Investigation into the strength and source of the memory effect for cyclopentane hydrate

Improved industrial induction time-based technique for evaluating kinetic hydrate inhibitors

Kinetic hydrate inhibitor laboratory testing before field application is one of the key priorities in the oil and gas industry. The common induction-time-based technique is often used to evaluate and screen for kinetic hydrate inhibitors (KHIs). However, the main challenge relates to the stochastic nature of hydrate nucleation observed in fresh systems, which often results in scattered data on hydrate formation with unacceptable uncertainties. A much more precise KHI evaluation method, called crystal growth inhibition (CGI), provides comprehensive insights into the inhibitory behavior of a kinetic hydrate inhibitor, including both hydrate formation and decomposition. Given that industry does not require this much information, it is not feasible to expend either much time or cash on this strategy. This study aims to provide a cost-effective technique that presents maximum data accuracy and precision with relatively little time and cost expenditure. Hence, the impact of water-hydrate memory on improving the accuracy and repeatability of the results of the induction-time-based technique (IT method) was examined. First, the concept of water-hydrate memory, which contains information about how it is created, was reviewed, and then, the factors influencing it were identified and experimentally investigated, like the heating rate of hydrate dissociation and the water-hydrate memory target temperature during heating. Finally, a procedure was developed based on the background information in the earlier sections to compare the consistency of the results, originating from the conjunction of water-hydrate memory with the IT technique. The results of replications at KHI evaluation target temperatures of 12.3-12.4°C and 11.5-11.7°C showed that more repeatable data were obtained by applying water-hydrate memory, and a more conclusive decision was made in evaluating KHI performance than with an IT method. It seems that combining the IT method with water-hydrate memory, introduced as the "HME method", can lead to more definitive evaluations of KHIs. This approach is expected to gain in popularity, even surpassing the accurate but complex and time-consuming CGI method.

Interfacial Heat and Mass Transfer Effects on Secondary Hydrate Formation under Different Dissociation Conditions

Secondary hydrate formation or hydrate reformation poses a serious threat to the oil and gas transportation safety and natural gas hydrate exploitation efficiency. The hydrate reformation behaviors in porous media have been widely studied in large simulators due to their importance in traditional industries and new energy resources. However, it is difficult to understand the interfacial effects of hydrate reformation on the surface and in micropores of the porous media via a basic experimental apparatus. In this work, in situ X-ray computed tomography (X-CT) technology is used to detect the period, distribution, volume, and morphology characteristics of secondary hydrate formation during hydrate dissociation under depressurization, thermal stimulation, and the combined conditions. It is found that the secondary hydrate formation is inevitable under any conditions of hydrate dissociation. The secondary hydrate morphology varies among porous, grain-enveloping, grain-cementing, granular, and patchy structures, which are closely correlated to the hydrate reformation region and gas/water saturated conditions during hydrate dissociation. Accordingly, we revealed that the interfacial superheating phenomenon before hydrate dissociation could provide a supercooling condition for hydrate reformation. The gas flow along the interface of pores and inside the liquid water, as well as gas accumulation in noninterconnected pores, would exaggerate the hydrate reformation by increasing the local pore pressure. Meanwhile, the hydrate reformation aggravates the nonuniform distribution of gas hydrates in pores. In order to avoid hydrate reformation during dissociation, we further compared hydrate reformation and dissociation behaviors under three hydrate dissociation conditions. It is revealed that the combination of thermal stimulation and depressurization is an effective method for hydrate dissociation by retarding secondary hydrate formation. This study provides visual evidence and an interaction mechanism between interfacial heat and mass transfer, as well as secondary hydrate formation behaviors, which can be favorable for future quantitative research on secondary hydrate formation in different scales under various dissociation conditions.

Fast Formation of Hydrate Induced by Micro-Nano Bubbles: A Review of Current Status

Hydrate-based technologies have excellent application potential in gas separation, gas storage, transportation, and seawater desalination, etc. However, the long induction time and the slow formation rate are critical factors affecting the application of hydrate-based technologies. Micro-nano bubbles (MNBs) can dramatically increase the formation rate of hydrates owing to their advantages of providing more nucleation sites, enhancing mass transfer, and increasing the gas–liquid interface and gas solubility. Initially, the review examines key performance MNBs on hydrate formation and dissociation processes. Specifically, a qualitative and quantitative assembly of the formation and residence characteristics of MNBs during hydrate dissociation is conducted. A review of the MNB characterization techniques to identify bubble size, rising velocity, and bubble stability is also included. Moreover, the advantages of MNBs in reinforcing hydrate formation and their internal relationship with the memory effect are summarized. Finally, combining with the current MNBs to reinforce hydrate formation technology, a new technology of gas hydrate formation by MNBs combined with ultrasound is proposed. It is anticipated that the use of MNBs could be a promising sustainable and low-cost hydrate-based technology.

Promoting Effect of Ultra-Fine Bubbles on CO2 Hydrate Formation

When gas hydrates dissociate into gas and liquid water, many gas bubbles form in the water. The large bubbles disappear after several minutes due to their buoyancy, while a large number of small bubbles (particularly sub-micron-order bubbles known as ultra-fine bubbles (UFBs)) remain in the water for a long time. In our previous studies, we demonstrated that the existence of UFBs is a major factor promoting gas hydrate formation. We then extended our research on this issue to carbon dioxide (CO2) as it forms structure-I hydrates, similar to methane and ethane hydrates explored in previous studies; however, CO2 saturated solutions present severe conditions for the survival of UFBs. The distribution measurements of CO2 UFBs revealed that their average size was larger and number density was smaller than those of other hydrocarbon UFBs. Despite these conditions, the CO2 hydrate formation tests confirmed that CO2 UFBs played important roles in the expression of the promoting effect. The analysis showed that different UFB preparation processes resulted in different promoting effects. These findings can aid in better understanding the mechanism of the promoting (or memory) effect of gas hydrate formation.

Contribution of Ultra-Fine Bubbles to Promoting Effect on Propane Hydrate Formation

To investigate experimentally how ultra-fine bubbles (UFBs) may promote hydrate formation, we examined the formation of propane (C₃H₈) hydrate from UFB-infused water solution using two preparation methods. In one method, we used C₃H₈-hydrate dissociated water, and in the other, C₃H₈-UFB-included water prepared with a generator. In both solutions, the initial conditions had a UFB number density of up to 10⁹ mL⁻¹. This number density decreased by only about a half when stored at room temperature for 2 days, indicating that enough amount of UFBs were stably present at least during the formation experiments. Compared to the case without UFBs, the nucleation probabilities within 50 h were ~1.3 times higher with the UFBs, and the induction times, the time period required for the bulk hydrate formation, were significantly shortened. These results confirmed that UFB-containing water promotes C₃H₈-hydrate formation. Combined with the UFB-stability experiments, we conclude that a high number density of UFBs in water contributes to the hydrate promoting effect. Also, consistent with previous research, the present study on C₃H₈ hydrates showed that the promoting effect would occur even in water that had not experienced any hydrate structures. Applying these findings to the debate over the promoting (or “memory”) effect of gas hydrates, we argue that the gas dissolution hypothesis is the more likely explanation for the effect.

Morphology Investigation on Cyclopentane Hydrate Formation/Dissociation in a Sub-Millimeter-Sized Capillary

The formation, dissociation, and reformation of cyclopentane (CP) hydrate in a sub-millimeter-sized capillary were conducted in this work, and the morphology of CP hydrate was obtained during above processes, respectively. The influences of the supercooling degree, i.e., the hydrate formation driving force, on CP hydrate crystals’ aspect and growth rate were also investigated. The results demonstrate that CP forms hydrate with the water melting from ice at the interface between the CP and melting water at a temperature slightly above 273.15 K. With the action of hydrate memory effect, the CP hydrate in the capillary starts forming at the CP-water interface or CP–water–capillary three-phase junction and grows around the CP–water interface. The appearance and growth rate of CP hydrate are greatly influenced by the supercooling degree. It indicates that CP hydrate has a high aggregation degree and good regularity at a high supercooling degree (or a low formation temperature). The growth rate of CP hydrate crystals greatly increases with the supercooling degree. Consequently, the temperature has a significant influence on the formation of CP hydrate in the capillary. That means the features of CP hydrate crystals in a quiescent system could be determined and controlled by the temperature setting.

Memory effect in tetra-n-butyl ammonium bromide semiclathrate hydrate reformation: the existence of solution structures after hydrate decomposition

The memory effect in TBAB semiclathrate hydrate reformation results from the residual solution structure composed of clusters and cluster aggregates.

Observation of the growth process of icy materials in interparticle spaces: phase-contrast X-ray imaging of clathrate hydrate

The visualization of temperature-controlled crystal growth and dissociation of tetrahydrofuran clathrate hydrates and ice in the interparticle spaces between beads is presented. Phase-contrast X-ray imaging using synchrotron X-ray radiation is a unique technique to study clathrate hydrates coexisting with both ice and liquid water and is used here to observe tetrahydrofuran hydrate and ice formation in situ. The nondestructive images obtained reveal a morphology change of tetrahydrofuran clathrate hydrate grown under isothermal temperature conditions at 253 K, which may be caused by the thermal history of crystallization of the clathrate hydrate. In addition, the water freezing process in the interparticle spaces between is observed using phase-contrast X-ray imaging. This method is useful for understanding the kinetics of clathrate hydrates in interparticle spaces.

Antifreeze proteins as gas hydrate inhibitors

Certain organisms survive low temperatures using a range of physiological changes including the production of antifreeze proteins (AFPs), which have evolved to adsorb to ice crystals. Several of these proteins have been purified and shown to also inhibit the crystallization of clathrate hydrates. They have been found to be effective against structure II (sII) hydrates formed from the liquid tetrahydrofuran, sI and sII gas hydrates formed from single gases, as well as sII natural gas hydrates using a mixture of three gases, as assessed using a variety of instrumentation including stirred reactors, differential scanning calorimetry, nuclear magnetic resonance, Raman spectroscopy, and X-ray powder diffraction. For the most part, AFPs are equal to or more effective than the commercial kinetic hydrate inhibitor (KHI) polyvinylpyrolidone, even under field conditions where saline and liquid hydrocarbons are present. Enclathrated gas analysis has revealed that the adsorption of AFPs to the hydrate surface is distinct from tested commercial KHIs and results in properties that should make these proteins more valuable in some field applications. Efforts to overcome the difficulties of recombinant protein production are ongoing, but in silico models of AFP adsorption to hydrates may offer the opportunity to design commercial KHIs for hydrocarbon recovery and transport with all the attributes of these AFP ”green inhibitors”, including their benefits for human and environmental safety.

Investigation into the Formation and Adhesion of Cyclopentane Hydrates on Mechanically Robust Vapor-Deposited Polymeric Coatings

Blockage of pipelines by formation and accumulation of clathrate hydrates of natural gases (also called gas hydrates) can compromise project safety and economics in oil and gas operations, particularly at high pressures and low temperatures such as those found in subsea or arctic environments. Cyclopentane (CyC5) hydrate has attracted interest as a model system for studying natural gas hydrates, because CyC5, like typical natural gas hydrate formers, is almost fully immiscible in water; and thus CyC5 hydrate formation is governed not only by thermodynamic phase considerations but also kinetic factors such as the hydrocarbon/water interfacial area, as well as mass and heat transfer constraints, as for natural gas hydrates. We present a macroscale investigation of the formation and adhesion strength of CyC5 hydrate deposits on bilayer polymer coatings with a range of wettabilities. The polymeric bilayer coatings are developed using initiated chemical vapor deposition (iCVD) of a mechanically robust and densely cross-linked polymeric base layer (polydivinylbenzene or pDVB) that is capped with a covalently attached thin hydrate-phobic fluorine-rich top layer (poly(perfluorodecyl acrylate) or pPFDA). The CyC5 hydrates are formed from CyC5-in-water emulsions, and differential scanning calorimetry (DSC) is used to confirm the thermal dissociation properties of the solid hydrate deposits. We also investigate the adhesion of the CyC5 hydrate deposits on bare and bilayer polymer-coated silicon and steel substrates. Goniometric measurements with drops of CyC5-in-water emulsions on the coated steel substrates exhibit advancing contact angles of 148.3 ± 4.5° and receding contact angles of 142.5 ± 9.8°, indicating the strongly emulsion-repelling nature of the iCVD coatings. The adhesion strength of the CyC5 hydrate deposits is reduced from 220 ± 45 kPa on rough steel substrates to 20 ± 17 kPa on the polymer-coated steel substrates. The measured strength of CyC5 hydrate adhesion is found to correlate very well with the work of adhesion between the emulsion droplets used to form the CyC5 hydrate and the underlying substrates.