Field Evaluation of Catalytic Deoxygenation Process for Oxygen Scavenging in Oilfield Waters

Abstract

Summary. An extended field test at the Grubb Lease, San Miguelito field, Ventura, CA, has shown that the catalytic deoxygenation (Cadeox TM) process is an effective new oxygen-scavenging method for oilfield injection waters. The process can reduce oxygen concentration from the saturation level in seawater (about 8 ppm) to substantially below the corrosive level (less than or equal to 0.01 ppm) with less than 60 seconds of contact time. The process is effective at temperatures as low as 5 deg. C [41 deg. F] and at process is effective at temperatures as low as 5 deg. C [41 deg. F] and at a water pH range of 4 to 10. The Cadeox process uses a simple hydrogen and oxygen reaction to remove corrosive oxygen from injection waters. Palladium-impregnated anion-exchange resin beads in a packed column are used as reaction sites for H2 and O2 to form water. Introduction Dissolved oxygen in water can cause destructive corrosion to metal pipes and process equipment. The corrosion byproducts, in turn, cause formation damage by plugging. Thus, oxygen needs to be removed from oilfield waters. In the past, chemical and/or mechanical methods have been used to remove oxygen. However, the chemical scavenging systems--e.g., sodium sulfite (Na2SO3)--introduce unnecessary dissolved solids (sulfates) to treating waters, and the commonly used vacuum deaerators are bulky and costly. The Cadeox method presented is a new process in oilfield oxygen scavenging. (This procedure was originally used for scavenging oxygen in boiler feedwaters in Europe.) Cadeox uses palladium-covered anion-exchange resin beads as the catalyst. palladium-covered anion-exchange resin beads as the catalyst. Oxygen dissolved in water is catalytically reduced by its reaction with hydrogen to form water. To test the applicability of this scavenging process for injection waters (seawater), an extended field test was conducted at the oceanwater treatment plant (OWTP) of the Grubb Lease, San Miguelito field. The test results indicated that Cadeox is a viable process for scavenging oxygen from seawater. Background Catalytic Reduction of Oxygen. Oxygen can be removed from water by its reaction with hydrogen under appropriate conditions to form water:(1) The reaction is exothermic; however, a catalyst is necessary to accelerate the process. Otherwise, the rate is too slow to be effective, particularly at low temperatures (less than 20 deg. C [less than 68 deg. F]). Catalysts that can be used in the reaction are palladium (Pd) or other metals from the eighth group of the periodic table. In our application, Pd was chosen because Pd-covered resins were commercially available. Polystyrene-based anion-exchange resins are used as the catalyst-carrier material because anion-exchange resins have very high chemical and mechanical stability. Finely dispersed metallic palladium is impregnated in the matrix of the resin. (The resin itself does not enter into the scavenging action.) The concentration of palladium in a resin matrix decreases from near the surface to the center of the bead. The catalyzed reaction between dissolved oxygen and hydrogen occurs stoichiometrically--8 g of 02 reacts with 1 g of H2. The final product is water. Therefore, this scavenging method does not add unwanted dissolved solids (to water) as do chemical scavenging methods. Theoretically, the catalyst should last indefinitely. The hydrogen gas is readily available. Thus, the catalytic reduction of oxygen can be an economical way to remove oxygen in water. Dissolved Oxygen in Water. The amount of dissolved oxygen in water depends on the solubility of oxygen in water. Solubility of oxygen is primarily a function of pressure, temperature, and salinity. When the partial pressure of oxygen vs. concentration is plotted, there is a linear relationship between the two. For example, plotted, there is a linear relationship between the two. For example, if at a partial pressure of 101 kPa [1 atm], 02 saturation is 41.3 mg/L, then at 21.2 kPa [0.21 atm], the saturation is 8.6 mg/L.4 Therefore, with the slope of the line known, the saturated oxygen concentration can be estimated at any partial pressure (at a given temperature). An increase in temperature generally causes a decrease in the solubility of a gas in a liquid. This inverse relationship is shown in Fig. 1. If the temperature of a gaseous solution is allowed to increase continuously (at atmospheric pressure), then complete degassing will ultimately occur (near the boiling point of water). When a strong electrolyte is present in a solution, the activity coefficient of the solution is increased. The concentration of dissolved oxygen decreases as the activity coefficient increases. Therefore, the solubility of oxygen in water decreases as total salt concentration increases. As a result, the hydrogen concentration necessary to achieve stoichiometric reaction also varies with temperature, pressure, and salinity. Experimental Procedures and Equipment Setup OWTP. At the Grubb Lease OWTP, the ocean water is pumped through a rotary screen to remove the large debris. The water is then treated with chlorine and alum as it enters a clarifier. Effluent from the clarifier goes into a downflow sand filter and into a clear well. A booster pump then lifts the water to a vacuum tower for deaeration. From the vacuum tower, the water is rechlorinated and transferred to the separate, lease-water-injection plants. The Cadeox feedwater is taken from the discharge of the clear-well booster pump (see Fig. 2). Cadeox Flow System. As shown in Fig. 3, the oxygen-saturated (6- to 8-ppm) feedwater enters the test system at 207 to 276 kPa [30 to 40 psig]. It is filtered through a 3-mu m cartridge filter. The seawater flow rate is controlled at 1.89 X 10–3 m3/min [0.5 gal/min] by a flow-control valve and measured with a rotometer. The water then enters a cell and is saturated with hydrogen entering at 20 to 25 mL/min, which is 5 to 10% above the stoichiometric amount of necessary H2. SPEPE P. 619

Catalytic Deoxygenation of Water: Preparation, Deactivation, and Regeneration of Palladium on a Resin Catalyst

“…Metals supported on ion exchange resins have been studied in several processes, such as the reduction of nitrates on drinking water, the elimination of oxygen dissolved in water, − alkene epoxidation and hydroformylation of styrene . Particularly, palladium supported on resins has been addressed in several publications and reaction systems − with particular attention on hydrogenations and nitrates reduction …”

“…Even though a commercial process has been developed and used in many applications, catalytic water deoxygenation has not been addressed in the open literature, neither in order to be applied for beverage production using drinking water nor for the treatment of demineralized water for heating circuits. Only short reports can be found related to this process. − …”