New Cement Formulation Helps Solve Deep Cementing Problems
Authors: Brothers L.E. , deBlanc F.X. 
Published: December 2007

Invert-emulsion muds are used in most deep, hot wells. The internal aqueous phase of these muds frequently contains high concentrations of salts. It is desirable to complete these wells with a cement slurry containing salt concentrations up to and including saturation to minimize compatibility problems between cement slurry and mud. Above their effective temperature range, however, saturated salt cements-though still considered desirable for their other properties-pose design difficulties regarding thickening time, fluid loss, and rheology. High salt concentrations tend to decrease the effectiveness of most common cement additives-e.g., retarders, fluid-loss additives, and dispersants. At high temperatures, concentrations of these additives can become unacceptably large, while the additives themselves are not as effective under these conditions. Development of and field experience with a new cementing formulation for deep, high-temperature, saturated-salt applications have helped resolve the cement design problems encountered in south Texas and southern and offshore Louisiana. A single synthetic-polymer additive provides cement retardation, fluid-loss control, and dispersant properties with normal design considerations as opposed to the lengthy design requirements of other cement systems. A particular benefit derived from use of the new cement system involves cementing of long liners. Such liners frequently require squeeze cementing at the liner top because the cement is designed for conditions at the bottom of the liner and is thus frequently over-retarded for the cooler temperatures encountered at the top of the liner. This over-retardation tendency is alleviated greatly by use of the new saturated-salt cement additive.

This paper presents case histories to document the effectiveness of this new cement additive for saturated-salt cements.

Introduction

Cementing of deep wells in the Fandango field in south Texas has typically required many hours of laboratory time and complicated field mixing procedures to produce successful results. Heavyweight (18.5- to 19.5-lbm/gal [2217- to 2337-kg/m3]) salt-saturated cement slurries previously used in this field have been difficult to mix and pump continuously because of high viscosity imparted by the high concentrations of cement additives required in salt-saturated cement. High additive levels have been necessary to produce desirable cement slurry properties because ofextreme temperatures encountered in south Texas deep-well cementing, andinterference with chemical interaction between cement and additives caused by high salt concentrations.

A goal of recent research efforts has been to develop a cementing system that is easier to mix and pump than these traditional materials, which have been pushed to their effective limits by extreme operating conditions. Laboratory work has centered on engineering a polymer with the temperature stability necessary for use as a cement additive under bottomhole conditions of 400F [204C] and higher and in the presence of high salt concentrations. As a result of these efforts, a polymer that imparts several desirable cement slurry properties to improve cementing results under high-temperature, saturated-salt conditions was developed.

High-Temperature Salt-Cement Usage

Technical papers concerning the use of salt cement date back to the early 1950's. Most of the early papers address the problem of casing failure opposite salt formations. Fewer papers deal with the properties of the cement used. Two papers that deal with this subject discuss the chemistry of salt cement and its field application. Papers concerning problems associated with the use of salt cement at low-to-moderate temperatures tend to deal with delayed compressive strength development, which is a concern in shallow-well cementing. The subject of the use of salt cements at very high temperatures has largely been ignored in the technical literature, perhaps because ofthe somewhat limited application of salt cements in this temperature range andthe relatively minor percentage of deep, hot wells actually drilled.

Motivation for the use of saturated and other high-salt-concentration cement designs is provided by the presence of salt zones, sensitive shale clay formations, and the use of salt-laden drilling fluids. Use of salt cement at very high temperatures complicates cement slurry design from a laboratory standpoint. To obtain desired thickening time, fluid loss, and compressive strength for deep-well conditions, excessively high additive concentrations are frequently necessary. High additive concentrations may interfere with desired properties-e.g., short-term bottomhole and top-of-liner compressive-strength development in the case of retarders. High fluid-loss-additive concentrations may affect rheological properties, in the case of densified cements, holding water ratios to a minimum to control the volume of densification additive needed to achieve a specific slurry density requirement is desirable. The total effect of these elevated additive concentrations is an overall difficulty in meeting individual slurry design parameters and a cement slurry that is difficult to mix. In the new cementing formulation, the number and volume of additives necessary to achieve critical design parameters may be lowered, which will lower cost, simplify slurry design, and increase early compressive-strength development. Central to this new formulation is a synthetic-polymer additive that acts as a dispersant, fluid-loss additive, and retarder. In addition to improved cement performance, this new formulation is easier to mix than conventional slurries and is more cost-effective than typical cement designs used in comparable conditions.

Chemical Background

Before development of a new polymer material could begin, chemical requirements for the operating environment were considered. In addition to the high temperatures and pressures in which the polymer would be required to function, other operating concerns included the high pH of a cement slurry and the high-ionic-strength environment of salt cement. The high ionic strength of water saturated with NaCl has a pronounced effect on the configuration of a polymer in solution. A polymer with ionizable hydrophilic groups will be coiled in the presence of high concentrations of dissolved salt because of charge shielding. This will reduce the hydration sphere of the polymer and must be considered. Resistance to hydrolysis of pendant groups in the polymer is a concern in the highly basic environment of the cement slurry aqueous phase. At 400F [204C] and pH 12, many materials bear little resemblance to their composition under ambient conditions. Many polymeric materials were evaluated for performance in the course of this investigation. Chemical composition variables and polymer configurations capable of withstanding the harsh chemical environment were identified in this evaluation process. Controllable variables included monomer selection, monomer ratios in copolymers and terpolymers, and molecular weight. An optimum polymer configuration that provides several benefits while withstanding the conditions of the rigorous operating environment was eventually identified and developed. Finally, the process of moving from laboratory synthesis to full-scale production and field application was undertaken.

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