Inhibitor Film Persistence Measurement by Electrochemical Techniques

Abstract

Summary. A technique that uses corrosion-potential, corr, measurements was developed to evaluate rapidly the film persistence of commercial adsorption-type inhibitors for downhole tubular treatment. It was demonstrated that the corr measurement revealed the progressive change with time of inhibitor film in aqueous moving fluids. The advantages of coor measurements are that film effectiveness can be monitored con-tinuously with a simple device and that they are nonintrusive. Additional results of the tests showed that dissolved O2, CO2, and fluid flow drastically degraded the effectiveness of inhibitor film, and higher concentrations of inhibitors provided better performance. These indicate that simulation of process conditions is a prerequisite when the film persistence of commercial adsorption-type inhibitors is determined in a laboratory. Introduction Downhole pitting corrosion of carbon-steel tubulars has prompted the consideration of an inhibitor program in various oil fields around the world. Most commercial inhibitors are film-forming chemicals that have organic polar molecules that adsorb and form a barrier on the metal. Excellent reviews of oilfield inhibitors that deal extensively with their basic mechanisms have been given previously. One of the crucial parameters in selecting an inhibitor is to know its effectiveness in service environments. It is also important to be able to estimate film life when an inhibitor is used for batch treatment because it determines the treatment time interval. The currently popular test method is the "film persistence wheel test" in which the test cell containing coupons and fluid is rotated on the wheel to provide agitation. Weight loss is compared between treated and untreated coupons. This method does not simulate the effect of flow velocity and thus is believed to provide inconsistent values when inhibitors are screened. Some researchers have used surface chemistry tools, such as Raman and infrared spectroscopy, to detect the presence of a corrosion inhibitor on a metallic surface. But these in-situ techniques cannot be used in high-temperature/high-pressure (HTHP) systems test method has now been developed that can simulate temperature, pressure, and flow conditions while continuously and nonintrusively measuring film persistence. Two test apparatus are used: one for ambient pressure and the other for high pressure. The high-pressure loop allows simulation of gas pressure and temperature. Two electrochemical parameters were chosen for measuring rim persistence: coor and galvanic current, Ig. These have been used to determine coating stability and in measurement of atmospheric conditions, respectively. Weight-loss measurements were also made to validate and to correlate with the electrochemical measurements. Experiments Inhibitors Tested. Two adsorption-type inhibitors were selected for testing. Table 1 lists the physical properties of Inhibitors 1 and 2. Inhibitor 1 is an oil- and water-dispersible inhibitor, and Inhibitor 2 is a higher-viscosity oil-dispersible and water-insoluble inhibitor. The nature of the active components of these inhibitors is proprietary. Description of Flow Loops. Two flow loops were used in this study. Fig. 1 is a schematic of an ambient-pressure circulating loop. The loop is made up of a 75-L Type 316 stainless-steel reservoir, centrifugal pump, and specimen chamber. These parallel chambers can compare the effectiveness of two different inhibitors under the same conditions. Flow rates are measured accurately by rotometers. Teflon unions are used to hold the 5-cm [2-in.] -long specimens. To provide a well-defined flow pattern at the specimens, long flow straighteners were used at the inlet and the outlet. Fig. 2a shows a schematic of the HTHP circulating loop. The basic flow loop consists of three components: the autoclave, the pump, and the specimen chamber. The autoclave is made of Type 316 stainless steel and has a 3.78-L capacity. The pump is a 0.6-kW [3/4-hp] centrifugal pump. The pump impeller and housing are made of Type 316 stainless steel. The pump has a capacity of up to 72 L/min against a low-pressure head. The specimen holder has room for three specimens and provides pressure-tight electrical connections to the outside (see Fig. 2b). A turbine flowmeter and an ultrasonic Doppler flowmeter are used. Specimen Preparation. A tubing specimen is made from 1018 steel bar and API Grade L-80 tubular for the ambient-pressure and the high-pressure tests, respectively. For the ambient-pressure tests, the inside wall of a 5-cm [2-in.] -long specimen with a 1.22-cm [0.48-in.] ID and 0.79-cm [0.3-in.] wall thickness is polished with a 600-grit SiC paper. rinsed with acetone and distilled water, and then dried in air. Likewise, for the high-pressure tests, the same procedure is applied to the outside wall of a 2.54-cm [1-in.] -long specimen with a 1.27-cm [0.5-in.] OD. Inhibitor Treatment. In service, the concentration of an inhibitor required to develop the optimum protective barrier on tubulars varies depending on well conditions. Polished and cleaned specimens are used to allow comparison of the inhibitors without the complication of surface anomalies. The polished specimen is immersed in an inhibitor or a mixture of inhibitor and crude oil from the North Slope. Three different concentrations of inhibitor were selected: 100, 50, and 10%. The specimen is immersed for 16 hours in a mixture of the inhibitor and diluent. Afterward, the specimen is removed and left to stand for 1 hour to drain excess inhibitor. Test Procedure. The selected test solutions, formation water from the North Slope, synthetic produced water. and 1.5% NaCl are sparged with N2 at room temperature for 16 hours while circulating through the loop. During this period, the preweighted untreated and treated specimens are assembled on the specimen holder and then mounted in the specimen chamber. The chamber is purged with N2 for about 3 hours before the equilibrated solution is introduced at the desired temperature, pressure, and gas composition. Upon termination of the run, the specimen holder is retrieved within a few minutes after the system is depressurized. Visual inspection and weight measurements are followed by scanning electron microscope (SEM) and optical microscope examination. The system is flushed with tap water after the test, and an N2 blanket is left on the system between runs. P. 395^