Scale prediction for iron, zinc and lead sulphides and its relation to scale test design

The accurate prediction and management of oilfield sulphide scales, such as iron sulphide, is an important issue in oil production. This has become particularly significant as high temperature high pressure (HTHP) fields are being brought into production, and the life of mature fields is being extended. In such systems, additional scales such zinc and lead sulphide (ZnS and PbS) are often being reported. This paper presents a detailed description of a sulphide modelling approach, leading to the prediction of saturation ratios (SRs) and masses of the formed sulphide scales, final solution compositions, final pH levels etc. The equilibrium equations for the sulphide system are presented and solved in a manner in which they are compared directly with the experimentally measured quantities. The actual Saturation Ratios (SRs) (e.g. SR = [Fe²⁺][S^2-]/Ksp,FeS) are calculated for the various experiments and the prediction model is used directly to the design the details of the sulphide scaling experiment in the blank solutions. Some calculated sulphide examples are presented and some key predictions of the sulphide scaling model are tested experimentally for FeS, ZnS and PbS systems. The quantitative agreement between the predictions of the model and the experiments are very good. The resulting sulphide test methodology is thus well underpinned theoretically and it is then applied to evaluate some examples of proposed commercial sulphide inhibitors/dispersants.