development of the high-ph lime-treated mud gave much bettertolerance for salt water, cement and shale solids. This mud came intowidespread use for drilling and was customarily left in the casing-tubingannulus upon completion. Although little attention was given to the corrosivityof mud at that time, the highly alkaline lime mud probably gave good corrosionprotection. Unfortunately, experience soon showed that this type of mud gelledexcessively or even solidified when exposed to downhole formation temperaturesfor any significant period of time. Laboratory studies reported by Gray, Neznayko and Gilkeson revealed that free hydroxyl in the high-ph mud reactedwith silica and silicates to form cementatious materials, resulting in gelationor solidification of the mud. In spite of years of research, no practicalsolution to this problem has yet been found. Raising the pH of a water-base mudthat is to be left in a well can lessen the corrosivity. If there is excessalkali and ample clay concentration in the mild, however, the above reactions will cause gelation and result in a time-consuming, expensive workoveroperation. There is insufficient clay in the packer mud, settling of weightingmaterial will necessitate washing-overtubing and make workover operationsexpensive. If the concentration of alkali in the mud is exceeded by that ofacid formers (such as the thinners and filtration control agents used in mostdrilling muds), the ph of the mud will eventually drop and the corrosion protection will be lost.