Salinity divalent effects

Be aware that the effect of alcohol and divalents on the optimum salinity and the effect on the effective salinity are opposite, as shown by Eqs. 7.63 and 7.65. When a divalent exists in the system, the optimum salinity in terms of monovalent concentration (C51) of the system should be lower than that had the system not had the divalent. However, when a divalent does exist in the system, because of the divalent contribution to the salinity effect, the effective salinity will become higher than the salinity of C51. The alcohol effect (contribution) is opposite to the divalent effect. 

There are several factors that can affect a given surfactant s performance in a reservoir environment. First, the effect of inorganic ions is significant. Most oil reservoirs have an aqueous phase of saline brine that may vary in concentration from 0.5% to upwards of 15% NaCl. Also, there are divalent ions, such as Ca" "" " and Mg "" " present in significant concentrations. Most of the experimentation, which serves as the basis for this paper, was conducted utilizing a brine of 3% NaCl with 100 ppm (mg/1) of Ca" "" ". This composition is typical of many natural reservoir brines, and those surfactants that will perform well with this brine will also do well in the majority of reservoirs. 

To combine the effects of monovalents and divalents, UTCHEM uses the concept of effective salinity, which is defined as... 

To consider the effect of divalent cations bound to micelles, the effective salinity is (Hirasaki, 1982a)... 

When fresh water displaces saline water, dilution occurs. During dilution, divalent ions are preferentially adsorbed in comparison to a monovalent ion. For example, dilution of the solution with distilled water is accompanied by an increase of the monovalent ions (Na ) relative to the divalent cations (Ca ) when the equilibrium with the exchanger is maintained. This effect follows from Eq. 3.19 ... 

Most often, the total amount of chloride is used because NaCl is the most common salt. The justification of using it is that the current technology really cannot describe the effect of every single ion on chemical EOR. For example, when HPAM reacts with multivalent metal ions, such as Cr ", and Ti ", in a solution, a weak gel is formed. In this case, we cannot simply use Eq. 5.2 to calculate effective salinity. Equation 5.2 shows that divalents have a larger effect on the effective salinity than monovalents at the same concentration. In general, the order of effect is Mg " > Csl > Na" > The activity of these ions is 10 to 20 kJ/mol, which is much less than the value for chemical reactions (about 200 kJ/mol). Therefore, the salt effect on polymer solution is a reversible electrostatic effect (Niu et al 2006). 

The effect of increasing salinity (NaCl) concentration is to increase the level of polymer adsorption, as shown in Figure 5.40, where the adsorption at 2% total dissolved solids (TDS) is higher than that at 0.1% TDS for each pair of data (Martin et al., 1983). This observation is consistent with the prediction made by Eq. 5.31. Adding a low concentration of divalent calcium ion, Csl, promotes HPAM adsorption on silica, as shown by data from Smith (1970) in Table 5.10, because the divalent ions compress the size of the flexible HPAM molecules and reduce the static repulsion between the polymer carboxyl group and silica surface. 

The input parameters—C50 (initial brine salinity), C60 (initial brine divalents), CSEL7 and CSEU7 (Cjei and when alcohol and divalents are 0) in UTCHEM input—are effective salinities in meq/mL water. 

The effective salinity in anion concentration in the presence of alcohol and divalents is defined as ... 

Experimental data suggest that the optimum salinity varies linearly with the cosolvent concentration. Therefore, p7 can be estimated from the slope of the straight line of normalized optimnm salinity (C5i,op/C i,op) versns f in the case without divalent cations, as schematically shown in Figure 7.20. To obtain the effect of cosolvent on the shift in optimum salinity, P, we need to measure the volume fraction diagram for at least two different cosolvent concentrations and must know C i op. According to the definition, ff is defined as V7/(V7 + V3). 

Variables identified as important in the achievement of the low IFT in a W/O/S/electrolyte system are the surfactant average MW and MW distribution, surfactant molecular structure, surfactant concentration, electrolyte concentration and type, oil phase average MW and structure, temperature, and the age of the system. Salager et al. (1979b) classified the variables that affect surfactant phase behavior in three groups (1) formulation variables those factors related to the components of the system-surfactant structure, oil carbon number, salinity, and alcohol type and concentration (2) external variables temperature and pressure (3) two-position variables surfactant concentration and water/oil ratio. Some of the factors affecting IFT-related parameters are briefly discussed in this section. Some other factors, such as cosolvent, salinity, and divalent, are discussed in Section 7.4 on phase behavior. Healy et al. (1976) presented experimental results on the effects of a number of parameters. 

Pp effective salinity parameter for divalents (calcium) to calculate... 

The salinity effect of different salts, particularly divalent cation salts, is expressed through the term bS in the correlation for non-ionic surfactants of the polyethoxylated phenol or alcohol type. No information is available yet on the salinity effect on other non-ionics such as alkyl-polyglucosides. The salinity effect on ionic surfactant systems is a more complex issue because the surfactant itself is also a (more or less) dissociated electrolyte. Its degree of dissociation is paramount as far as its hydrophilicity is concerned. For instance sodium salts of alkyl sulphonic acids are essentially completely dissociated, hence they act as the sulphonate ion, and it is essentially the same with the salt of potassium or ammonium. The presence of multivalent anions produces an interference with the monovalent anionic surfactant ion, such as an alkyl benzene sulphonate, but it is essentially an ideal mixing rule. 

Salinity Salinity plays at least two important roles, namely it maintains the integrity of the reservoir and it balances the physicochemical environment so that surfactant formulation stays close to optimal. Thus, ultra-low interfacial tension and oil solubilisation are very sensitive to salinity. Mixing of the surfactant slug with connate water may alter the surfactant formulation mainly due to dilution and to the incorporation of new electrolytes to the formula. Adsorption and desorption of electrolytes, particularly divalent cations, onto or from solid materials such as clay, will also change the salinity of the aqueous phases to some extent and may cause surfactant precipitation, which is however not always an adverse effect [151]. In order to attenuate the undesirable salinity effects on formulation, surfactants able to tolerate salinity changes [109], high salinity [150] and the presence of divalent ions [112] maybe used. 

Not many investigations have been dedicated to the salinity effect when the electrolyte is not sodium chloride, maybe becau.se the effect is more complex, not amenable to a simple expression in paiilcular with divalent catioiis and ionic surfactants. However some trends are available in applied publications (27,6.1), Worth noting is a systematic study of the effect of the electrolyte anion on the equivalent salinity of different sodium salts, that showed that the correlation is followed for all sodium salts and that the effective or equivalent molar salinity only depends on the valency of the anion t64). 

The effect of monovalent and divalent salts on the solubility of these hydrophobically associating polymers (HAPs) is similar to that of ionic surfactants. An increase in salt content decreases solubility. With increasing salinity, the hydrocarbon chains are forced into closer proximity to the point where the subtle balance between hydrophobic associative forces and hydrophilic hydration forces breaks down, and phase separation results. Divalent cations have a larger effect on decreasing polymer solubility than do alkaline earth or monovalent cations. This is particularly so when the polymer contains anionic functionality such as acrylate or sulfonate. Another interesting phenomena occurs in mixed salts with certain polymer compositions when the ratio of divalent to monovalent cation is varied. A window of solubility is observed similar to that found with anionic surfactant solutions. 

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