Stiff-Davis index

The Stijf-Davis index attempts to overcome the shortcomings of the LSI with respect to waters with high total dissolved solids and the impact of "common ion" effects on the scale formation driving force. Like the LSI, the Stiff-Davis index has its basis in the concept of saturation level. The solubility product used to predict the pH at saturation (pHs) for a water is empirically modified in the Stiff-Davis index. The Stiff-Davis index will predict that water is less scale forming than the LSI calculated for the same water chemistry and conditions. The deviation between the indices increases with ionic strength. Interpretation of the index is by the same scale as for the LSI. 

The calcium carbonate group includes calcite saturation level, the Langelier saturation index, the Stiff-Davis index, and the Oddo-Tomson index. 

Langelier Saturation Index is valid up to about 4,000 ppm TDS. At higher TDS concentrations, the Stiff-Davis Saturation Index (SDSI) is used, Equation 3.12 ... 

The program calculates total dissolved solids (TDS), conductivity, and the Langelier and Stiff-Davis Saturation Indexes (see Chapter 3.9). The screen allows the designer to use sodium, calcium, magnesium, chloride, sulfate, or bicarbonate to balance the water analysis. Water quality for up to five feed streams can be entered and blended together to make the total, combined feed water to the system. 

For high-salinity water (>5 g-T ), the calculation of the stability index must be modified. The effect of the salts must be taken into account and the Stiff and Davis index must be used (for brackish water and seawater). 

The Ryznar index can haidly be used because it is based on the Langelier graph. The latter is valid only for salinity lower than 5 g l". The Stiff and Davis index must therefore be used. 

The Oddo-Tomson index accounts for the impact of absolute pressure and partial pressure of carbon dioxide on the pH of water, and on the solubility of calcium carbonate [20]. This empirical model also incorporates corrections for the presence of two or three phases (water, gas, and oil). Interpretation of the index is by the same scale as for the LSI and Stiff-Davis indices. 

The solubility product of salt changes with temperature and the concentration of the ions present in solution. In oilfield brines, if the surface solubility produced is smaller than their solubility product under down-hole formation, scale formation would occur. Under such conditions, the water is called unstable water. The StabiUty Index (S/) is used to predict the tendency of ions to form scales from oilfield brines which are highly corrosive and encountered during drilling process. The Stiff-Davis Modification of the Langelier equation is used to predict calcium carbonate precipitation from the brines and is given by... 

Calculations based on Stiff and Davies stability index (Ref. 11 indicated that under normal producing temperatures formation water 1s not expected to form scale. However, the high skin temperature (up to 150° C) of the crude heaters will cause severe CaCO-scaling. This expectation was confirmed by laboratory tests using synthetic formation water. The result indicates a requirement to Inject scale inhibitor upstream of the heaters. A polyphosphate scale inhibitor was found to be effective in the laboratory tests. 

Other indices proposed that have tried to overcome the basic limitations of LSI include the Stiff and Davies Saturation Index, Larson and Buswell Index, Puckorius (Practical) Scale Index, Oddo-Tomson Index, and Larson-Skold Corrosivity Index, some of which are discussed briefly here. 

The scaling tendency of a given feed water is often evaluated using the Langelier Saturation Index (LSI) for brackish water and the Stiff and Davis Stability Index (S DSI) for seawater (LSI is acceptable for seawater). LSI and S DSI are defined below ... 

Stiff and Davis have simphfied the work of Langeher on the scaling index of oil-field waters, i.e. their tendency to deposit calcium carbonate scale. They defined the stability index (SI) as follows ... 

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