A review of wettability alteration using surfactants in carbonate reservoirs

Enhanced oil recovery promoted by aqueous deep eutectic solvents on silica and calcite surfaces: a molecular dynamics study

Enhanced oil recovery (EOR) plays a critical role in optimizing oil extraction from existing fields to satisfy global energy demands while mitigating environmental impact. One promising EOR technique involves injecting water with reduced surface tension utilizing deep eutectic solvents (DESs). Despite early experimental support, the efficacy of aqueous-DES EOR varies and depends on factors such as connate water saturation, water salinity, and reservoir wettability. The recovery mechanisms for aqueous DESs are poorly understood due to the intricate nature of oil components and reservoir formation. In this paper, we investigate the role of DESs in the EOR process through molecular dynamics (MD) simulations. Three different types of DES molecules, such as choline chloride : urea (ChCl : U), choline chloride : ethylene glycol (ChCl : EG), and menthol : salicylic acid (M : SA) are used, for the recovery of dodecane (C12H26) oil from silica and calcite confined surfaces. We have demonstrated the structural characteristics of these systems by examining various physical properties, including interaction energies, density profiles, hydrogen bonds, and interfacial tension (IFT). Different concentrations (10 and 25 wt%) of DESs have been considered to unravel the effect of concentration on oil removal. The wettability of the substrate and the IFT between oil and aqueous DESs are critical physical properties that play a crucial role in influencing EOR phenomena. The IFT between water and oil decreases with the addition of DESs for all DES molecules, leading to a shift in surface behavior from oleophilic to oleophobic and ultimately facilitating the removal of oil from the substrate. Additionally, hydrogen bond formation between DESs and water has been calculated to elucidate its influence on the water/oil interface and substrate wettability. The study provides insights into the fundamental aspects of EOR processes for more effective and sustainable oil extraction.

Effect of Organosilane Structures on Mineral Surface Energy and Wettability

The use of organosilanes has been shown to be an effective method for wettability alteration. This work explored for the first time how the structure of organosilanes impacts their ability to modify the wettability of different mineral surfaces, including pure quartz, pure calcite, sandstone, and limestone. Seven organosilanes were selected with different numbers of hydrolyzable groups, alkyl chain lengths, alkyl chain structures, and number of silicon atoms. Contact angle measurements, residual fluid saturations, and capillary pressure curves consistently showed that more hydrolyzable groups create more hydrophobic surfaces. As the number of carbon atoms increases in the silane alkyl chain, the hydrophobicity increases. The structure of the alkyl chain does not have an observable impact on the degree of wettability alteration. Finally, dipodal silanes with two silicon atoms create a much less hydrophobic surface than a single silicon atom silane. By understanding organosilane structure-property relationships with sandstone and limestone surfaces, it is possible to design tailored treatments for specific subsurface applications. Particularly in geosystems engineering, the results presented here can offer insights into enhanced oil recovery processes such as improving gas well deliverability and addressing injectivity issues during water-alternating-gas injection, as well as geological carbon sequestration processes such as improving storage capacity and caprock integrity.

An experimental study of the physical mechanisms of fluid flow in tight carbonate core samples by binary surfactants

Binary surfactants present a promising approach to modifying the petrophysical mechanisms of rock formations to enhance fluid flow, particularly in challenging environments like carbonate rocks. Carbonate rocks exhibit a complex surface charge, which makes it difficult to generalize the use of traditional single surfactants. Hence, the application of binary surfactant systems is proposed as a more effective alternative. This study investigates fluid-rock interactions through adsorption, wettability alteration, and spontaneous imbibition tests. First, static adsorption tests were conducted on eight different surfactant systems to compare the adsorption behaviors of the binary surfactant systems with those of individual surfactants. The results showed a significant influence of the nonionic surfactant with a considerable reduction in adsorption values of 53% and 28% in its anionic-nonionic and cationic-nonionic blends, respectively. Although contact angle measurements taken after aging oil-treated carbonate discs in binary surfactant solutions indicated that wettability was not significantly altered, the binary systems demonstrated the highest efficiency in terms of oil production during spontaneous imbibition tests. Specifically, the zwitterionic-nonionic surfactant system recovered 58% of the initial oil in core samples, compared to 31% and 25% when zwitterionic and nonionic surfactants were used individually. Thus, the use of binary surfactant systems shows great potential for improving oil recovery efficiency, and the findings may have broader implications for optimizing filtration mechanisms in carbonate reservoirs.

An investigative and simulative study on the wetting mechanism of alkaline dust suppressant acting on long-flame coal

In order to tackle the issue of excessive coal dust and hydrogen sulfide (H2S) concurrently in underground coal mines, an alkaline dust suppressant was formulated by combining surfactant sodium sec-alkyl sulfonate (SAS60) and sodium carbonate (Na2CO3) at a certain mass ratio, meant for injection into coal seams. This study principally targeted long-flame coal extracted from Hequ County, located in Shanxi Province, China. The objective was to investigate and analyze the underlying process of how alkaline dust suppressants affect the wettability of coal. A comprehensive strategy including contact angle determination, Fourier transform infrared absorption spectrometer(FTIR) analysis, low-temperature nitrogen adsorption tests, scanning electron microscope(SEM) experimental studies, and molecular dynamics simulations was employed for this examination. The results revealed that merging Na2CO3 with SAS60 could reduce the coal's contact angle. Despite the core structure of the coal surface staying unchanged after alkaline dust suppressant treatment, a rise in the number of hydrophilic functional groups was observed. This count notably surpassed the amount of hydrophobic functional groups, consequently boosting the coal's hydrophilicity. The permeability of the examined coal specimens was chiefly affected by the existence of macropores and mesopores. Processing with 0.05 wt% SAS60 and 1.0 wt% Na2CO3, the coal acquired additional pores and cracks, causing an upswing of average pore size by 25.79% and a 30.64% increase in the maximum gas adsorption. This facilitated more straightforward infiltration of water into the coal dust. Molecular dynamics simulation outcomes indicated a closer affiliation between coal and water following the incorporation of Na2CO3. It led to a heightened activity in water molecule movement, fortifying intermolecular electrostatic interactions, and fostering the creation of hydrogen bonds. Consequently, this improved the coal's wettability. The increase in the mass fraction of Na2CO3 directly corresponds to a more considerable enhancement in the solution's ability to wet the coal. The outcomes of the molecular dynamics simulation validated the experimental results' precision.

Wettability of rock minerals and the underlying surface forces: A review of the implications for oil recovery and geological storage of CO2

The wettability of subsurface minerals is a critical factor influencing the pore-scale displacement of fluids in underground reservoirs. As such, it plays a key role in hydrocarbon production and greenhouse gas geo-sequestration. We present a comprehensive and critical review of the current state of knowledge on the intermolecular forces governing wettability of rock minerals most relevant to subsurface fluid storage and recovery. In this review we first provide a detailed summary of the available data, both experimental and theoretical, from the perspective of the fundamental intermolecular and surface forces, specifically considering the roles played by the surface chemistry, fluid properties, as well as other significant factors. We subsequently offer an analysis of the effects of chemical additives such as surfactants and nanoparticles that have emerged as viable means for manipulating wettability. In each example, we highlight the practical implications for hydrocarbon production and CO2 geo-storage as two of the most important current applications. As the physico-chemical mechanisms governing the wetting phenomena are the main focus, special emphasis is placed on nano-scale experimental approaches along with atomic-scale modeling that specifically probe the underlying intermolecular and surface forces. Lastly, we discuss the gaps in the current state of knowledge and outline future research directions to further our fundamental understanding of the interactions and their impact on the wetting characteristics of Earth's minerals.

Advancements in Surfactant Carriers for Enhanced Oil Recovery: Mechanisms, Challenges, and Opportunities

Enhanced oil recovery (EOR) techniques are crucial for maximizing the extraction of residual oil from mature reservoirs. This review explores the latest advancements in surfactant carriers for EOR, focusing on their mechanisms, challenges, and opportunities. We delve into the role of inorganic nanoparticles, carbon materials, polymers and polymeric surfactants, and supramolecular systems, highlighting their interactions with reservoir rocks and their potential to improve oil recovery rates. The discussion includes the formulation and behavior of nanofluids, the impact of surfactant adsorption on different rock types, and innovative approaches using environmentally friendly materials. Notably, the use of metal oxide nanoparticles, carbon nanotubes, graphene derivatives, and polymeric surfacants and the development of supramolecular complexes for managing surfacant delivery are examined. We address the need for further research to optimize these technologies and overcome current limitations, emphasizing the importance of sustainable and economically viable EOR methods. This review aims to provide a comprehensive understanding of the emerging trends and future directions in surfactant carriers for EOR.

Unveiling the revolutionary role of nanoparticles in the oil and gas field: Unleashing new avenues for enhanced efficiency and productivity

Prominent oil corporations are currently engaged in a thorough examination of the potential implementation of nanoparticles within the oil and gas sector. This is evidenced by the substantial financial investments made towards research and development, which serves as a testament to the significant consideration given to nanoparticles. Indeed, nanoparticles has garnered increasing attention and innovative applications across various industries, including but not limited to food, biomedicine, electronics, and materials. In recent years, the oil and gas industry has conducted extensive research on the utilization of nanoparticles for diverse purposes, such as well stimulation, cementing, wettability, drilling fluids, and enhanced oil recovery. To explore the manifold uses of nanoparticles in the oil and gas sector, a comprehensive literature review was conducted. Reviewing several published study data leads to the conclusion that nanoparticles can effectively increase oil recovery by 10 %-15 % of the initial oil in place while tertiary oil recovery gives 20-30 % extra initial oil in place. Besides, it has been noted that the properties of the reservoir rock influence the choice of the right nanoparticle for oil recovery. The present work examines the utilization of nanoparticles in the oil and gas sector, providing a comprehensive analysis of their applications, advantages, and challenges. The article explores various applications of nanoparticles in the industry, including enhanced oil recovery, drilling fluids, wellbore strengthening, and reservoir characterization. By delving into these applications, the article offers a thorough understanding of how nanoparticles are employed in different processes within the sector. This analysis may prove highly advantageous for future studies and applications in the oil and gas sector.

Sulfonamide Derivatives as Novel Surfactant/Alkaline Flooding Processes for Improving Oil Recovery

Over time, oil consumption has increased along with a continuous demand for petroleum products that require finding ways to increase hydrocarbon production more economically and effectively. So, enhanced oil recovery technologies are believed to be very promising and will serve as a key to meeting the future energy demand. This paper aims to introduce an innovative method to boost the EOR by using two novel types of surfactants synthesized from sulfonamide derivatives. Types I and II surfactants were analyzed using Fourier transform infrared spectroscopy, and their characterization was further performed using 1H NMR and 13C NMR spectroscopy. Additionally, the evaluation of these surfactants included interfacial tension measurements at concentrations up to 0.9 wt %. The combination of types I and II surfactants with alkaline (NaOH) was also investigated by the measurements of interfacial tension. A series of coreflood and sandpack tests under high-salinity conditions were carried out to assess the effects of a surfactant alone and alkaline-surfactant as a combination on improving oil recovery. The rock wettability was evaluated using relative permeability saturation curves, and the oil displacement efficiency was determined using fractional flow curves. The coreflood results demonstrated that alkaline-surfactant flooding with the chemical formula 0.2 wt % surfactant type II plus 0.5 wt % NaOH achieved a higher oil recovery of 74% OOIP compared to surfactant flooding with the chemical formula 0.5 wt % surfactant type II (64% OOIP) and waterflooding (saline solution with a 35,000 ppm salinity: 48% OOIP). Moreover, the experimental results showed that under both core and sandpack flood conditions, there was a noticeable reduction in oil-water interfacial tension, a change in rock wettability to more water-wet, and higher efficiency of oil displacement when alkaline was added to the surfactant. Based on current research, the alkaline-surfactant formulation is strongly recommended for chemical flooding because of its high efficacy and relatively low cost.

Experimental Investigation on Spontaneous Imbibition of Surfactant Mixtures in Low Permeability Reservoirs

Spontaneous imbibition of surfactants could efficiently enhance oil recovery in low permeability sandstone reservoirs. The majority of studies have considered the application of individual surfactants to alter wettability and reduce interfacial tension (IFT). However, a significant synergistic effect has been reported between different types of surfactants and between salts and surfactants. Therefore, this study systematically studied the capability of a binary surfactant mixture (anionic/nonionic) and a ternary surfactant mixture (anionic/nonionic/strong base-weak acid salt) in imbibition enhanced oil recovery (IEOR). The interfacial properties and the cores' wettability were explored by IFT and contact angle measurements, respectively. Subsequently, the imbibition performances of different types of surfactant solutions were discussed. The results suggested that the surfactants' potential to enhance oil recovery followed the order of ternary surfactant mixture > binary surfactant mixture > anionic > nonionic > amphoteric > polymer. The ternary surfactant mixture exhibited strong capacity to reverse the rock surface from oil-wet (125°) to strongly water-wet (3°), which was more significant than both binary surfactant mixtures and individual surfactants. In addition, the ternary surfactant mixture led to an ultralow IFT value of 0.0015 mN/m, achieving the highest imbibition efficiency (45% OOIP). This research puts forward some new ideas on the application of the synergistic effects of surfactants in IEOR from low-permeability sandstone reservoirs.

Effect of reservoir characteristics and chemicals on filtration property of water-based drilling fluid in unconventional reservoir and mechanism disclosure

The research objective of this investigation is to explore the influence of filtrate reducer and reservoir characteristics on the filtration reduction of drilling fluid during the drilling process, and the filtration reduction mechanism of drilling fluids is also revealed. The results obtained that a synthetic filtrate reducer can significantly reduce the filtration coefficient than that of the commercial filtrate reducer. Moreover, the filtration coefficient of drilling fluid constructed from synthetic filtrate reducer is reduced from 4.9 × 10^-2 m³·min^1/2 to 2.4 × 10^-2 m³·min^1/2 with an increase in the filtrate reducer content, which is much lower than that of the commercial filtrate reducer. The weaker filtration capacity of the drilling fluid containing the modified filtrate reducer is attributed to the combined action of the filtrate reducer containing multifunctional groups adsorbed on the sand surface and the hydration membrane adsorbed on the sand surface. Furthermore, the increase in reservoir temperature and shear rate increases the filtration coefficient of drilling fluid, indicating that low temperature and shear rate are conducive to improve the filtration capacity. Thus, the type and content of filtrate reducer are preferred during drilling in oilfield reservoir, but increasing reservoir temperature and shear rate are not recommended. It is necessary to confect the drilling mud with appropriate filtrate reducer such as the chemicals prepared herein during drilling operation.

Surface Energy and Wetting Behavior of Dolomite in the Presence of Carboxylic Acid-Based Deep Eutectic Solvents

This study endeavors to apply experimental and theoretical analyses to assess the viability of wettability alteration for two carboxylic acid-based deep eutectic solvents (DESs). To prepare these chemicals, oxalic acid and citric acid were used as hydrogen bond donors mixed with choline chloride as the hydrogen bond acceptor in an equimolar ratio. In the theoretical part, dolomite and crude oil were characterized using a three-phase setup. Then, the adhesion propensity of brines/crude oil toward dolomite was evaluated by calculating the work of adhesion. Contact angle and interfacial tension measurements were conducted in the experimental part to investigate the impact of chemicals on brine-crude oil and brine-rock interactions. Results revealed that the oxalic acid-based DES outperformed the citric acid-based DES in terms of interfacial tension reduction. In addition, choline chloride/oxalic acid (1:1) could effectively restore the wettability of the dolomite sample to its original state with a wettability alteration index of 82%. Theoretical calculations also confirmed the wettability alteration potential of DESs. Finally, a correlation was proposed to predict the contact angle of brine on the dolomite surface in the presence of crude oil using surface-energy components of brine, crude oil, and dolomite.

Hydrate as a by-product in CO2 leakage during the long-term sub-seabed sequestration and its role in preventing further leakage

Subsurface sequestration of CO₂ produced by industrial production is an effective way to control the excessive emission of greenhouse gases and alleviate the potential global warming. However, CO₂ leakage is likely to occur in its long-term sub-seabed sequestration, posing a great threat to the marine environment and the marine ecology. Previous investigations mainly focused on the implementation of CO₂ sequestration project, ignoring the subsequent potential environmental hazards such as CO₂ leakage, let alone the post-treatment of these accidents. In the present work, secondary sequestration mechanism of hydrate-bearing sediment for leaked CO₂ was explored, and the effect of two important factors (hydrate saturation and sediment thickness) on it was then analyzed. It is expected to provide reference for exploring engineering measures to secondary sequestrate the leaked CO₂ and avoid the catastrophic environmental accidents. The experimental results demonstrate that the role of hydrate in secondary sequestration for leaked CO₂ is mainly due to its filling and occupation of the migration channels in sediment. In addition, due to the secondary sequestration of CO₂ hydrate, change in seawater pH value caused by dissolution of leaked CO₂ in water can be significantly weakened. Besides, with the increasing hydrate saturation and sediment thickness, CO₂ hydrate plays a progressively obvious role in secondary sequestration of CO₂ and avoiding great change in the marine environment. In this way, the leaked CO₂ can be secondary sequestrated by designing/optimizing the characteristics of hydrate-bearing sediment. The investigation demonstrates that most of the leaked CO₂ can be secondary sequestrated only when the hydrate saturation exceeds 0.30 and/or thickness of hydrate-bearing sediment exceeds 3.0 cm. The experimental investigation herein can provide technical support for avoiding environmental disasters caused by the leakage of long-term sequestrated CO₂.

Influence of organoboron cross-linker and reservoir characteristics on filtration and reservoir residual of guar gum fracturing fluid in low-permeability shale gas reservoirs

To effectively reduce the filtration rate of water-based fracturing fluid and promote the pressure holding effect of fracturing fluid in underground unconventional reservoirs, an efficient and clean organic-boron cross-linker was synthesized with boric acid and low alcohols. The results obtained that the synthesized organoboron cross-linker exhibits better fluid loss performance to water-based fracturing fluid than the commercially available cross-linker. This organoboron cross-linker allowed decreasing filtration coefficient more than 0.74 × 10^-2 m³·min^1/2 as a result of the network structure formed by the organoboron cross-linker and guar gum molecule. However, commercially available cross-linker exhibits a relatively large filtered mass of water more than 1.33 × 10^-2 m³·min^1/2 at the same condition. Meanwhile, the cross-linked guar gum fracturing fluid can significantly improve the fluid loss property with the increase of cross-linker content and pressure, and an increased fluid filtration gradually was revealed with increasing the reservoir temperature and current speed. Moreover, the damage of shale reservoir caused by the prepared boron cross-linker was only 11%, which was lower than 18% of the commercial boron cross-linker under the same conditions.

Research on Percolation Characteristics of a JANUS Nanoflooding Crude Oil System in Porous Media

According to the research status of low-permeability reservoir development, in order to find an intelligent and efficient displacement method, a Janus smart nanocapsule (embedding nanomaterials with surfactant function into the polymer) is developed in this paper. There are two kinds of phase fluids in the migration of porous media, the initial Janus intelligent microcapsule slow-release liquid (dissolved and undissolved AP-g-PNIPAAM and Janus functional particle ternary dispersion) and the later AP-g-PNIPAAM and Janus functional particle binary dispersion. Comprehensively, using the indoor oil displacement experiment, the seepage characteristics of the Janus smart nanocapsule (JSNC) in porous media are studied, and their macro and micro oil displacement mechanisms are revealed. Research shows that Janus intelligent microcapsules have good mobility control ability in low-permeability heterogeneous reservoirs. The displacement performance of stepped differential pressure shows that the displacement medium can expand the swept volume. The research results presented can show that the JSNC oil displacement system has great application potential for the development of low-permeability reservoirs.

Experimental investigation on the high-pressure sand suspension and adsorption capacity of guar gum fracturing fluid in low-permeability shale reservoirs: factor analysis and mechanism disclosure

Guar fracturing technology has been considered as a kind of popular EOR technology, but the weak static suspension capacity becomes a challenge due to the poor temperature resistance and stability of guar fracturing fluid. The main goal of this investigation is to explore the effect of different factors on the high-pressure static sand suspension of guar gum fracturing fluid by a synthetic efficient nano-ZrO₂ cross-linker. In particular, a mechanism of static suspended sand of nano-ZrO₂ cross-linker is analyzed by microscopic simulation. The adsorption performance of guar fracturing fluid on the shale surface is also studied for analyzing the environmental pollution and damage of guar gum fracturing fluid to shale reservoirs after cross-linking in this investigation. The results obtained that the inclusion of a small content of nano-ZrO₂ cross-linker (0.4%) leads to an apparent increase of fracturing fluid viscosity and decrease in the falling quality of gravel (104 mPa·s and 0.3 g) compared to the classical cross-linker (63 mPa·s and 3.5 g). The lower adsorption capacity of guar fracturing fluid containing nano-ZrO₂ cross-linker on the shale surface means that it has a weaker pollution ability to the shale reservoir than the commercially available cross-linker. Meanwhile, the grid structure density formed by nano-cross-linker and guar gum is considered to be the key factor to significantly change the suspended sand capacity. The investigation of nano-cross-linker cannot only provide necessary theoretical technology and data support for the stability of water-based fracturing fluid, efficient sand carrying, and the development of water-based fracturing technology, but also effectively protect the underground shale reservoir.

Assessment of shear-thinning fluids and strategies for enhanced in situ removal of heavy chlorinated compounds-DNAPLs in an anisotropic aquifer

The removal of chlorinated organic hydrocarbons (COHs) -DNAPLs was studied in permeability-contrasted sandboxes with an egg-box shaped substratum. Aqueous solutions were compared to viscous shear-thinning fluids (xanthan solution and foam). Interfacial and viscous effects were compared by increasing the capillary number of injected fluids. Non-spatially targeted DNAPL recovery (NSTR) where the driving force was the injection pressure, was compared to spatially targeted DNAPL recovery (STR) where a pumping system allowed the controlled flow. A historical contamination made of a complex mixture of COHs and hexachlorobutadiene (HCBD) as a model were used. NSTR results showed that DNAPL recovery with non-viscous liquids did not exceed 40%. The best results were obtained for xanthan solutions with surfactant ~ 1.3 ×CMC for which pure phase recovery amounted to 88% and 93% for HCBD and for the historical DNAPL, respectively. The STR strategy showed similar recovery yields, whereas xanthan concentrations were 10-times lower. Mass balances on DNAPL showed that at most, 0.15% of COHs was dissolved in the aqueous effluents. NZVI (1 g.l^-1) were delivered in xanthan in view of the chemical degradation of residual COHs and showed a 65% transmission through the low permeability soil.

Affecting analysis of the rheological characteristic and reservoir damage of CO2 fracturing fluid in low permeability shale reservoir

The fracturing property of liquid CO₂ fracturing fluid varies greatly due to the rheology of fracturing fluid during fracturing process. The main objective of this investigation is to study the rheology property of thickened liquid CO₂ by measuring the viscosity of thickened liquid CO₂ in different physical parameters of this prepared thickener and explain the causes of rheological changes. The results show that thickener content, branching content, and molecular weight of a thickener for all could significantly improve the rheology of liquid CO₂; the consistency coefficient K increased as they rose, but the rheological index n presented a decreased trend. Meanwhile, the mesh structure is proposed as a model to explain the rheological changes, and the large wetting angle means an excellent backflow, low reservoir damage, and low adsorption property. These results herein provide a basic reference to improve the CO₂ fracturing technology and molecular design of CO₂ thickener.

Fundamentals and utilization of solid/ liquid phase boundary interactions on functional surfaces

The affinity of macroscopic solid surfaces or dispersed nano- and bioparticles towards liquids plays a key role in many areas from fluid transport to interactions of the cells with phase boundaries. Forces between solid interfaces in water become especially important when the surface texture or particles are in the colloidal size range. Although, solid-liquid interactions are still prioritized subjects of materials science and therefore are extensively studied, the related literature still lacks in conclusive approaches, which involve as much information on fundamental aspects as on recent experimental findings related to influencing the wetting and other wetting-related properties and applications of different surfaces. The aim of this review is to fill this gap by shedding light on the mechanism-of-action and design principles of different, state-of-the-art functional macroscopic surfaces, ranging from self-cleaning, photoreactive or antimicrobial coatings to emulsion separation membranes, as these surfaces are gaining distinguished attention during the ongoing global environmental and epidemic crises. As there are increasing numbers of examples for stimulus-responsive surfaces and their interactions with liquids in the literature, as well, this overview also covers different external stimulus-responsive systems, regarding their mechanistic principles and application possibilities.

Experimental investigation on hydrate dissociation in near-wellbore region caused by invasion of drilling fluid: ultrasonic measurement and analysis

As we all know, development and utilization of clean energy is the only way for society to achieve its sustainable development. Although natural gas hydrates is a new type of clean energy, uncontrollable hydrate dissociation and accompanying methane leakage in drilling operation threaten drilling safety, as well as marine environment. However, the dissociation range of natural gas hydrates around wellbore cannot be reasonably determined in previous investigations, which may lead to the inaccurate estimation of borehole collapse and methane leakage. Then, the marine environment will be greatly damaged or affected. The purpose of the present work is to experimentally explore the dissociation characteristics of gas hydrates around wellbore in drilling operation and analyze the influence law and mechanism of various factors (such as hydrate saturation) on hydrate dissociation. It is expected to provide reference for exploring effective engineering measures to avoid the uncontrolled hydrate dissociation, borehole collapse and accompanying methane leakage. The experimental results demonstrate that acoustic velocity of hydrate-bearing sediment can be accurately expressed as quadratic polynomial of hydrate saturation, which is the theoretical basis for determination of hydrate saturation in subsequent experiments. Owing to the fact that hydrate dissociation is an endothermic reaction, hydrate dissociation gradually slows down in experiment. Throughout the experiment, the maximum dissociation rate at the beginning of the experiment is 8.69 times that at the end of the experiment. In addition, sensitivity analysis found that the increase in the stabilizer concentration in drilling fluid can inhibit hydrate dissociation more effectively than the increase in the hydrate saturation. Hydrate dissociation was completely inhibited when the concentration of soybean lecithin exceeds 0.60wt%, but hydrate dissociation definitely occurs in the near-wellbore region no matter what hydrate saturation is. In this way, based on the requirements of drilling safety and/or environment protection, hydrate dissociation and accompanying methane leakage can be controlled by designing and adjusting the stabilizer concentration in drilling fluid.

Influence of Polymer Viscoelasticity on Microscopic Remaining Oil Production

To investigate the impact of polymer viscoelasticity on microscopic remaining oil production, this study used microscopic oil displacement visualisation technology, numerical simulations in PolyFlow software, and core seepage experiments to study the viscoelasticity of polymers and their elastic effects in porous media. We analysed the forces affecting the microscopic remaining oil in different directions, and the influence of polymer viscoelasticity on the displacement efficiency of microscopic remaining oil. The results demonstrated that the greater the viscosity of the polymer, the greater the deformation and the higher the elasticity proportion. In addition, during the creep recovery experiment at low speed, the polymer solution was mainly viscous, while at high speed it was mainly elastic. When the polymer viscosity reached 125 mPa·s, the core effective permeability reached 100 × 10⁻³ μm², and the equivalent shear rate exceeded 1000 s⁻¹, the polymer exhibited an elastic effect in the porous medium and the viscosity curve displayed an ‘upward’ phenomenon. Moreover, the difference in the normal deviatoric stress and horizontal stress acting on the microscopic remaining oil increased exponentially as the viscosity of the polymer increased. The greater the viscosity of the polymer, the greater the remaining oil deformation. During the microscopic visualisation flooding experiment, the viscosity of the polymer, the scope of the mainstream line, and the recovery factor all increased. The scope of spread in the shunt line area significantly increased, but the recovery factor was significantly lower than that in the mainstream line. The amount of remaining oil in the unaffected microscopic area also decreased.