Effects of Flow Velocity on Inhibitors Used in Oil and Gas Production

Abstract

ABSTRACT This paper describes an experimental facility designed to study velocity effects on corrosion inhibitor films. These studies have application to corrosion problems found in offshore oil and gas production equipment. Initial results concerning velocity effects on the performance of oilfield corrosion inhibitors are presented. The experimental facility consists of a flow loop designed to produce two-phase flow conditions for a variety of gas-liquid ratios at velocities in the test section up to 150 fps. Nominal pressure and temperature of the loop are 100 psi and 160°F, respectively. The major loop components, the loop operation, and the data acquisition system are described. The test section is 2-inch pipe with elbows and straight segments in which electrodes for making electrochemical measurements of corrosion activity are mounted to conform to the curvature of the piping. Performance testing of the loop verified that it can produce the desired range of two-phase flow regimes including stratified, slug, froth, and mist flows. Data are presented showing the erosive effects of high flow velocity on inhibitor film performance. The effect of surface roughness on corrosion rates for two inhibitors is examined. INTRODUCTION Oil and gas production equipment is subject to corrosion damage by a variety of corrosive substances in the produced fluids. Corrosion damage has been found in production tubing, couplings, flow lines, fittings, valves, and other production equipment. Erosion plays a role too. Some investigators have suggested that dynamic effects of the flow (and possibly solid particles entrained in the flow) cause the removal of protective corrosion products from the metal surfaces resulting in accelerated corrosion rates. In many cases, the corrosion rate is thought to be related to the flow velocity of the produced fluids. Movement of the fluid across the surface of a corroding metal can accelerate the removal of the protective corrosion scale by a dissolution process or by mechanical erosion. Countermeasures for corrosion in production equipment are many and include such strategies as the use of corrosion-resistant materials, the limitation of production velocities, the use of chemical additives known as inhibitors, and others. The purpose of an inhibitor is to control the rate of corrosion by retarding electrochemical activity taking place at the metal surface. Oilfield inhibitors, used in oil and gas production, are applied to the surfaces of the materials to be protected. In a batch application, bulk quantities of the inhibitor are transported to the well tubing inlet by a carrier fluid and produced along with the production fluids. In its journey through the production equipment, the inhibitor adsorbs to exposed surfaces of the equipment to provide the desired protection. The process is repeated periodically to renew the treatment and maintain the protection. How long a treatment may provide protection depends on many factors. It is believed by some investigators [1] that velocity of the produced fluid plays an important role in determining the useful lifetime of a batch-applied inhibitor treatment. If velocities are high enough, the inhibitor may get stripped away from the metal surface by mechanical forces applied to the protected surface by the flow.