Hydrate Deposits Research Articles

An improved computational fluid dynamics (CFD) model for predicting hydrate deposition rate and wall shear stress in offshore gas-dominated pipeline

Gas hydrates in pipelines is still a flow assurance problem in the oil and gas industry and requires a proactive hydrate plugging risk predicting model. As an active area of research, this work has developed a 3D 10 m length by 0.0204 m diameter horizontal pipe CFD model based on the eulerian-eulerian multiphase modelling framework to predict hydrate deposition rate in gas-dominated pipeline. The proposed model simulates the conditions for hydrate formation with user defined functions (UDFs) for both energy and mass sources implemented in ANSYS Fluent, a commercial CFD software. The empirical hydrate deposition rates predicted by this model at varying subcooling temperatures and gas velocities are consistent with experimental results within ±10% uncertainty bound. At lower gas velocity of 4.7 m/s, the model overpredicted the hydrate deposition rates of the experimental results in Aman et al. (2016) by 9–25.7%, whereas the analytical model of Di Lorenzo et al. (2018) underpredicted the same experimental results by a range of 27–33%. Consequently, the CFD model can enhance proactive hydrate plugging risk predictions earlier than the analytical model, especially at low gas productivity. Similarly, at a velocity of 8.8 m/s and subcooling temperatures of 2.5 K, 7.1 K and 8.0 K, the CFD model underpredicted the hydrate deposition rates of the regressed experimental results in Di Lorenzo et al. (2014a) by 14%, 6% and 4% respectively, and overpredicted the results by 1% at a subcooling temperature of 4.3 K. From the CFD model results, we also suggest that hydrate sloughing shear stress is relatively constant, and the wall shedding shear stress by hydrate vary during deposition. Finally, the CFD model also predicted the phase change during hydrate formation, agglomeration, and deposition.

Gas Sources, Migration, and Accumulation Systems: The Shallow Subsurface and Near-Seafloor Gas Hydrate Deposits

Compared with the deeply buried marine gas hydrate deposits, gas hydrates in the shallow subsurface, close to and at the seafloor, have attracted more attention owing to their concentrated distribution, high saturation, and easy access. They accumulate at relatively shallow depths <100–120 m and occur as gas hydrate-bearing mounds (also known as hydrate outcrops, pingoes) at the seafloor derived from the growth of hydrates in the shallow subsurface or as pure hydrate chunks formed by gas leakage. This paper reviews and summarizes such gas hydrate systems globally from the perspective of gas sources, migration pathways, and accumulation processes. Here, we divided them into four categories: fault-chimney-controlled, diapir-fault-controlled, fault-controlled, and submarine mud volcano-controlled deposits. Gas chimneys originate immediately above the restricted regions, mostly affected by faults where high gas concentrations trigger elevated pore fluid pressures. Diapirism derives a dendritic network of growth faults facilitating focused gas discharge and hydrate formation near the seafloor. Furthermore, pre-existing faults or fractures created by overpressured gas from greater depths in accretionary tectonics at convergent margins act as preferential pathways channeling free gas upwards to the seafloor. Gas flux rates decrease from the submarine mud volcano center to its margins, creating a concentric pattern of distributing temperature, gas concentrations, and hydrate contents in shallow sediments around the mud volcano. Hydrate-bound hydrocarbons are commonly of thermogenic origin and correspond to high-background geothermal conditions, whereas microbial gas is dominant in a few cases. The presence of heavier hydrocarbons mitigates the inhibition of hydrate formation by salt or heat. Fluid migration and pathways could be compared to the “blood” and “bones” in an organic system, respectively. The root of a pathway serves as the “heart” that gathers and provides considerable free gas concentrations in a restricted area, thereby triggering pore fluid pressures as one important drive force for focused fluid flow in impermeable sediments (the organic system). Besides the suitable temperature and pressure conditions, a prerequisite for the formation and stability of hydrate deposits in the shallow subsurface and at the seafloor is the sufficient supply of gas-rich fluids through the hydrate stability zone. Thus, the proportion of gas migrating from deep sources is significantly larger than that trapped in hydrates. As such, such marine hydrate deposits seem more like temporary carbon storage rather than the main culprit for climate warming at least in a short period.

Hydrate based carbon capture and sequestration (HBCCS): An innovative approach towards decarbonization

The surge in global greenhouse gas emissions (mainly CO2) has introduced the world to the significant problem of climate change. A paradigm shift towards sustainable energy sources is necessary to meet the sustainable development goals (SDGs). Applying carbon capture and sequestration (CCS) technologies could be a suitable approach for minimizing anthropogenic CO2. Implementation of CCS involves enormous capital expenditure along with transportation, sequestration, and purification cost making its adaptability economically unfeasible. A novel approach to hydrate-based CO2 capture and sequestration (HBCCS) has gathered significant attention due to its potential to provide long-term CO2 sequestration. Herein, the methane-carbon dioxide sweeping process (CH4-CO2 replacement) has gained considerable interest as it produces clean energy (natural gas) from natural gas hydrate deposits while sequestering anthropogenic CO2. In this review paper, we presented a thorough assessment of the HBCCS process while highlighting the critical factors along with the economic/environmental/technical barriers controlling its deployment in actual field applications.

Durable Hydrate-phobic Coating with In Situ Self-Replenishing Hydrocarbon Barrier Films for Low Clathrate Hydrate Adhesion.

Clathrate hydrate growth, deposition, and plug formation during oil and gas transportation causes blockage of pipelines. An effective strategy to solve this problem is to mitigate the hydrate formation and reduce its adhesion on pipe walls through a coating process. However, durability failure, corrosion, inability to self-heal, high cost, and strong hydrate adhesion remain unsolved issues. To address these challenges, in this work, we present an in situ self-replenishing nonfluorinated durable hydrate-phobic coating of candle soot particles. The candle soot coating reduces hydrate adhesion by promoting a thick barrier film of hydrocarbons between the hydrate and the soot coated substrate. The hydrocarbons permeating the soot coating display a high contact angle for water and inhibit the formation of water bridges between the hydrate and soot coated substrate. The spherical cyclopentane hydrate slides off easily on the candle soot coating inside the cyclopentane environment. The hydrate former, cyclopentane-water emulsion, and THF-water mixture have high contact angles as well as low hydrate adhesion on soot coating simultaneously. In addition, the coating is flow-induced long-term slippery, durable, low cost, anticorrosion, self-cleaning, and suitable for practical applications.

Analysis of the influence of the hydrate-bearing rocks properties on the prospects their industrial development

Along with renewable energy and hydrogen, gas hydrates may become the most significant energy resource in the coming years. The reserves of gas in the hydrate state exceed all the combined world reserves of traditional energy resources. At the same time, the gas hydrates properties in the conditions their natural occurrence in the composition of hydrate-containing rock cause significant difficulties in their extraction. In this regard, the industrial use of colossal renewable gas resources in the gas hydrate state is just beginning. Based on this, the methods of developing gas hydrate deposits are the object of research. Based on the analysis and generalization of the currently known examples results of experimental and industrial development of gas hydrate deposits, as well as the results of studying the hydrate-bearing rocks properties, an assessment of the prospects for the industrial implementation of gas hydrate deposit development methods is given. Extraction of methane from gas hydrate deposits causes difficulties due to their solid form. Existing promising methods of their development involve the dissociation of gas hydrate into gas and water. Currently implemented research and industrial development projects of gas hydrate deposits have shown a number of problems related, first of all, to the instability of the hydrate-bearing rock after dissociation of the gas hydrate (at the same time, in the vast majority, the natural gas hydrate becomes metastable and weakly cemented). Therefore, there is still no commercially attractive technology for obtaining natural gas from gas hydrate deposits. At the same time, the depressurization method is considered the most promising. Based on this, the improvement of the technology of influence on the hydrate-bearing rock for the natural gas extraction should concern the provision of the rock removal the into the well. At the same time, effective and competitive development of marine gas hydrates deposits can be realized only if taking into account the geological features of the distribution of hydrate-bearing rocks, as well as the gas hydrates properties in their natural occurrence.

Gas recovery enhancement from fine-grained hydrate reservoirs through positive inter-branch interference and optimized spiral multilateral well network

A spiral multilateral well network is a promising production method to enhance long-term gas recovery from prevalent fine-grained hydrate reservoirs. However, practical application is greatly restricted before the optimal well network parameters are determined and the mechanism behind a unique phenomenon in multilateral wells, namely inter-branch interference, is clear. In this study, we numerically optimized the well configuration and spacing when spiral multilateral wells were deployed in two typical fine-grained hydrate reservoirs, i.e., ultra-low permeability hydrate reservoirs (ULPHR, <1 mD) and low-permeability hydrate reservoirs (LPHR, >1 mD). The mechanism behind inter-branch interference was innovatively revealed. The results indicated that the number of spiral branches should be increased, and equidistant branches should be deployed uniformly in the lower ULPHR or throughout LPHR to enhance production efficiency. A wide spacing of spiral multilateral wells with long branches contributed to long-term productivity in fine-grained hydrate reservoirs with any permeability; however, narrow spacing was more favorable for short branches or short-term production. Our study found three inter-branch interference stages during gas production, namely, “no effect” stage, “positive” stage, and “negative” stage; all the three stages are controlled by reservoir permeability, production distance, and production time. Owing to the “positive” interference effect, longer equal-length branches resulted in superior long-term production enhancement in ULPHR, particularly for lengths greater than 30 m. Gas production from LPHR using only two optimal spiral multilateral wells exhibited high production performance similar to that of the sandy hydrate deposits in Japan, suggesting that the optimal spiral multilateral well network is promisingly suitable for commercial production in the future.

Effect of wax crystal on the kinetic and morphology of gas hydrate deposition in water-in-oil emulsions

The deposition of hydrate and wax in the oil–gas pipelines are two of the flow assurance issues during the subsea oil production, they could reduce the cross-sectional area and increase the pressure drop, even the complete blockage could be caused by either of them in extreme conditions. Thus far, the study of the interaction between the hydrate and wax is still insufficient, especially in the process of the co-deposition of the two species. In this study, by utilizing the high-speed camera, the effect of wax content on the hydrate formation and deposition was explored, the macro- and micro- morphologies of the hydrate deposit layer in both the liquid and gas phase could be directly observed. It was found that with the increase of the wax content, the gas amount encapsulated in the hydrate increased first and then decrease in the tested wax content range. When hydrate was deposited on the wall, the morphologies of the deposit layer in the liquid phase and the growth pattern of the layer in the gas phase were altered by the presence of wax, the corresponding mechanisms were proposed based on the micro-behaviors of the hydrate layer. Finally, the adhesive forces of the hydrate formed in the emulsion with different wax content were measured by a self-developed method, the formed hydrate under 1 wt% wax content was discovered to show the lowest adhesive force when the deposition occurred. This work provides insights on the interaction of hydrate and wax in the W/O emulsion, and therefore could improve the flow-assurance strategies in the oil–gas production.

Evolution Process of Natural Gas Hydrate Deposits in Water-Dominated Systems

Evolution Process of Natural Gas Hydrate Deposits in Water-Dominated Systems

Investigation on synergistic deposition of wax and hydrates in waxy water-in-oil (W/O) flow systems

Elucidating the synergistic effect of wax and hydrates, involving formation, aggregation and deposition, is imperative to the operation and transportation safety for offshore petroleum fields. To understand the characteristics and mechanism of synergistic deposition of wax and hydrates, flow and deposition experiments of systems with different wax contents (0–2.89 wt%), initial flow rates, pressures and temperatures were conducted in a high pressure visual flow loop. According to the flow rate and pressure drop data as well as the visual window observation, four different types of plugging scenarios of wax-hydrate coexisting systems with different flow properties and wall deposition state were summarized, including rapid plugging, transition plugging, gradual plugging type I and gradual plugging type II. Compared with the wax-free system after hydrate formation, even with the addition of anti-agglomerant (AA) with the same concentration, wax-hydrate coexisting systems could not reach stable hydrate slurry flow state, indicating that the existence of wax deteriorated the performance of AA. Aside from the influence of wax crystals on hydrate agglomeration, it was found that wax deposition layer would alter the adhesion and bedding of hydrates, resulting in the variation of flow properties and wall deposition state. For low wax content systems (0.75 wt%) where rapid plugging occurred, the synergistic effect between wax and hydrates promoted the formation of wax-hydrate coupling aggregates, resulting in severe local deposition when the coupling aggregates attained critical deposition size and consequently decreasing flow rate, forming a vicious circle of decreasing transportability. Since bedding of coupling aggregates was hindered by the uniformly coated wax deposition layer on pipe wall, gradual plugging rather than rapid plugging occurred in medium wax content systems (1–1.25 wt%), predominately caused by the gradual increment in viscosity of waxy hydrate slurry. For relatively high wax content systems (2.89 wt%), hydrate formation and plugging did not occur, due to the insulation effect of wax deposition layer. A physical model for the synergistic deposition of wax and hydrates was also presented, which was meaningful to the development of a mathematical model for the prediction of blockage formation and risk analysis.

Experimental study on the dual-gas co-production from hydrate deposit and its underlying gas reservoir

Short gas production periods and high production costs are two main challenges for the commercial exploitation of natural gas hydrates (NGHs). The NGHs deposit with enriched underlying free gas has drawn more attention, because the dual-gas co-production from hydrate zone and underlying free gas zone are expected to increase gas yields. We designed a method to prepare this kind of hydrate deposits that a hydrate layer over a free gas layer, and performed a series of dual-gas co-exploitation experiments to investigate its gas production characteristics via depressurization method. Results demonstrate that the temperature of hydrate layer and free gas layer exhibit different variation tendency during gas production process and lower production pressure attributes to higher gas production rate and gas recovery ratio. The wellbore set at underlying free gas layer is recommended because it can avoid lower initial local permeability around the well and prevent ice plug or hydrate reformation during gas production process. Meanwhile, it enhances the movement of hydrate decomposition front and further improves gas production efficiency. Compared with the NGHs reservoir with a single hydrate zone, the dual-gas co-production has higher gas production efficiency and lower temperature decrease to maintain the driving force of hydrate dissociation.

In situ inelastic neutron scattering of mixed CH4–CO2 hydrates

An abundant source of CH4 can be found in natural hydrate deposits. Recent demonstration of CH4 recovery from hydrates via CO2 exchange has revealed the potential as a fuel source that also provides a medium for carbon sequestration. It is vital to understand the structural and dynamic impacts of guest variation in CH4, CO2, and mixed hydrates and link the results to the stability of various deposits in nature, harvesting methane, and sequestering CO2. Molecular vibrations are examined in CH4, CO2, and mixed CH4-CO2 hydrates at 5 and 190 K and Xe hydrates for comparison. Inelastic neutron scattering (INS) is an ideal spectroscopy technique to observe the dynamic modes in the hydrate structure and enclathrated CH4, as it is extremely sensitive to 1H. The presence of CO2 in hydrates tightens the lattice. It introduces more active librational modes to the host lattice, while hindering the motion of CH4 in mixed CH4-CO2 hydrate at 5 K. At 190 K, a large broadening of the CH4 librational modes indicates disorder in the structure leading to dissociation.

Analytical Transient Thermal Model for Predicting Cooldown Temperature in a Subsea Pipe-in-Pipe Flowline System

Background: For the reliable operation of deep-water production systems transporting hydrocarbons, the critical flow assurance problems to manage are wax and hydrate formation and their deposition due to the heat transfer between the production system and surroundings. Wax and hydrate deposition can restrict the flow with significant production loss. Method: The adequate thermal insulation of such systems can allow the retention of thermal energy in the production system to ensure that the fluid cooldown temperature remains above the wax and hydrate formation temperatures. This may provide sufficient time for taking the preventive measures during an emergency shut-in operation. The purpose of this paper is to predict the cooldown temperature of a subsea Pipe-in-Pipe flowline system transporting crude oil from the reservoir to the host facility using a recently developed transient thermal model. The model incorporates the energy redistribution inside the system through the internal temperature gradient and allows to determine the requirement of thermal insulation of the Pipe-in-Pipe system to retain sufficient heat before the fluid temperature falls below the hydrate and wax temperatures. Result: The model results are compared with an industry accepted commercial simulator results to illustrate the accuracy of the model for emergency shut-in operations. Conclusion: With limited pipeline configurations and fluid properties, the model can successfully predict the requirements of thermal insulation and cooldown temperature efficiently and economically.

Development of methods of operative determination of parameters of repeated hydrate formation in layer systems of gas hydrate deposits

The object of research is the methods of laboratory setting of the parameters of hydrate formation of well production and the design features of the equipment for its implementation. Methane hydrate is becoming a promising topic for a new energy resource. At the same time, hydrostatic formation is one of the most problematic areas in ensuring the transport of well products, and this primarily concerns the production of gas hydrate deposits. An analysis of the thermobaric parameters of the well production of gas hydrate deposits shows that when they are moved by technological lines, they are close to hydrate equilibrium, but due to the intensity of the process, the system does not have time to reach it. In addition, reservoir system water has a memory of hydrate structures, or a certain amount of gas hydrate solid phase is also present in the flow water. In this regard, a set of laboratory studies was carried out to assess the nature of the behavior of this type of systems during the re-crystallization of gas hydrate and its dissociation. Based on the results of the research, a method for the operational laboratory setting of the parameters of repeated hydrate formation in reservoir systems of gas hydrate deposits was developed. It provides for setting the parameters of mass crystallization of gas hydrate by visual fixation of the moment of appearance of the solid phase at the interfacial contact «liquid – gas». The design features of the laboratory facility for its implementation have also been developed and substantiated. The technique makes it possible to reduce the duration of the study of one sample by almost an order of magnitude (from several days to 8–10 hours). In addition to the information on the equilibrium parameters of hydrostatic formation, provided by traditional methods of laboratory research, an additional characteristic of the behavior of reservoir systems in non-equilibrium conditions has been obtained, which will help to quickly assess the risks of technogenic hydrate formation. The developed technique is important for systems that, at least, have a memory of hydrate structures. However, the preliminary transfer of a part of the water of the test sample through the gas hydrate form allows estimating the parameters of hydrate formation of any reservoir system.

Optimization of Critical Parameters of Deep Learning for Electrical Resistivity Tomography to Identifying Hydrate

As a new energy source, gas hydrates have attracted worldwide attention, but their exploration and development face enormous challenges. Thus, it has become increasingly crucial to identify hydrate distribution accurately. Electrical resistivity tomography (ERT) can be used to detect the distribution of hydrate deposits. An ERT inversion network (ERTInvNet) based on a deep neural network (DNN) is proposed, with strong learning and memory capabilities to solve the ERT nonlinear inversion problem. 160,000 samples about hydrate distribution are generated by numerical simulation, of which 10% are used for testing. The impact of different deep learning parameters (such as loss function, activation function, and optimizer) on the performance of ERT inversion is investigated to obtain a more accurate hydrate distribution. When the Logcosh loss function is enabled in ERTInvNet, the average correlation coefficient (CC) and relative error (RE) of all samples in the test sets are 0.9511 and 0.1098. The results generated by Logcosh are better than MSE, MAE, and Huber. ERTInvNet with Selu activation function can better learn the nonlinear relationship between voltage and resistivity. Its average CC and RE of all samples in the test set are 0.9449 and 0.2301, the best choices for Relu, Selu, Leaky_Relu, and Softplus. Compared with Adadelta, Adagrad, and Aadmax, Adam has the best performance in ERTInvNet with the optimizer. Its average CC and RE of all samples in the test set are 0.9449 and 0.2301, respectively. By optimizing the critical parameters of deep learning, the accuracy of ERT in identifying hydrate distribution is improved.

Molecular simulation of methane hydrate growth confined into a silica pore

The growth of a methane hydrate seed within a silica slit pore of fixed width has been studied using All-Atom Molecular Dynamics (AA-MD). An AA force field has been used to describe the molecules of the solid silica substrate, with α-quartz crystalline structure. The crystallisation of hydrates in confined geometries is not well understood yet, and the objective of this work is to study the hydrate growth inside a silica pore using molecular simulation. Both NVT and NpT ensembles were used in the AA-MD simulations to analyse the hydrate growth from an initial seed. Results showed that the boundary conditions imposed by the nanometric slit pore yielded a hydrate with structural defects, filling the accessible space between the silica walls. The water molecules which were not incorporated to the initial seed hydrate formed a high density water layer trapped between the silica walls and the crystallised hydrate. These results provide an interesting insight into the hydrate crystallisation process in confined geometries, resembling those found in natural hydrate deposits.

Permeability Models of Hydrate-Bearing Sediments: A Comprehensive Review with Focus on Normalized Permeability

Natural gas hydrates (NGHs) are regarded as a new energy resource with great potential and wide application prospects due to their tremendous reserves and low CO2 emission. Permeability, which governs the fluid flow and transport through hydrate-bearing sediments (HBSs), directly affects the fluid production from hydrate deposits. Therefore, permeability models play a significant role in the prediction and optimization of gas production from NGH reservoirs via numerical simulators. To quantitatively analyze and predict the long-term gas production performance of hydrate deposits under distinct hydrate phase behavior and saturation, it is essential to well-establish the permeability model, which can accurately capture the characteristics of permeability change during production. Recently, a wide variety of permeability models for single-phase fluid flowing sediment have been established. They typically consider the influences of hydrate saturation, hydrate pore habits, sediment pore structure, and other related factors on the hydraulic properties of hydrate sediments. However, the choice of permeability prediction models leads to substantially different predictions of gas production in numerical modeling. In this work, the most available and widely used permeability models proposed by researchers worldwide were firstly reviewed in detail. We divide them into four categories, namely the classical permeability models, reservoir simulator used models, modified permeability models, and novel permeability models, based on their theoretical basis and derivation method. In addition, the advantages and limitations of each model were discussed with suggestions provided. Finally, the challenges existing in the current research were discussed and the potential future investigation directions were proposed. This review can provide insightful guidance for understanding the modeling of fluid flow in HBSs and can be useful for developing more advanced models for accurately predicting the permeability change during hydrate resources exploitation.

Coupled flow and geomechanical analysis for gas production from marine heterogeneous hydrate-bearing sediments

For natural gas hydrate exploitation, revealing the interaction between flow and geomechanics is the key to accurate prediction of gas production and formation stability. In this work, based on the site GMGS3-W19 of the South China Sea, a numerical model with multilayer hydrate deposits was established to analyze the evolution of geological parameters during depressurization. The results demonstrated that the reservoir underwent sequential process of expansion and compression, which resulted in the corresponding changes of porosity and permeability. The different initial geological parameters of the three hydrate layers showed respective coupling characteristics between flow and geomechanics. Additionally, we also analyzed the geological stability of reservoir. The calculation results of horizontal and vertical effective stresses were within the elastic region away from the Mohr-Coulomb yield function until 1800 d. In contrast, the layer with high hydrate saturation was more likely to produce shear failure. Finally, the gas-water production was determined after the coupled flow and geomechanical analysis. The result showed that gas productions of the two horizontal wells were on par, while water production of upper well was >7 times that of lower well. The numerical model and analysis could provide useful insight into the marine heterogeneous hydrate exploitation.

In situ measurements of tetrahydrofuran hydrate and ice growth in the presence of copper surface with fringe projection: Growth kinetics and interfacial morphology

In recent years, study on the gas hydrate in the presence of solid surfaces has attracted much interest due to the importance of hydrate deposition on the pipe wall and shutdown of these pipelines. The present work examined the in situ and online monitoring of tetrahydrofuran (THF) hydrate and ice growth on a solid surface using a high-resolution interferometry method. Michelson interferometer was applied to directly measure the thickness of ice and hydrate films using the fringe-shifting technique. Furthermore, the change in the interface morphology during hydrate growth was developed as a function of distortion in the fringe intensity distribution in terms of universal scaling laws. Universal exponent β for the growing interface of THF hydrate was experimentally studied and reported for the first time. Dynamic exponent β was derived as 0.2042, which is in general agreement with other experimental studies of local growth processes. Further, the value of the growth exponent β for the ice growth was obtained by 0.2971, which is close to the value predicted by ballistic deposition and Eden models. Based on the measured dynamic exponents, we suggest that THF hydrate and ice growth can lead to self-similar morphologies and be classified according to their dynamical behavior.

Heat Control Effect of Phase Change Microcapsules upon Cement Slurry Applied to Hydrate-Bearing Sediment

This study aims to develop a novel low-heat cement slurry using phase change microcapsule additives to reduce the decomposition of hydrate-bearing sediments during cementing. Microcapsules were prepared by coating mixed alkanes with polymethyl methacrylate, and lipophilic-modified graphite was incorporated to enhance the thermal conductivity of microcapsules. The effects of microcapsules upon the hydration heat, pore distribution, and compressive strength of the cement slurry/stone were studied through a variety of tests. The results showed that the phase-change temperature, thermal enthalpy, and encapsulation efficiency of the microcapsules were 8.99–16.74 °C, 153.58 Jg−1, and 47.2%, respectively. The introduction of lipophilic-modified graphite reduced the initial phase-change temperature of microcapsules by 0.49 °C, indicating an improvement in their temperature sensitivity. The maximum hydration heat of cement slurry decreased by 41.3% with 7% dosage of microcapsules; the proposed microcapsules outperformed comparable low-heat additives. Moreover, the presence of microcapsules could reduce the number of large pores in (and thereby improve the compressive strength of) cement stone. The innovation of this study is that it comprehensively and intuitively confirms the feasibility of the application of low-heat cement slurry with MPCM as the key in hydrate sediments rather than just focusing on the reduction of hydration heat; furthermore, a self-made cementing device was developed to simulate the cementing process of hydrate deposition. The results show that the thermal regulation of microcapsules inhibited the temperature increase rate of the cement slurry, significantly reducing the damage caused to the hydrate. These findings should improve the safety and quality of cement in offshore oil and gas well applications.

Effects of hydrate inhibitors on the adhesion strengths of sintered hydrate deposits on pipe walls

The effects of hydrate inhibitors on the adhesion of sintered hydrate deposits on pipe walls are still unexplored. Herein, a custom-built adhesion strength measurement apparatus was utilized to quantify the adhesion strengths of sintered cyclopentane (CyC5) hydrate deposits with thermodynamic inhibitors (ethylene glycol, glycerol) and low-dosage inhibitors (dodecylbenzene sulfonic acid (DBSA), sorbitan oleate (Span 80)). It was found that the hydrate adhesion strengths decreased by 69.82%-97.06% and 40.24%-94.36% with the concentration of ethylene glycol and glycerol increased from 2 wt% to 6 wt%, respectively. For DBSA and Span 80, the hydrate adhesion strength increased with concentration less than 0.01 wt% due to the acceleration on hydrate growth. The further increment of concentration leads to a dramatic reduction in adhesion strengths. Furthermore, the relatively large deviations with the predicted strengths led to the discussions of the effects of change in hydrate formation rate, crystal morphology, and also the adaption of the fitting model. Two modified models were proposed to give a better prediction/explanation of the hydrate adhesion with thermodynamic inhibitors and low-dosage inhibitors, respectively. This work provides a fundamental understanding of the adhesion mechanism of hydrate deposits with hydrate inhibitors, which is important in advancing the management of hydrate formation for preventing plugging in pipelines.