A Novel Approach for Monitoring of CaCO3 and BaSO4 Scale Formation

Abstract

Abstract To date much emphasis on scale inhibitor evaluation and studies of scale kinetics has been given to studies using static beaker tests. This paper describes an electrochemically based technique, which allowed the extent of scaling on a solid surface to be determined in presence and in absence of scale inhibitor. The ability of the technique to provide kinetic information on surface scaling will be demonstrated with two kinds of scale. Finally the potential for this technique as a method for on-line monitoring of scale formation downhole will be discussed. Introduction The formation of scale (e.g. carbonates, sulphates) on the surface of production equipment and in the pores of rock is a problem which imposes massive costs to oilfield operators1,2. The deposition of scale can have severe consequences in which the efficiency of oil recovery is reduced or in the extreme cases the reservoir can become plugged. One of the most troublesome scales found in oil production processes is barium sulphate, the formation of which is often due to the mixing of two incompatible waters during seawater injection when the sulphate in the seawater combines with the high concentration of barium ions in the offshore reservoir3,4. In the case of inverse solubility salts such as calcium carbonate, their formation is often associated with heat transfer surfaces5 and as such they present a problem in a wide range of process industries from oil production to desalination3,6,7. Extensive research effort has been concentrated in the field of scaling, or fouling as it is often referred, and this has been divided into three main areas : kinetics of precipitation, prediction models and inhibitor development and technology8,9. As a result, there have been significant advances in the understanding of the fundamental mechanisms of precipitation and in the chemistry of scale inhibitors. However, scaling continues to cause operational problems in the oil industry in particular where access to scaled components is often difficult and requires expensive shut down periods for maintenance10. It has recently been stated by Hasson11 that one of the main areas which requires attention in this field is the relationship between scale formation by precipitation in the bulk solution and scale formation at a solid surface. In his paper he stated that there is little known about how inhibition occurs in both of these processes and that field experience has shown that often inhibition does not follow the same trend as was predicted by laboratory tests using assessment of the bulk solution precipitation rates11–13. It is against this that the current research was undertaken. A new technique has been developed which enables the extent of surface scaling (measured as a percentage coverage of the surface) to be determined by an electrochemical assessment14,15. By combining the information obtained from the study of scale formation at a solid surface with measurements of the solution turbidity and amount of precipitate formed, a comparison can be made of the scale kinetics on the surface and in the bulk solution. The efficiency of inhibition in both processes can also be assessed. In this paper the formation of CaCO3 and BaSO4 on a solid surface and in the bulk solution are studied using a combined electrochemical/bulk chemistry approach. It has been shown that the efficiency of the commercial scale inhibitor PolyPhosphinoCarboxylic Acid (PPCA) in the bulk and at the surface can be dependent on the type of scale formed. The purpose of this paper is not to compare the kinetics of BaSO4 and CaCO3 scale formation but is to compare the inhibitor efficiency in controlling surface and bulk scaling of the two species. The electrochemical measurements made on the surface also enable the formation of a film of inhibitor absorbed at the surface to be determined and some preliminary results demonstrating this phenomena are presented.