Effects of Commonly, Used Oilfield Chemicals on the Rate of Oxygen Scavenging by Sulfite/Bisulfite

Abstract

Summary. The effect of common oilfield biocides, corrosion inhibitors, scale preventives, and alcohols on the rate of O2 scavenging by sulfite/bisulfite is described. Emphasis is placed on the effect of the functional group of each of the chemical types. An attempt is made to explain the results in terms of the free-radical mechanism. Introduction In many EOR floods, it is difficult to control corrosion and polymer degradation without sufficient removal of O2 from the polymer degradation without sufficient removal of O2 from the water system. The most common treatment for this removal uses the oxidation-reduction reaction of sulfite/bisulfite with O2. In many of these applications, successful treatment includes not only reducing the O2 to acceptable levels but reaching these levels within a limited time. Therefore, it is important to understand any conditions that can affect the rate of the scavenging reaction. Since Backstrom's and Alyea and Backstrom's studies of the thermal and photo-oxidation of sodium sulfite solutions, this scavenging reaction has been known to proceed by a chain mechanism. The literature suggests that this reaction is quite sensitive to the presence of additional chemicals. This sensitivity is not surprising because, as a chain reaction, the process involves highly reactive chain carriers. Because of this extreme sensitivity, the exact reproducibility necessary for a complete mechanistic analysis is a difficult task. In many cases, however, the reproducibility of +/- 20% that we experienced is adequate to indicate the mechanism by which the additive affects the reaction. This paper reports preliminary investigations into the effects of common oilfield chemicals on the O2-scavenging reaction, with insights into mechanistic explanations when possible. These water-soluble oilfield chemicals include biocides, corrosion inhibitors, scale preventives, and alcohols, which are used as solvents and freeze-point depressants. Many of the actual commercial chemicals are complex multicomponent formulations or contain active species that are complex multifunctional chemical structures. Because isolation of the effect of one component or functional group in these complex formulations is difficult, simple model compounds have been used in this study. These model compounds have been chosen with consideration to purity, simplicity, and the presence of the appropriate chemical functional group. Experimental The reaction cell consisted of a water-jacketed 250-mL beaker and a rubber stopper. Fitted into the stopper wasa combination pH electrode attached to an Orion 407A(TM) specific ion meter, pH electrode attached to an Orion 407A(TM) specific ion meter,a YSI 5750(TM) O2 probe attached to an O2 meter, andtwo gauge-18 syringe needles for solution injection and volume relief. Both pH readings and O2 levels were simultaneously recorded on a dual-channel recorder. Temperature was regulated at 25 degrees C [77 degrees F] with a circulating water bath through the outside jacket of the cell. All measurements were performed with 1% KCl as the test solution. The pH of the performed with 1% KCl as the test solution. The pH of the reacting solution was regulated to +/ 0.1 unit by the manual addition of a dilute NaHCO3 solution. Method of Analysis The rate of a chain reaction of long chain length may be represented symbolically by a chain-initiating step, I, a chain-propagating step, P, and a chain-terminating step, T: (1) where R is the rate of a chain reaction and n is a small integer, usually 1 or 2. The rates of initiation and termination are equal-i.e., I= T. The factor I/T is introduced to cancel from the rate law the concentrations of chain carrier(s) that appear in P and T. If the termination involves one chain carrier, n will be unity; if it involves two chain carriers, n will be two. Analysis of rate expressions in terms of Eq. 1 can help in the determination of the role of additives in the chain process. This method of analysis may not lead to a unique, unambiguous resolution into initiation, propagation, and termination expressions in all cases. The following guidelines, however, have been used in an attempt to produce a consistent and physically realistic interpretation.1. The additive is allowed to affect only one process.2. The concentration of the additive to the first power is present in the rate-law expression for the affected process. This is consistent with a bimolecular collision process. SPEPE P. 137