Scale and corrosion testing under high surface shear flow conditions, and selection of effective inhibitors, is of increasing importance in oilfield production. Extremely high wall shear stresses, up to 10,000 Pa, can be generated in production systems, for example in inflow control devices, and as such there is a need for high shear corrosion and chemical testing techniques. A jet impingement test technique is presented that enables performance testing of both scale and corrosion inhibitors under these challenging conditions and demonstrates the effect of elevated shear on inhibitor performance. A test system has been developed that allows routine laboratory evaluation of scale and corrosion processes at moderate to high shear stresses using a jet impingement method. A series of tests has been conducted to determine the extent of scale deposition and corrosion observed with a mild scaling system, both independently and combined, and then to assess the effectiveness of inhibitors under high shear conditions. Static tests and low shear rotating cylinder tests have been carried out with the same brine system to allow comparison of results from the different methods. The results show that the jet impingement approach is effective in generating scale deposition and corrosion under the high shear, field representative conditions. The effect of the scale and corrosion processes occurring simultaneously in the same system was demonstrated. Both scale inhibitor and corrosion inhibitor performance were found to be affected by the flow conditions with higher concentrations of each being required as surface shear stress was increased although this was also dependent on the chemical nature of the inhibitors. Comparison of the jet impingement results with those obtained from static and rotating cylinder methods showed that the developed technique is more suitable and allows both scale and corrosion processes to be observed in the same system, under the same hydrodynamic conditions. This test method therefore provides an additional laboratory technique that can be used to evaluate both scale deposition and corrosion under very high shear stresses that cannot be readily achieved by alternative approaches. This work also highlights the importance of testing under field representative conditions and advances the understanding of inhibitor performance in these systems.
Top of the line (TOL) corrosion presents a major challenge for many oil and gas operating companies, especially those producing or exporting gas. It is known to occur during multiphase flow, such as transport of wet gas, as a result of water vapour condensing on the upper, internal surfaces of the pipe which may not be protected by conventional corrosion inhibitors. The dissolution of corrosive gases present in the gas stream (mainly CO2 and/or H2S) into the condensed water can result in severe general or localised corrosion. The design and testing of inhibitors to protect against TOL corrosion is a key area of development, and as yet no industry standard test methodology is available for measuring TOL corrosion and assessing inhibitor performance under these conditions. In order to combat TOL corrosion, effective inhibitors are required to possess two somewhat contradictory properties: namely, to establish stable films on the steel surface while also possessing sufficient volatility to be transported to all locations where protection is required. Recently, we have developed a laboratory test method involving a relatively rapid screening stage followed by testing at more field-representative conditions, to determine TOL corrosion rates as well as to qualify the effectiveness of corrosion inhibitors designed to provide effective control against both TOL and bottom of the line (BOL) corrosion. The method, which includes techniques for ambient-pressure testing as well as alternative techniques for elevated pressures, has been shown to reliably determine the effectiveness of TOL corrosion inhibitor formulations. Both approaches have been validated by comparison with field cases. The application of this test methodology is already providing improved inhibitor selection for TOL corrosion, complementary to more standard test methods for inhibitor selection for BOL corrosion.
The use of inflow-control valves (ICVs) and inflow-control devices (ICDs) to improve production rates in horizontal wells has become increasingly common in recent years. These devices have small apertures resulting in high, local high flow rates which results in regimes of very high shear and turbulence, potentially resulting in materials failure due to accelerated corrosion rates, erosion and potentially erosion-corrosion.To meet the challenge of testing accelerated corrosion rates, various laboratory methods have been developed to study the effects of increasing shear on corrosion. Common test methods such as rotatingcylinder electrode (RCE) tests can provide useful data at moderate shear stresses (up to 100 Pa) and ambient pressures, while rotating-cage autoclave tests (RCA) and rotating-cylinder electrochemical autoclave (RCEA) tests allow moderate shear tests to be conducted at elevated temperatures and pressures. However, achieving the very high wall shear stresses seen with certain oilfield jewellery, such as ICVs and ICDs, is significantly more challenging.In contrast, jet impingement (JI) methods have enabled materials testing at up to 10,000 Pa, and by coupling these with the ability to conduct these tests under increasingly higher pressures and temperatures, very-high-shear systems can be tested under conditions closely matched to those in the field. The approaches developed in our laboratories, which use both weight loss and electrochemical corrosion measurements, have also proved to be robust even in extremely corrosive environments, such as in the presence of stimulation acids (both uninhibited and inhibited) and over extended exposure times (Ͼ 7 days).These jet-impingement test methods have enabled enhanced understanding of the susceptibility of various materials to corrosion and erosion under extremely high shear conditions, and how effectively (or not) film-forming corrosion inhibitors perform. The application of these advanced laboratory techniques is currently playing a vital role in evaluating suitable methods for preventing corrosion under very challenging conditions before deployment in the field.
Current scale risk analysis focuses on thermodynamic calculations to determine the risk of scale, ignoring system kinetics and the impact of flow regimes on scale precipitation from mildly oversaturated systems. It is however recognised that flow regimes affect scale precipitation. Surface growth is influenced by mass transport and diffusion which are susceptible to shear stress and turbulence. Little work has been reported which examine these factors under conditions that can be readily tuned to match field production conditions. Scale inhibitor evaluation exercises therefore often rely on conventional low shear/static or laminar flow conditions which have been demonstrated in many papers to be largely inadequate for mildly oversaturated systems. This work addresses this concept and focuses on scale deposition and growth at metal surfaces as well as bulk (liquid phase) nucleation and growth in mildly oversaturated brines as a function of increasing shear. A series of controlled experiments have been conducted under “mildly oversaturated” conditions to examine the effect of; no shear conventional “static” tests, moderate shear mixed statics and much higher shear regimes including rotating cage and jet impingement approaches with calculated shear stresses up to 500 Pa and higher. This builds on previous work published by the authors in this area1 and further illustrates the importance of conducting tests at field representative shear conditions. Since shear and turbulence have a governing effect on the critical scaling tendency (the level of oversaturation below which brines remain stable under normal production conditions) the ability to correlate between shear and the propensity for scaling in mildly oversaturated systems is critically important in determining the risk of scale at different locations in the production stream. New test methods have been validated which allow the impact of shear and turbulence to be observed under conditions more representative of production conditions. These methodologies lead to scaling in mildly oversaturated brine systems without having to adjust brine chemistry or otherwise increase the scaling regime, i.e. by adjusting the flow regime to reproduce the shear expected at critical locations in the production system. Improved methodologies are therefore presented which allow more appropriate scale inhibitor qualification, taking into account the impact of shear and turbulence under field representative conditions. The work shows that this is critically important for mildly oversaturated conditions.
Laboratory-based assessments of corrosion inhibitors for chemical qualification generally rely on approaches designed to replicate all key aspects of the corroding environment (ppCO2, field crude, brine composition, system temperature, pressure, wall shear, etc.). However, the time and costs associated with testing under fully field representative conditions (e.g. High Pressure/ High Temperature (HP/HT) autoclave or flow loop tests), often restrict the use of such tests to chemicals at dose rates pre-qualified by simpler screening tests, especially when many different formulations are submitted for potential qualification. This paper includes illustrative examples of how apparently small changes to the test methodology in these preliminary screening tests can have significant influence on the relative ranking and absolute performance of the chemicals tested, specifically: the effect of pre-corrosion; adjustment of brine chemistry to avoid artefacts from scale deposition; appropriate pH control (representative of the in situ field conditions where high ppCO2 may be present) partitioning between hydrocarbon and aqueous phases can be affected by minor procedural differences. The net effect is that product ranking can be significantly affected, potentially leading to the qualification of less effective products, or more significantly to the de-selection of otherwise effective products, prior to the more detailed field representative autoclaves / flow loops. More so, since many of the artefacts can be transferred from the screening tests to the field representative tests, product performance and ranking in the more field representative HP/HT tests can also be affected, if critical control of the test conditions is not adhered to. Examples are provided from both generic corrosion inhibitor formulations as well as case studies using field specific formulations. This will assist in improving the design of corrosion inhibitor screening programs and eliminating common errors and artefacts.
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