Numerous computer programs and calculations exist to predict the type(s) and severity of scale deposition, This paper compares scale index calculations, two scaling indices computing programs, and two computer predictive models on two wells with the actual field experience. The computer programs? advantages and disadvantages are considered along with their accuracy in predicting field scale problems. The programs reviewed by this paper are tools to be used in conjunction with personal experience of water chemistry, field conditions,-a program?s reproducibility, and one?s confidence level deposition, severity, and treating regimes.
Numerous publications have been written explaining the basis of scale index calculations and computer modeling programs from simple saturation levels of the reactants to including multiple parameters such as common ion effects. Most of the calculations and programs available yield deposition potentials (scale indices) based upon several user defined parameters such as ion concentrations, temperature, pH, pressure, carbon dioxide partial pressures, organic acids, etc. The trend with oil companies and oilfield service companies is to routinely use single point water analysis results to predict the potential and severity of scale problems of a well or water handling system. Small changes in the water chemistry, operating parameters, and/or inaccurate analytical results may invalidate the results of these single point evaluations; therefore, many users of the scale indices perform multiple calculations using two main parameters, pH and temperature, to bracket the potential effects.
This paper compares single point water analysis results on Wells Eand F from the Medrano Field at Cyril, Oklahoma, Both wells E and F have experienced severe barium sulfate scaling problems and Wells E and F exhibited mild and severe calcium carbonate scale problems respectively. Calcium sulfate scale was not significant in these wells. The well conditions and parameters are listed in Table 1. Studies provided by a third party chemical supplier for scale prediction and prevention resulted in the application of a polyacrylate scale inhibitor and later a phosphonate scale inhibitor at dosages of 30 parts per million based upon total fluid production, Unfortunately, the scale inhibitors and the dosage level proved to be incompatible with the produced fluid. One treatment caused a severe secondary scale problem that was identified as being an iron-calcium-poly acrylate complex.