Low-Adhesion Coatings Provide Novel Gas-Hydrate-Mitigation Strategy

Topics: Flow assurance
Fig. 2—Images taken through the window in the rocking cell during experiments using an uncoated surface (left) and a surface with the omniphobic coating (right). Hydrates deposited on the uncoated surface but did not deposit on the omniphobic surface. CS=carbon steel.

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When certain thermodynamic conditions exist in oil- and gas-production flowlines, gas-hydrate plugs can form and greatly restrict flow. One potential strategy for hydrate management involves allowing hydrates to form but mitigating their ability to deposit on the flowline walls by deploying a low-adhesion, protective surface coating on the inside wall of the flowline. In this study, two coatings were produced and evaluated to determine their effect on hydrate adhesion onto carbon-steel surfaces.

Introduction

In order to test the adhesion of hydrates to the two low-surface-energy coatings, micromechanical-force (MMF) apparatuses were used at low and high pressures. MMF apparatuses have been used historically to investigate both hydrate/hydrate interparticle forces and hydrate/surface interactions. The primary mechanism of hydrate cohesion and adhesion has been determined to be capillary bridging.

Many of the parameters in capillary bridge theory can be altered through the modification of the system (e.g., the addition of surfactants or other chemicals or modification of the hydrate or solid surface). Hydrate/surface interactions have been studied previously; however, the surfaces used in these previous adhesion studies were always either stainless-steel or mineral surfaces, such that surface degradation by corrosion was not taken into account.

In addition to MMF studies measuring adhesion forces, experiments were performed in a rocking cell to observe more-macroscopic effects of the coatings on deposition. Rocking-cell tests give a simulated flow environment and may provide insight into how the coatings respond in the bulk, rather than single-particle measurements performed with an MMF apparatus.

Materials

An omniphobic-coating system was developed to protect metallic surfaces against corrosion and accumulation of oil and gas precipitates such as carbonate and sulfide scales. The coating consists of a composite polymer coating that provides a low-interfacial-energy exterior with a fixed base that is impermeable to both water and olefin phases. By acting as a barrier to wetting and chemical contact with the protected surface, this coating provides protection against extreme acidic and basic chemicals and limits pitting corrosion in oxygenated and anaerobic carbonic acid environments. Additionally, the low-surface-energy exterior minimizes wetting, reduces drag/turbulence in mixed flows, and limits any surface features that could act as nucleation sites for precipitate formation and adhesion.

Unlike many other coating systems, the coating used here has been designed specifically to be applied to already-worn, corroded surfaces, and has shown excellent abrasion resistance against impingement from sand and debris collected within the flowline. Application over corroded surfaces has been shown to arrest the spread of corrosion. 

A second proprietary coating was developed specifically to prevent ice nucleation, adhesion, and accumulation on metal surfaces. This is achieved through the addition of highly nonpolar nanomaterials to a given substrate, greatly reducing its interfacial energy and imparting superhydrophobicity and near-zero water-roll-off angles. This repulsive behavior is so strong that a pseudobarrier layer against direct water contact with the surface is formed, and it has been shown to greatly reduce the rate of corrosion as well as fluid-flow drag over a wide range of temperatures and pressures. This coating requires an applied thickness of <37.5 µm and, consequently, does not increase thermal resistance or electrical impedance.

MMF Measurements

For these surface experiments, cantilevers were created by giving a glass capillary tube two right-angle bends then affixing the coated or uncoated surfaces to the end so that they sit below the level of the cyclopentane in the experimental cell. 

The surfaces were secured to the glass capillary tubes using a two-part epoxy. Surfaces were soaked in a mixture of cyclopentane and water before each experiment. Without the soaking, no measurable adhesion forces would be present regardless of the coating or surface type used. The soaking time was varied for different experiments from 2 to 34 hours. 

Hydrates were formed by freezing water droplets in liquid nitrogen before adding them to an environment containing hydrate formers (i.e., cyclopentane). Particles were typically annealed for 30 minutes. After this period, the surface was moved from the cyclopentane/water bath into the experimental cell. While the soaking solution contains water-saturated cyclopentane, the experimental bath contains cyclopentane with no water present. 

Forty pull-off trials were performed for each experiment; each reported data point is the average of a minimum of three experiments. A pull-off trial is depicted in Fig. 1.

Fig. 1—Procedure for a pull-off trial. ΔD=distance between surface and hydrate; ΔP is related to the contact-initiation force.

 

First, the surface (which is remotely controlled using a micromanipulator) is moved into contact with the hydrate particle. After a 10-second waiting period, the surface is pulled away from the particle at constant velocity. Once the hydrate separates from the surface, the distance between them is measured and used in conjunction with the spring constant of the cantilever to calculate the force according to Hooke’s law.

Rocking Cell

Rocking cells are in use to study high-pressure gas and liquid systems and can include salts and antiagglomerants. To perform an experiment, the testing section (with the carbon-steel surface) was loaded with water and mineral oil at the desired water content and liquid loading. The system was pressurized to 500 psi using a gas mixture of 74.7 mol% methane and 25.3 mol% ethane and maintained at ambient temperature and 500 psi until the liquid phase was saturated with gas. The temperature was set at 1°C to provide a large driving force for hydrate formation. The cell was kept at constant volume during the hydrate formation and deposition experiments (i.e., pressure dropped as hydrates form). Once the experiment concluded (indicated by a stable pressure and no further hydrate formation or after a predetermined time), the hydrate--dissociation process was initiated by setting the temperature to 20°C. The temperature and pressure during the experiment are recorded at an interval of 30 seconds. 

Results

Experiments were performed for six different surface preparations (bare, omniphobic, and superhydrophobic for both pristine and precorroded surfaces), with each surface soaking in a cyclopentane/water mixture for 4 hours. For the pristine surfaces, the forces decreased with the presence of the coatings. For the omni-phobic coating, the force decreased by 53% compared with the uncoated scenario. For the superhydrophobic coating, the decrease from the uncoated baseline was 29%. 

For the precorroded surfaces, the effect of the coatings was even more pronounced. The bare-surface forces increased more than 10 times, while the coated surfaces each saw a much smaller increase. The result of the large increase for the uncoated case was that the reductions in force seen from the coatings were much more significant than for the pristine surfaces. The omniphobic and superhydrophobic coatings, respectively, showed forces 96 and 95% lower than in the uncoated precorroded baseline case. 

Rocking-cell tests with 5 vol% water were performed to investigate whether the coating could prevent hydrate from depositing on the surface sample at lower hydrate volume fractions. When a large volume fraction of water is present in the rocking cell, it is difficult to distinguish deposition (hydrate adhering to the surface) from plugging (large hydrate that physically fills the cell). In either case (deposition or plugging), the hydrates would be observed as stationary within the cell. By limiting the water content in the cell, there is not enough water to form a plug, thereby making deposition easier to observe. The omniphobic coating was chosen for evaluation in this study because of its superior performance in lowering hydrate-adhesion force and its enhanced ability to limit corrosion of the substrate, reducing sample-to-sample variability.

The omniphobic-coating surface at 5 vol% water content delayed hydrate-deposit formation for 24 hours (the maximum length of the experiment), whereas the uncoated surface led to hydrate--deposit formation within the experimental time. The precorroded surface also showed favorable performance using the coating surface. Fig. 2 above shows a comparison of experiments using coated and uncoated surfaces in the rocking cell. In the left image, which was taken at the end of an experiment after rocking was stopped, hydrate agglomerates are observed growing or depositing on the sides of the carbon-steel surface. During these experiments, hydrates did not move when the cell was rocked, indicating that they were adhered to the surface rather than simply resting in contact. 

In contrast, the right image shows the window of the rocking cell during an experiment (i.e., while the cell is still rocking). This image shows hydrates present in the liquid phase, but there is no evidence of hydrates sticking to the omniphobic-coating surface. When the experiment was stopped, the hydrates would settle onto the surface, but the hydrates did not adhere to it. In some experiments, the hydrates would agglomerate, forming a snowball-like aggregate because of the rocking motion of the cell. This agglomeration behavior did not affect the outcome of the tests; the omniphobic coatings reduced hydrate deposition significantly under a relatively short time scale. 

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper OTC 27874, “Low-Adhesion Coatings as a Novel Gas-Hydrate-Mitigation Strategy,” by Erika Brown, SPE, Sijia Hu, Shenglong Wang, and Jon Wells, Colorado School of Mines; Matthew Nakatsuka and Vinod Veedu, SPE, Oceanit Laboratories; and Carolyn Koh, SPE, Colorado School of Mines, prepared for the 2017 Offshore Technology Conference, Houston, 1–4 May. The paper has not been peer reviewed. Copyright 2017 Offshore Technology Conference. Reproduced by permission.

Low-Adhesion Coatings Provide Novel Gas-Hydrate-Mitigation Strategy

01 November 2017

Volume: 69 | Issue: 11

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