Quantitative Determination Of Lignosulfonates in Drilling Fluids By Ultraviolet Absorption Analysis
Authors: Carney Leroy L. , Skelly William G. , Gullickson Richard 
Published: December 2007

The method described here for determining the concentration of modified lignosulfonates in drilling fluids has been applied to field muds to show, among other things, the effect of thermal degradation on lignosulfonate and the distribution of lignosulfonates in the feed to and the effluents from a centrifuge.

Introduction

Lignosulfonates have been used in drilling fluids for years without benefit of a method for determining the amount present in a given system at a specific time.

High temperatures cause lignosulfonates to undergo certain degradational changes that alter their effectiveness. It would therefore be desirable to have a rapid method for monitoring the effective concentration of lignosulfonate at various times and points within a drilling fluid system. The method should be rapid enough to determine the concentration of lignosulfonate in mud from the suction pit and in the same mud at the flowline after one circulation, so that any change during one circulation can be noted.

Lignosulfonates are known to have an ultraviolet absorption maximum at about 281 nanometers (nm). The magnitude of this absorption peak can be related to the concentration of lignosulfonate in an aqueous medium, even at extreme dilution, and it has been found that the magnitude of this peak is not greatly affected by the pH of the solution.

These observations formed the basis for developing a method to determine lignosulfonate concentrations in water-base drilling muds. A correlation was made between the rheology and lignosulfonate concentration of muds prepared in the laboratory and those of muds taken from the field. (This and subsequent references to lignosulfonate apply to modified lignosulfonates of the chrome or ferrochrome type.)

Methodology

The first step of the method basically consists of suspending a small sample of the drilling fluid in water; 1 ml is usually sufficient. Ammonium acetate is added to the dilute water suspension to buffer the solution and to flocculate the clay minerals that are present. The suspension is then centrifuged until the present. The suspension is then centrifuged until the clay minerals and other solids are separated from the lignosulfonates. The supernatant solution is examined by ultraviolet absorption, and the optical density (O.D.) difference due to the 281-nm peak is determined (Figs. 1 and 2). This difference can then be related to the concentration of lignosulfonate present in the original sample.

The following general procedure is applicable to most water-base drilling muds containing modified lignosulfonate:

Pipette 1 ml of mud from a well stiffed suspension. Dilute the mud with water to 200 ml, add 5 ml of 1N ammonium acetate and dilute with water to 250 ml (higher and lower dilutions may be used, depending on the concentration of lignosulfonate in the mud). Centrifuge aliquots of the well mixed suspension at 13,000 rpm for 10 to 15 minutes. Place a portion of the clear supernatant solution in a 10-mm silica cell. Scan and record the spectrum from 320 to 240 nm. Any scanning instrument capable of operating from the 500- to 240-nm wavelengths is suitable. (We used Beckman DBG instruments.)

JPT

P. 651