Prediction of Sulfate Scaling Tendency in Oilfield Operations (includes associated papers 23469 and 23470 )
Geochemistry(18%), Health safety and environment(14%)
Summary. This paper presents a model for predicting the scaling tendencies of barium, strontium, and calcium sulfates resulting from the mixing of injected and formation waters and from temperature and pressure effects. The model also predicts competitive simultaneous coprecipitation of BaSO4, CaSO4, and SrSO4, where sulfate is the common ion, reflecting the precipitation of more than one sulfate mineral. The supersaturations and amounts of precipitation of the sulfates are calculated from their solubilities, which in turn are calculated by the Pitzer equation for electrolyte ion activity coefficients.
In North Sea operations where seawater injection is a common development practice, barium, calcium, and strontium sulfate scale deposition is a concern. Barium sulfate and related scale occurrence is considered a serious potential problem that causes formation damage near the production-well zone. Sulfate scales may result from changes m temperature and/or pressure while water flows from one location to another, but the major cause of sulfate scaling is the chemical incompatibility between the injected seawater, which is high in sulfate ions, and the formation water, which originally contains high concentrations of barium, calcium, and/or strontium ions. An accurate, convenient, and fast model capable of predicting such scaling problems may be helpful in planning a waterflood scheme. It may also aid in selection of an effective scale prevention technique through the prediction of scaling tendency, type, and potential severity. The model presented here is considered an improvement on the previous models.
Previous models neglected various aspects that affect scaling, and as a result, large errors may occur in scale prediction at certain conditions. Early prediction methods did not consider the pressure effect on scaling. Another shortcoming of some of the previous models lies in the assumption of salt solubility as the unique function of sodium chloride concentration or ionic strength. Pucknell did consider the specific ion effect on solubility caused by the existence of Mg2+ and SO42-ions, but his solution is rather empirical and not very reliable. Most of the models predict scale formation of only one mineral without considering the effect of the potential formation of other minerals in the same solutions. This simplified treatment may lead to erratic conclusions when different scaling ions compete for a common ion component to form scale. For example, in a solution containing Ba2 +, Ca2 +, and SO42 - ions, SO4 2 - ions are shared by Ba2 + and Ca2 + ions in forming BaSO4 and CaSO4 scales, and a separate scale prediction for either BaSO4 or CaSO4 must give incorrect results by assuming that all the SO4 2 - ions in the solution are involved in the scale formation of only one sulfate mineral. Vetter et al. reported a model for predicting simultaneous precipitation of CaSO4, BaSO4, and SrSO4- Fig. 2 of Ref. 6 suggests that the effect of scaling of a less soluble sulfate, such as BaSO4, on the precipitation of more soluble SrSO4 and CaSO4 was considered, but the reverse effect (such as CaSO4 on BaSO4 and SrSO4) was not taken into account.
The model described in this paper was developed from a solubility-prediction model that generates sulfate solubilities at various solution compositions, temperatures, and pressures. The solubilities used in the scaling-tendency prediction are calculated from the Pitzer equation for electrolyte mean activity coefficients in aqueous solutions, which has been widely used since it was proposed by Pitzer and his coauthors. Solubilities of sparingly soluble salts (such as BaSO4, CaSO4, and SrSO4) over a-wide range of ion concentrations and compositions have been accurately calculated at 25 degrees C with the Pitzer equation. In the model presented in this paper, the Pitzer equation was successfully used in calculating CaSO4, BaSO4, and SrSO4 solubilities over wide ranges of temperatures and solution compositions. The calculated solubilities agree reasonably well with the published data.
An iterative model based on the solubility prediction model was developed for predicting sulfate scaling tendency.
In the North Sea, it is common for two or three Ba +, Sr2 + , and Ca2+ ions to coexist in a formation water and to precipitate with SO4(2-)ions. In such a situation, the separate calculation of one of scale without taking into account the effect of the consumption of SO4(2-) ions by the other scale precipitation is unable to reflect the competition for SO4(2-)anions between the scaling cations. On the other hand, sequential calculation of the precivitation of the three sulfates starting from the least soluble BaSO4, then SrSO4, and finally the most soluble CaSO4 also ignores the effect of the more soluble scale formation (CaSO4) on the less soluble scale formation (BaSO4). To avoid these deficiencies, the model includes an iteration method for calculating the simultaneous scaling problems of more than one sulfate mineral. The model predicts sulfate scaling problems based on the thermodynamic solubilities. The influence of kinetic or dynamic factors is not taken into account.
A detailed description of the Pitzer equation, the parameterization of the virial coefficients in the equation, solubility prediction, and the development of the scaling prediction model are given here. The predicted results for single solutions and mixed waters, the temperature and pressure effects, and the scaling-tendency change with the mixing ratio of two waters are discussed and come with the results from previous models.
The general form of the Pitzer equation for electrolyte mean activity coefficient is
In ymx = [zmzx]fy+(2vM/v) ma[BMa+(Emz)CMa a
+(vX/vM) Xa]+(2vX/v) Mc[BcX+( mz)CcX c
+(vM/vX) Mc]+ mcma[ zMzX B'ca c a +v-1(2vMzMCca+vM Mca+vX caX)]
+1/2 McMc'[(vX/v) cc'X+ zMzX 'cc'] c c' +1/2 MaMa'[(vM/v) Maa'+zMzX 'aa'],.......(1) a a'
where mz= McZc= Ma Za . c a
Eq. 1 is composed of two parts. The first part is a modified Debye-Huckel term shown in Eq. 2 that accounts for the long-range electrostatic interaction between ions:
fY = -A [I1/2 /(l+l.21 1/2)+(2/1.2)ln(1+1.21 1/2)].......... (2)