A study of surfactant flooding at high salinity and hardness

Interactions of Hardness Ions with Polymeric Scale Inhibitors in Aqueous Systems

The interactions of hardness ions (i. e., Ca, Mg, Ba) with anionic polymeric scale inhibitors (SI) containing different functional groups has been investigated using a turbidity technique. It has been found that all SI tested form insoluble salts with hardness ions under the experimental conditions used (pH 7.0 to 10.0, 25 to 65 °C, 100 to 1,000 mg/L Ca). The results indicate that compatibility of hardness ions with SI decreases with increasing pH, temperature, and hardness ion concentrations and increases with increasing NaCl concentrations. It has also been found that compatibility of hardness ion with SI can be improved by incorporating bulkier and hydrophobic monomer into SI structure. In addition, the data suggest that SI architecture (i. e., molecular weight, monomer functional group, and nature of polymerization solvents) influences the interactions of hardness ions with SI in aqueous systems.

Surfactant Flooding With Hard Water: A Case Study Solved by HLB Gradient

Summary.

Surfactant formulations were designed for field application of micellar processes in a salty environment on the basis of a mixture of one synthetic sulfonate with two types of nonionic agents. This formulation led to the required Winsor-III-type phase diagram in the presence of reservoir fluids at reservoir temperature. The main difficulty in this study was the high level of surfactant retention in the reservoir rock. The sacrificial agents tested did not reduce these surfactant losses significantly. The solution found to be efficient consisted of creating a hydrophile-lipophile balance (HLB) gradient of the additives used in situ by injecting a desorbing agent with high HLB behind the micellar slug containing the nonionic cosurfactants with lower HLB. The interpretation proposed for such a procedure shows that the mechanisms involved produce proposed for such a procedure shows that the mechanisms involved produce the same beneficial effects as the salinity gradient technique. The good results obtained indicate the potential importance of the HLB gradient when the salinity gradient cannot be used in the micellar processes under consideration.

Introduction

The implementation of micellar processes using hard water like sea-water, containing about 1,500 ppm bivalent ions (Ca++ and Mg++), can result in the elimination of alcohol- and petroleum- sulfonate-based formulations from use because of the risk that these surfactants will be precipitated during propagation of the micellar slug in the reservoir. Other types of surfactants or cosurfactants better suited to calcic salty media should be used e.g., modified anionic agents containing both polar and ethylene oxide groups in their molecule. Such products are more costly than sulfonates, however, and may be used optimally at high salinities or temperatures. At medium temperatures and in a salinity range close to seawater, sulfonate mixtures combined with nonionic surfactants that are much less sensitive to bivalent ions should be considered. This type of formulation has been studied systematically to match the conditions of the application discussed here i.e.. a reservoir at 43C [109F] and an injection brine similar to seawater. Among the many nonionic agents available, the ones chosen come from the chemical family of ethoxylated fatty alcohols with the general formula

C H -(O-C H ) -OH. n 2n+1 2 4 m

The left part of the molecule brings a lipophilic trend in balance with the right part, which provides the hydrophilic character. The well-known concept of HLB is an expression of this dual structure of tensio-active molecules. The HLB of the ethoxylated nonionics investigated here can be calculated simply by dividing the ethylene oxide weight content in their molecule by five. The advantage of these products, which cost less than sulfonates, comes from the wide range of HLB (8 to 20) that they can cover. This makes the adjustment of formulations to specific reservoir conditions more flexible and more accurate.

Characteristics of the Formulations Suitable for Formation Fluids

Maini and Batycki's approach to match one sulfonate to two non-ionic surfactants that have a different affinity for water and oil was used. To make this approach more exact, the HLB concept was applied to the nonionic agents used. The average HLB of the two ethoxylated alcohols used in the formulations was computed according to the method explained previously pro rata to their weighted proportions. This average HLB previously pro rata to their weighted proportions. This average HLB was varied steadily in the mixtures of synthetic sulfonate (60% active material) and nonionic agents used to determine the optimum phase behavior.

Phase Diagrams.

The phase diagrams were determined at 43C Phase Diagrams. The phase diagrams were determined at 43C [109F], the studied reservoir's temperature, with crude stock-tank oil (specific gravity=0.838 and viscosity=6 mPas [6 cp]) and a synthetic injection water (see composition in Table 1). The type of diagram obtained depended on the sulfonate proportions in the mixtures (50/50 here) and on the average HLB of the nonionic agents associated with the sulfonate. For an average HLB of 12.24, the phase diagram is of the Winsor I type. Fig. 1 and Table 2 give the properties and compositions of the micellar phases in equilibrium for different initial preparations. Microemulsion compositions in Table 2 were calculated by material balance from phase volume and specific-gravity determinations at equilibrium. All three surfactants were assumed to be in the lower phase. phase. As Fig. 1 shows, the representative points appear close to the experimentally determined phase boundary line of the phase diagram. By decreasing the hydrophilic balance of the two nonionic agents associated with the sulfonate i. e., with an HLB = 11.6 the diagram of the Winsor III type was obtained (see Fig. 2). Figs. 1 and 2 also give the kinematic viscosities (at 43C [109F]) for a series of single-phase micellar systems as well as the interfacial tensions (IFT's) measured at reservoir temperature with the spinning-drop apparatus. In Fig. 2, IFT values found for two-phase systems are indicated in micronewtons per meter within the Winsor II lobe of the diagram. Other measurements were also carried out on triphasic systems; /E or E/w were always in the range 0.5 to 5 N/m. A pseudoternary diagram has been used in Fig. 2 for convenience. Actually, the analysis of triphasic systems in equilibrium revealed (left side of Table 3) a partition of the salts between the excess water and the micellar-phase water. The injection water thus is not a true pseudocomponent. pseudocomponent. Changes in Equilibrium Data on Contact With Reservoir Rock

Chemical interactions between surfactants (sulfonate) and reservoir rock have been shown by several investigators through oil recovery experiments that used sandstone and shale core samples. Therefore, to complete the investigation of phase equilibria, we used a static test to assess the possible effects of such interactions on equilibrium data. In this test, 2 g of disintegrated reservoir rock (average specific surface area of 0.23 m2/g) was placed in tubes containing 10 g of fluid mixture. Under these conditions. the phase volumes decanted were found to be appreciably different no matter what type of micellar system was obtained.

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