5.1 Corrosion inhibitors continue to play a key role in controlling internal corrosion associated with oil and gas production and transportation. This results primarily from the industry's extensive use of carbon and low alloy steels, which, for many applications, are economic materials of construction that generally exhibit poor corrosion resistance. As a consequence, there is a strong reliance on inhibitor deployment for achieving cost-effective corrosion control, especially for treating long flowlines and main export pipelines (1).⁶

5.2 For multiphase flow, the aqueous-oil-gas interphases can take any of an infinite number of possible forms. These forms are delineated into certain classes of interfacial distribution called flow regimes. The flow regimes depend on the inclination of the pipe (that is, vertical or horizontal), flow rate (based on production rate), and flow direction (that is, upward or downward). The common flow regimes in vertical upward flow, vertical downward flow, and horizontal flow are presented in Figs. 1-3 respectively (2, 3).

Note 1—ρ_G and ρ_L are gas and liquid densities and U_L and U_G are superficial velocities or the volume of flow rates of the liquid and gas per unit cross-sectional area of the channel (2).

FIG. 1 Flow Regimes for Vertical Upward Multiphase Flow

FIG. 2 Flow Regimes for Vertical Downward Flow (2)

Note 1—Boundary conditions given by two studies are presented.(2)

FIG. 3 Flow Regimes for Horizontal Flow

5.3 Depending on the flow regime, the pipe may undergo various forms of corrosion, including general, localized, flow-induced, and erosion-corrosion. One of the predominant failure mechanisms of multiphase systems is pitting corrosion.

5.4 The performance of a corrosion inhibitor is influenced primarily by the nature of inhibitor, operating conditions of a system, and the method by which it is added. Two types of inhibitors are used in the oil field, continuous and batch. Water-soluble and oil-soluble, water-dispersible inhibitors are added continuously. Oil-soluble inhibitors are, in general, batch treated. The test methods to evaluate the inhibitors for a particular field should be carried so that the operating conditions of the system are simulated. Thus during the evaluation of a corrosion inhibitor, an important first step is to identify the field conditions under which the inhibitor is intended to be used. The environmental conditions in the field locations will dictate the laboratory conditions under which testing is carried out.

5.5 Various parameters that influence corrosion rates, and hence, inhibitor performance in a given system are (1) composition of material (2) composition of gas and liquid (3) temperature (4) flow and (5) pressure.

5.6 Composition of material, composition of gas and liquid (oil and water), temperature, and pressure are direct variables. Simulation of them in the laboratory is direct. Laboratory experiments are carried out at the temperature of the field using coupons or electrodes made out of the field material (for example, carbon steel). The effect of pressure is simulated by using a gas mixture with a composition similar to the field for atmospheric experiments and by using partial pressures similar to those in the field for high pressure experiments.

5.7 In multiphase systems there are three phases, oil, aqueous (brine water), and gas. Corrosion occurs at places where the aqueous phase contacts the material (for example, steel). The corrosivity of the aqueous phase is influenced by the composition and the concentration of dissolved gases (for example, H₂S and CO₂). In evaluating corrosion inhibitors in the laboratory, aqueous phase is usually used with a positive pressure of gas mixture to simulate the gaseous phase. The oil may have a major effect on the corrosion rate and inhibitor efficiency. The presence of oil phase in the test environment can have significantly different effects (4). The primary effect of the oil phase is apparently on the protectiveness of the corrosion inhibitor. The oil phase may have the following effects: (1) partitioning of inhibitor between phases (2) changing the contact time of the aqueous phase on the pipe (3) changing the wetting behaviour of the pipe surface (4) introducing protective compounds that are naturally occurring in the oil.

5.8 Flow is an indirect variable, and simulation of flow in the laboratory is not direct. For this reason, the hydrodynamic flow parameters are determined, and then the laboratory corrosion tests are conducted under the calculated hydrodynamic parameters. The fundamental assumption in this approach is that, when the hydrodynamic parameters of different geometries are the same, then the corrosion mechanism will be the same. Under these conditions, the corrosion rate and the efficiency of corrosion inhibition in the laboratory and in the field are similar. The commonly used hydrodynamic parameters are wall shear stress, Reynolds number, and mass transfer coefficient (3, 5).

5.9 Neither the flow rate (m/s) nor dimensionless parameters can be directly related to the local hydrodynamic forces at the material surface that may be responsible for accelerated localized attack. Local hydrodynamic forces are influenced by several factors including pipe inclination, position (that is, 3, 6, 9 o'clock), presence of bends, deposits, edges, welds, expansion, and contraction. The flow rate and dimensionless parameters describe only bulk, or average, properties of the dynamic system. Thus the wall shear stress and mass transfer coefficient can be calculated only as averages at the surface with an average surface roughness.

5.10 Inhibitors are first screened in the laboratory, then evaluated in the field, and finally used in engineering operations. The laboratory methodologies, therefore, should be carried out in a compact system with the capacity to evaluate various products quickly with the flow pattern and regime characterized. The results obtained should be relevant to field operation, should be predictive of field performance in terms of inhibitor efficiency, and should be scalable, that is, the experiments can be carried out at various hydrodynamic conditions.

5.11 Flow loops are used to evaluate corrosion inhibitors either in the laboratory or by attaching to a live pipe. The loop simulates the flow regime, but the apparatus is relatively sophisticated, and experiments are expensive and time consuming. The loop is considered sophisticated to be an ideal laboratory methodology under the scope of this guide.

5.12 This guide discusses test facilities and considers the necessary elements which need to be built into a laboratory strategy for testing corrosion inhibitors for multiphase systems. The emphasis is on those methodologies that are compact and scalable, hydrodynamically well characterized, and relatively inexpensive to use. The laboratory methodologies are (1) rotating cylinder electrode (RCE) (2) rotating cage (RC) and (3) jet impingement (JI). These methodologies can be used under both atmospheric and high pressure conditions. Detailed description of RCE and JI are presented in NACE-5A195.

5.13 Laboratory tests for inhibitor evaluation consist of two main components–laboratory methodology and measurement technique. The combinations of laboratory methodology and measurement technique for inhibitor evaluation for multiphase systems are presented in Table 1.

5.14 To develop an inhibitor selection strategy, in addition to inhibitor efficiency, several other key performance factors need to be evaluated: (1) water/oil partitioning, (2) solubility, (3) emulsification tendency, (4) foaming tendency, (5) thermal stability, (6) toxicity, and (7) compatibility with other additives/materials.