Behavior of Chromate in Drilling Fluids Containing Chrome Lignosulfonate
Authors: Skelly W.G. , Dieball D.E. 
Published: December 2007

Abstract
The effect of chrome lignosulfonate dispersants on the rate of reduction of added chromate in drilling fluids was determined by polarographic analysis. Analyses were made after aging the fluids at ambient and elevated temperatures. A study was made of the comparative corrosion rates of mild steel in chrome lignosulfonate solutions before and after aging. From the data obtained, it was inferred that chromate, in the presence of chrome lignosulfonate, is reduced and complexed with the lignosulfonate molecule. The data show that the reaction is quite slow at room temperature, but is accelerated at elevated temperatures. The rate of reaction is also dependent on the lignosulfonate concentration. The corrosion rate studies showed that the corrosion protection provided by the added chromate is protection provided by the added chromate is dissipated after aging. It is concluded that the acceleration of corrosion rates often observed at very low chromate concentration is obviated by the presence of lignosulfonate. The reason for this is presence of lignosulfonate. The reason for this is that chromate in very low concentrations is rapidly reduced in this environment, particularly at slightly elevated temperatures.

Introduction
Since the advent of chrome lignosulfonates as dispersants for water-base drilling fluids about a decade ago, chromium has served an important function in the control of these fluids. In addition to the chromium present in the lignosulfonate molecule, soluble chromates are added to drilling muds as standard practice to improve their thermal stability. The addition of chromates to aqueous systems, generally for corrosion protection, is a practice of long standing. It has been reported in practice of long standing. It has been reported in the past, however, that chromates intensify corrosion if added in insufficient quantities. Several authors have investigated various aspects of metal lignosulfonates in drilling fluids. Jessen and Johnson studied metal exchange effects of ferrochrome lignosulfonates on sodium and calcium clays. More recently, X-ray and ion exchange data obtained by McAtee and Smith led them to suggest two mechanisms for the action of metal lignosulfonates. In view of its importance, a study of some additional aspects of chromium in drilling fluids was undertaken. It is known that chromate, a strong oxidizing agent, will ultimately oxidize chrome lignosulfonates in drilling mud systems. Polarographic analyses of chrome lignosulfonate Polarographic analyses of chrome lignosulfonate solutions were, therefore, made to determine how fast this reaction occurs. To clarify the results of these analyses, some ultracentrifugation ultrafiltration experiments were undertaken. Finally, some comparative corrosion rates of aged and heated chrome lignosulfonate solutions were made to determine if the reducing properties of the chrome lignosulfonates would alleviate the corrosion intensification effects reported for low concentrations of chromate in aqueous systems.

EXPERIMENTAL
The following experimental procedures were used.Polarographic analysis of modified (chrome) lignosulfonate (MLS) solutions and muds before and after heating, with and without added chromate.Ultracentrifugation (32,000 × g) and ultrafiltration (0.45-micron filter) of MLS solutions and muds. Chromium analysis of supernatants and solids.Comparative corrosion rates of MLS solutions and muds as determined by a corrosion rate meter before and after aging and heating.

POLAROGRAPHIC ANALYSES
POLAROGRAPHIC ANALYSES A polarograph was used to determine the half-wave potentials for chrome lignosulfonate solutions with potentials for chrome lignosulfonate solutions with and without bentonite. The values obtained are shown in Table 1.
SPEJ
P. 140