October 2020, Vol. 247, No. 10
Features
Molecular Plasma Coating Process for Mitigating Deposit Adhesion
Michael A. Miller, Institute Scientist, Southwest Research Institute (SwRI)
The deposition of solids, such as asphaltenes, scale and wax, in production systems can cause challenges to oil and gas producers. Allowed to accumulate, these solids may begin to restrict, or completely block, flow. Such restrictions can result in production deferment and decreased revenue for an asset.
The flow assurance community has developed a suite of mitigation and remediation tools to manage these deposition risks. Depending on the solid of concern, these technologies include, among others, chemical inhibitors, remediation solvents, thermal insulation and routine pigging.
While often effective, these methods have downsides, including increased costs, added system complexity, added hardware, and added installation and operational health, safety and environment exposure.
An alternative to these conventional means is now available for applicable conditions. Southwest Research Institute (SwRI) and its sponsor Shell International Exploration and Production have developed a novel coating and scalable process that decreases the deposition of asphaltenes, scale and wax.
This process technology and family of coating chemistries are collectively named LotusFlo.1,2 Applied to the fluid-wetted portions of well tubulars, flowlines and production hardware (e.g., downhole jewelry and valves), the hydrophobic coating reduces and possibly prevents many of the disadvantages of conventional solutions.
Through benchtop lab tests, flow-loop tests and field trials, the coating has been shown to be effective at mitigating asphaltene, scale and wax deposition under certain conditions. The first full-field deployment of the coating is underway, and application of the coating at industrial scales started in 2019 with installation of the hardware in a Gulf of Mexico deepwater asset.
Starting with an organosiloxane precursor (or co-mixtures of organosiloxane and perfluoroepoxide precursors), the coating is applied to electrically conducting substrates through a plasma process. In this process, the volatile precursors are introduced into an evacuated pipe [parallel process lines, 40-foot (12-meter) pipe sections per line], along with an inert gas, and are electrically stimulated to ignite a plasma composed of highly ionized gas molecules inside the entire length of the pipe structure (Figure 1).
The plasma emits light, and the output of the excitation source may be programmed to excite the mixture in the pipe according to a tailored pulse sequence by selecting an appropriate set of characteristics parameters, namely, transient DC voltage pulse, pulse-width duration and frequency.
These particular parameters may be tailored to evolve a select ensemble of activated fragment ions of each respective chemical precursor such that particular metastable fragment species, or adducts of two precursor fragments, as cations, anions, radials and radical ions are preferentially formed in the plasma phase. Prior to acceleration onto the substrate, the fragments may form new fragments, depending on their mass and lifetime.
The positive ions are accelerated very rapidly onto the internal surface of the pipe. When the ions collide on the interior surface, they undergo a polymerization reaction that results in a partially inorganic, glass-like coating.
The resulting molecularly bonded coating shows hydrophobic properties with water contact angles (in oil) [WCA/O] of greater than 150 degrees (superhydrophobic) (Figure 2).
Hydrophobic properties have long been thought to decrease the adhesion force between a surface and a deposit. Subjected to a flowing fluid, a deposit will be stripped from a surface when the shear force produced by the flow is equal to or greater than the adhesion force between the deposit and the surface.
The flow rate at which the deposit begins to separate from the surface is typically referred to as the “critical flow rate.” Lowering the adhesion force between a deposit and a surface reduces the critical flow rate and expands the flow rate envelope in which deposits will not stick to a surface.
In addition to WCA/O, the shear adhesion strength of a water-ice droplet on the surface is quantified as a measure of coating performance to assess the propensity for dislodgement of polar particle deposits under liquid flow.3
The shear stresses required to dislodge a water-ice drop from LotusFlo-coated surfaces have been shown to range between 0.04 and 0.17 MPa. This result is approximately one order of magnitude of the shear stress required to dislodge a water-ice drop on an uncoated electropolished stainless steel surface.
While not yet validated through field trials, such extremely low water-ice shear stresses exhibited by these coatings may be beneficial in mitigating obstructions caused by another challenging deposit encountered in offshore operations – methane hydrates.
Through benchtop tests, flow-loop tests and a field trial, the coating has shown to be effective at mitigating the deposition of asphaltenes, scale (calcite and halite) and wax from the fluids investigated.4 Benchtop tests with two crude oils showed decreased asphaltene deposition for coated components.
Lab results for scale also show a benefit from the coating. Calcite and halite depositions were dramatically lower on coated coupons and flow-loop components, respectively, compared to their uncoated counterparts.
Barite deposits on the coating were easier to remediate compared to the uncoated surface. The wax flow-loop results have been consolidated into an operating envelope describing conditions where there are no deposition with coating, reduced deposition with coating and unaffected deposition (i.e., coated and uncoated behave similarly).
A wellbore field trial with a waxy African crude validated the flow-loop results. Wax deposition in the wells is typically managed with frequent scaping, sometimes as often as several times per day. With the coating applied to the inner wall of the tubular, the coated well showed a 75% decrease in scraping frequency relative to comparable wells with no coating.
Based on the results of the performance testing, a commercial-scale application facility has been designed and constructed by Shawcor at its Channelview, Texas, facility based on SwRI’s pilot scale facility. Deployment of the coating is underway, where conditions are applicable, throughout the industry.
Recognizing the case-specific nature of solids deposition from reservoir fluids, the efficacy, universality and longevity of this novel coating will continue to be evaluated both in-house and through independent laboratories.
Ongoing studies unrelated to petroleum production being performed at SwRI further suggest that this family of coatings is also beneficial in mitigating the adhesion of urea-derived deposit formation within selective catalytic reduction (SCR) aftertreatment systems used in diesel engines.
References:
- Miller, M.A., Wei, R. and Hatton, G., “Superhydrophobic compositions and coating process for the internal surface of tubular structures,” US Patent US9701869 B2, 2015 and 2017.
- Miller, M.A., Wei, R. and Hatton, G., “Coatings formed from the deposition of plasma-activated adducts,” US Patent Pending, filed May 6, 2016.
- Zou, M., Beckford, S., Wei, R., Ellis, C., Hatton, G. and Miller, M.A., “Effects of surface roughness and energy on ice adhesion strength,” Applied Surface Science 257 (2011): p. 3786-3792.
- 4. Bethke, G.K., Snook, B., Herrera, G., Kelly, A.E., Joshi, S., Jain, S., Choudhary, S., Hammami, A. and Evans, L., “A Novel Coating to Reduce Solids Deposition in Production Systems,” Offshore Technology Conference (2018); DOI: 10.4043/28902-MS.
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