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Filtration control is important to ensure safe and high efficient drilling. The aim of the current research is to explore the feasibility of using basil seed powders (BSPs) to reduce filtration loss in water-based drilling fluid. The effect of BSP concentration, thermal aging temperature, inorganic salts (NaCl and CaCl2) on the filtration properties of bentonite/basil suspensions was investigated. The filtration control mechanism of BSP was probed via water absorbency test, zeta potential measurement, particle size distribution measurement, and filter cake morphologies observation by scanning electron microscope. The incorporation of BSPs into the bentonite suspension generated acceptable rheology below 1.0 w/v%. The BSPs exhibited effective filtration control after thermal aging at 120 °C, but less efficiency at 150 °C. After thermal aging at 120 °C, the bentonite suspension containing 1.0 w/v% BSPs could resist NaCl and CaCl2 pollution of 5.0 w/v% and 0.3 w/v% respectively. Besides general filtration control behaviors, the exceptional water retaining capability formed by numerous nanoscale 3D networks in the basil seed gum and considerable insoluble small particles in BSPs might further contribute to the filtration control. The excellent filtration properties bring basil seed a suitable and green candidate for the establishment of high-performance drilling fluids.

Basil seed, containing anionic heteropolysaccharides in its outer pericarp, swells as gelatinous hydrocolloid when soaked in water. In this study, basil seed powder (BSP) was used as a multifunctional additive for water-based drilling fluids. The chemical composition, water absorbency, rheological properties of aqueous suspension of BSP were tested. The effect of BSP on the rheological and filtration of bentonite-based drilling fluid before and after thermal aging was investigated. The inhibition characteristics were evaluated by linear swelling, shale cuttings dispersion and shale immersion test. Lubricity improvement by BSP was measured with extreme pressure lubricity test. The results revealed that incorporation of BSP into bentonite suspension improved rheological and filtration properties effectively after thermal aging of 120 °C. BSP exhibited superior inhibitive capacity to xanthan and synergistic effect with KCl. BSP could reduce friction by forming hydration layer. The nanoscale three-dimensional network structures enable BSP to maintain high water retention and absorb strongly on bentonite and metal surface, contributing to enhanced rheology, filtration, inhibition and lubrication properties. The versatile characteristic of BSP, as well as biodegradation makes it a promising additive using in high performance water-based drilling fluid and a potential alternative to conventional synthetic polymers.

Summary Pore throat blockage due to fines migration during drilling and completion is one of the leading causes of damage to unconsolidated sandstone reservoirs. Therefore, it is necessary to explore an effective control method for fines migration. Five types of nanoparticles in suspension with aqueous NaCl solutions of six different ionic strengths were chosen. Their ability to control the migration of quartz and kaolinite fines in quartz sand as the porous medium is discussed in this work. Results show that nanoparticles can effectively adsorb and fix fines, thus successfully suppressing their migration. Among these nanoparticles, Al2O3 showed the best performance, and nanoparticle suspensions with higher ionic strengths were preferable. A surface element integration method was used to establish a mathematical model for calculating the interaction energy between the formation fines and the rock pore surface with adsorbed nanoparticles. Through atomic force microscopy and zeta potential measurements, the effect of nanoparticle adsorption on the heterogeneity of the pore surface was analyzed in terms of roughness and electrical properties. The interaction energy between the formation fines and the heterogeneous pore surface was calculated; it revealed the microscopic mechanism of how nanoparticles control fines migration. The results indicated that the nanoparticles form an adsorption layer, which enhances the physical and chemical heterogeneities of the pore surface and provides favorable conditions for the adsorption and fixation of fines. As a result, the interaction energy curves of the fines and the pore surface shift downward, and their repulsive barriers decrease or even disappear, exhibiting higher attractive potential energy. These variations promote adsorption and fixation of fines at the pore surface, as confirmed by the experimental results reported in this work, thus successfully preventing formation damage.

The development of offshore oil and gas requires environmental protection during the drilling process. However, the existing drilling fluid additives cannot form an efficient environmentally friendly drilling fluid system. At the same time, some environmentally friendly drilling fluid additives cannot be widely used due to their high cost and complicated production process. In this paper, a natural material named wild jujube pit powder (WJPP) was used to improve the performance of the drilling fluid for the first time. The viscosity, shear force, shear thinning, rheology, filtration loss, and lubrication properties of the drilling fluid are discussed. By means of microstructure analysis, infrared spectroscopy, thermogravimetric analysis, and particle size analysis, the properties of WJPP and the drilling fluid system containing WJPP (i.e., rheological property, lubrication property, and filtration loss property) were studied. The results show that the microscopic appearance of WJPP was spherical, fibrous, block, and flake, the thermal decomposition temperature was 273.9 °C, and the suspension of WJPP was slightly acidic. WJPP can increase the viscosity, reduce the filtration loss, enhance the shear thinning and thixotropy, and reduce the lubrication coefficient of the drilling fluid. With the decrease in the particle size of WJPP, the shear thinning and thixotropy of the drilling fluid are enhanced, the viscosity and shear force increased, and the filtration loss and lubrication coefficient decreased. With the increase in the dosage, the shear thinning of the drilling fluid was enhanced, the filtration loss and coefficient of friction decreased, the viscosity and shear force increased, while the thixotropy did not change significantly. The microstructure test of mud cake showed that WJPP could form a grid structure. Combined with the blocking action of particles, the structure could prevent water molecules from passing through to reduce the filtration loss. WJPP can improve the rheological property, filtration property, and friction property of the drilling fluid, which has certain application prospects.

Due to the rapid degradation of conventional biopolymer or synthetic polymeric additives at high temperature (HT) or ultra-high temperatures (ultra-HT), effective control of water-based drilling fluid filtration in HT or Ultra-HT environment is still a great challenge in drilling operation. β-cyclodextrin polymer microspheres (β-CPMs), generally using for drug release and waste water treatment, are evaluated as environmentally friendly ultra-HT filtration reducer. The impact of the microspheres on water-based drilling fluids’ properties including rheology and filtration prior to and after hot rolling at different temperatures ranging from 120 to 240°C was investigated. The high temperature and high pressure (HTHP) filtration properties of the microspheres compared to several commercial high temperature filtration reducers were conducted according to the API recommended procedures. The filtration controlling mechanism was analyzed from zeta potential measurement, particle size distribution measurement, and scanning electron microscope observation of filter cake. The results indicated that the β-CPMs exhibited peculiar filtration behavior differently from conventional additives. When the hot rolling temperature was below 160℃, β-CPMs performed a 30% filtration reduction at 1 w/v% content in comparison with control sample. Once the hot rolling temperature was above 160℃, the capacity of filtration control was further improved with increasing temperatures. This is contrast with conventional filtration reducers that the filtration control capacity deteriorate with increasing temperatures. The microspheres still exhibited superior filtration control after exposure to 240℃. Furthermore, β-CPMs showed little effect on the drilling fluid's rheology. When the temperature was below 160℃, the filtration reduction was obtained by water absorption and swelling of β-CPMs. When the temperature was above 160℃, hydrothermal reaction occurred for β-CPMs. Numerous micro- and nano-sized carbon spheres formed, which bridge across micro and nanopores within filter cake and reduce the filter cake permeability effectively. When the temperature was higher than 160℃, hydrothermal reaction occurs. Carbon spheres generated by the hydrothermal degradation of the β-CPMs, which are responsible for the effective filtration control. The hydrothermal reaction changes the adverse effect of high temperature into favorable improvement of filtration control, which provides a novel avenue for HT and ultra-HT filtration control. The β-CPMs show potential application in deep well drilling as environmental friendly and high temperature filtration reducers.

With the depletion of the conventional shallow oil/gas reservoirs and the increasing demand for oil and gas, deep drilling become more and more essential to extract the oil/gas from deep formations. However, deep drilling faces many complex challenges. One of the complexities is the degradation of polymers and flocculation of bentonite particles, leading to hardly control the rheological and filtration properties of water-based drilling fluids, especially under ultra-high temperature and high pressure (HTHP) conditions. Therefore, an experimental investigation is performed to study how bentonite-hydrothermal carbon nanocomposites will influence the rheological and filtration properties of water-based drilling fluids under ultra-HTHP conditions. Bentonite-hydrothermal carbon nanocomposites are proposed as non-polymer additives to solve the ultra-HTHP challenge in water-based drilling fluid. The nanocomposites are synthesized by facile hydrothermal reaction, in which biomass starch and sodium bentonite are used as the precursor and template, respectively. In this study, the effect of the nanocomposites on the rheology and filtration properties of water-based drilling fluid are investigated before and after hot rolling at 220 °C and 240 °C. The structure characterization indicates that carbon nanospheres can successfully deposit on the bentonite surface after hydrothermal reaction and finally form as nanocomposites. The elemental carbon content, zeta potential and particle size distribution of the nanocomposites could be adjusted according to the reaction conditions. After thermal aging at 220 °C and 240 °C, addition of nanocomposites can improve the rheological properties significantly where a stable and minor change of rheological properties is observed, which is desirable for ultra-HTHP drilling. Regarding filtration control, after adding 1.0 wt% nanocomposite materials, the filtration loss is reduced by 41% and 44% respectively after aging at 220 °C and 240 °C, which is better than the conventional natural materials that lose their function in this case. The identification of microstructure shows that the hydrothermal reaction endows nanocomposites with a unique surface morphology and an improved surface charge density. The interaction between nanocomposites and bentonite particles forms a rigid connection network, which is the main mechanism to facilitate effective rheology and filtration control under ultra-HTHP conditions. The green and facile synthetic routes and environmentally friendly features of the nanocomposites, coupled with the excellent performance in ultra-HTHP rheology and filtration control, indicate that the nanocomposites have a high promise for water-based drilling fluid in ultra-HTHP drilling. Moreover, it provides a new way to design high performance additives with high temperature stability.

It is difficult to define safe drilling mud density window for shale sections. To solve this problem, the general Biot effective stress principle developed by Heidug and Wong was modified. The Weibull statistical model was used to characterize the hydration strain-related strength damage. Considering drilling fluid sealing barrier on shale, a calculation method of safe drilling mud density has been established for shale formation under drilling fluid sealing-inhibition-reverse osmosis effect, combined with a flow-diffusion coupling model. The influence of drilling fluid sealing and inhibiting parameters on safe drilling mud density window was analyzed. The study shows that enhancing drilling fluid sealing performance can reduce the pore pressure transmission and solute diffusion; the inhibiting performance of drilling fluid, especially inhibition to strength damage, is crucial for the wellbore collapse pressure of shale section with significant hydration property. The improvement of drilling fluid sealing and inhibition performance can lower collapse pressure and enhance fracturing pressure, and thus making the safe drilling fluid density window wider and the collapse period of wellbore longer. If there is osmosis flow in shale, induced osmosis flow can make the gap between collapse pressure and fracturing pressure wider, and the stronger the sealing ability of drilling fluid, the wider the gap will be. The safe drilling mud density window calculation method can analyze the relationships between collapse pressure, fracturing pressure and drilling fluid anti collapse performance, and can be used to optimize drilling fluid performance. Key words: shale, drilling fluid, collapse pressure, fracturing pressure, wellbore stability, safe mud density window, shale gas development

Invert emulsion fluids stabilized by organoclays are important for applications in thixotropic fluids and petrochemical industry, especially in the field of oil extraction. Among these applications, improvement of the rheological properties of invert emulsion fluids at high temperatures lacks detailed study. In the current work, three different organoclays, including organo-sepiolite (OSEP), organo-hectorite (OSHCA) and organo-montmorillonite (OMMT) were prepared through chemical modification of unmodified clays using DODMAC. The results of X-ray diffraction (XRD) and scanning electron microscopy (SEM) show that DODMAC is not only adsorbed on the surface of particles, but also inserted into the layers of OMMT and OSHCA. For OSEP particles, DODMAC can only be adsorbed on the surface of particles. Interestingly, the results of optical microscopy and rheology indicate that the OSHCA particle-stabilized invert emulsion fluid provides better rheological properties than the other two organoclays at high temperature. Such high rheological properties is ascribed to the weak flocculation of OSHCA particles. Due to its nonreactive Mg−OH surface groups, OSHCA maintains the rheology of high-temperature aged invert emulsions by reconstructing weak flocculation structure. On the other hand, on the particle surface of OMMT and OSEP, Al−OH may react with OH− under alkaline conditions, which will destroy the weak flocculation due to increased repulsion interaction. These results clearly shed light on the relationship between the performance of invert emulsion at high aged temperature and particle weak flocculation, and also are beneficial to the applications which need stable emulsion at high temperature.

The injection of CO2 into deep coal beds can not only improve the recovery of CH4, but also contribute to the geological sequestration of CO2. The adsorption characteristics of coal determine the amount of the greenhouse gas that deep coal seams can store in place. Using self-developed adsorption facility of supercritical fluids, this paper studied the adsorption behavior of supercritical CO2 and CH4 on three types of coal (anthracite, bituminous coal A, bituminous coal B) under different temperatures of 35 °C, 45 °C and 55 °C. The influence of temperature, pressure, and coal rank on the Gibbs excess and absolute/real adsorption amount of supercritical CO2/CH4 on coal samples has been analyzed. Several traditional isotherm models are applied to interpret the experimental data and Langmuir related models are verified to provide good performances. However, these models are limited to isothermal conditions and are highly depended on extensive experiments. To overcome these deficiencies, one innovative adsorption model is proposed based on machine learning methods. This model is applied to the adsorption data of both this paper and four early publications. It was proved to be highly effective in predicting adsorption behavior of a certain type of coal. To further break the limit of coal type, the second optimization model is provided based on published data. Using the second model, one can predict the adsorption behavior of coal based on the fundamental physicochemical parameters of coal. Overall, working directly with the real data, the machine learning technique makes the unified adsorption model become possible, avoiding tedious theoretical assumptions, derivations and strong limitations of the traditional model.

As one of the most widely used preventive treatment of lost circulation, wellbore strengthening techniques need to be further investigated to improve its efficiency. It is crucial to accurately pre...

In recent years, developing high-performance shale inhibitors with excellent shale swelling and dispersion inhibition properties has attracted great interests in the drilling industry. 1-Octyl-3-methylimidazolium bromide (OMB), a common imidazolium-based ionic liquid, was introduced as a promising shale inhibitor. Shale hydration inhibition capacity was investigated via sodium bentonite (Na-BT) plate soaking test, linear swelling experiment and hot-rolling dispersion experiment. The inhibition mechanism was analyzed by Fourier transforms infrared spectroscopy (FT-IR) analysis, X-ray diffraction (XRD) analysis, thermogravimetric (TGA) analysis, contact angle test, zeta potential test and surface tension measurement. The results demonstrated the shale hydration inhibition property of OMB was obviously superior to other widely used shale inhibitors. OMB could be easily adsorbed onto sodium montmorillonite (MMT) through electrostatic interaction, and some interlayer water molecules were expelled due to its intercalation. Meanwhile, the surface of MMT became more hydrophobic after adsorbed by OMB, and an effective shield against water intrusion was formed. In addition, OMB could effectively reduce the surface tension even at low concentrations, and a lower capillary suction pressure could be beneficial for reducing fluid invasion. The results obtained from the work indicated that OMB had the potential to become a high-performance shale inhibitor for drilling shale formations.

Hydrate dissociation poses a significant problem during drilling operations in hydrate-bearing sediments. The drilling fluid is in direct contact with hydrates, and understanding its inhibitory effect on hydrate dissociation is important for stabilizing hydrates during drilling operations. In this work, an apparatus was designed for investigating hydrate dissociation in drilling fluids, and the effects of thermodynamic hydrate inhibitors (THIs), polyvinyl pyrrolidone (PVP), and soybean lecithin on hydrate dissociation were studied. The inhibitory effect of a water-based drilling fluid in the presence of the potential hydrate dissociation inhibitors was also studied. An unexpected inhibitory effect occurred with ethylene glycol (EG) at concentrations below 10 wt%, resulting from a range of factors including the dissociation driving force, and mass and heat transfers in the EG solution. PVP and lecithin reduced the hydrate dissociation rate and increased the time required for complete dissociation of hydrates. These can, therefore, be used in water-based drilling fluids to inhibit hydrate dissociation. A combination of 0.1 wt% PVP and 0.5 wt% lecithin was selected as an optimal hydrate dissociation inhibitor, considering its inhibitory effect and compatibility with the drilling fluid. Using EG at a concentration below 10 wt% as a THI instead of NaCl can also help inhibit hydrate dissociation. The findings of this work provide a basis for the development and design of drilling fluids for drilling in hydrate-bearing sediments.

Immense oil and gas reserves exist in the deepwater of the South China Sea, and the first priority for implementing successful drilling and completion operations without reservoir damage is to understand the damaging mechanisms to the reservoir formation. With the use of deepwater reservoir cores from the western South China Sea, the mineral composition, microstructure, porosity, and permeability characteristics of the reservoir formation were analyzed. With the use of core flow tests, potential fluid sensitivity was analyzed to determine how and which fluids can damage the reservoir formation during drilling and completion. The results show that the reservoir formation is composed of unconsolidated siltstone with a clay mineral content of 6–19%, and has high porosity and permeability with large pore throats. Sensitivity damage due to the increasing flow rate is strong, and an unusual salinity sensitivity exists with a critical salinity at 40,000–45,000 mg/L. Moderate alkali fluids sensitivity damage exists, with water sensitivity being weak. The main mechanisms of formation damage by drilling and completion fluids were analyzed as following: plugging of pore throats by solid invasion occurs because the sizes of the solid materials in the drilling fluids are just ideal for the formation of an external filter. Rock-fluid incompatibilities occur due to the increasing flow rate and salinity of the fluids because the formation rock is unconsolidated and the fines will easily detach and migrate, plugging the pore throats. Therefore, effective temporary plugging of the pore throats with a large diameter is important for protecting the reservoir. This study provides basic data for the design and implementation of reservoir protection measures in deepwater oil and gas exploration.

Formation damage is an inevitable problem during drilling in the sandstone gas reservoir. The invasion of drilling fluid could cause the decreasing of permeability of the formation and induce the reduction of gas production. In this paper, one kind of microemulsion is designed to improve the performance of drilling fluid in Sulige gas field, China. The fundamental chemical/physical properties and damage mechanism of targeted tight sandstone formation is analyzed using a combination of several experimental methods, including X-ray diffraction, scanning electron microscopy, core inhibition test, and etc. It shows that the formation rock is composed of medium to coarse particles and massive sensitive minerals, distributed with tiny connected pore throats. The rock has a high potential for water blocking damage and shows a medium to a strong performance of water sensitivity and stress sensitivity. Aiming to avoid these damages, one protective additive (microemulsion) is specifically designed. Through adsorption morphology test and core inhibition test, it shows that the newly developed microemulsion (0.5% concentration) could effectively weaken the inhibition effects induced by capillary pressure and decrease the flow back resistance of drilling fluid. Finally, a new drilling fluid using this new additive is designed and evaluated. The new drilling fluid works effectively in preventing water blocking by decreasing surface tension and transforming the wettability of tight sandstone from hydrophilic to intermediate wet. This contributes to the significant recovery of formation permeability (more than 92.1%) and the increasing of gas production.

Filtration control of drilling fluid plays an important role in the process of wellbore construction and ensures the success of drilling operation. A swellable polymer microsphere (SPM) of methylmethacrylate (MMA), butyl acrylate (BA), and lauryl methacrylate (LMA) was synthesized with suspension polymerization. Its swelling property was characterized by oil adsorption capacity measurement. First, the effect of SPM on the filtration and rheological properties of virgin organic clay dispersion was investigated. Then the filtration and rheological properties of mineral oil-based drilling fluid in the presence of SPM were elucidated under the respective conditions of hot aging temperature, oil to water ratio and density. Furthermore, the filtration control properties between SPM and two traditional filtration loss additives including modified lignite and asphaltic additive was compared. Addition of SPM exhibited obvious influence on the viscosity of oil-based drilling fluid especially at high concentration of 3 w/v%, therefore, a concentration of lower than 1.5 w/v% was recommended. After hot aging at 200 °C, the oil-based drilling fluid containing 1 w/v% SPM exhibited an API filtration loss decrease of 85%, and HTHP filtration loss decrease of 79% in comparison with the base fluid. After hot rolling at 180 °C, the API filtration loss decreased by 2%, 24% and 53%, and the HTHP filtration loss decreased by 42%, 54% and 65% respectively when 1 w/v% asphaltic additive, modified lignite and SPM are in the presence of the base fluid. Besides decreasing the filtration loss volume under high temperatures, incorporation of SPM also improves the filter cake quality of oil-based drilling fluid. SPM is capable of adsorbing considerable base oil and become swellable. The swellable polymer microspheres with favorably deformable and compressible properties can effectively fill the voids of filter cake and reduce the permeability, which leads to low filtration loss and minimization of solids invasion.

Owing to superior temperature stability in comparison with water-based drilling fluids, oil or synthetic-based drilling fluids are generally preferred for high temperature and high pressure (HTHP) formations. However, the thermal degradation of emulsifiers and polymeric components under HTHP conditions that results in loss of rheological and filtration control, barite sag or even fluid phase separation also occurs. It is a challenge to sustain these properties stable under such harsh condition. Since nanoparticles have potential to provide better thermal stability, improved filtration loss as well as emulsion stability, the aim of this study is to investigate the effect of nano carbon spheres on the properties of oil-based drilling fluids under high temperature conditions. The nano carbon spheres were synthesized with the hydrothermal reaction of glucose. The influence of nano carbon spheres on the rheological, filtration, emulsion stability, settlement stability, as well as lubricity of a typical mineral oil-based drilling fluid with oil to water ratio of 80:20 was investigated before and after thermal aging at 180 and 200°C, respectively. The structure characterization showed that the uniform hard nano carbon spheres exhibited intermediate wettability. Laboratory performance test indicated that, for the oil-based drilling fluid, the addition of nano carbon spheres improved the rheological properties in terms of yield point and the ratio of yield point to plastic viscosity, which is beneficial for transporting of drilling cuttings. After thermal aging at 200 °C, the filtration loss volume was reduced as high as 70%, and desirable filter cake quality was obtained by incorporation of 1.0 wt% spheres, meanwhile the electrical stability was improved both before and after thermal aging. Furthermore, the fluid formulated with the nano carbon spheres generated better barite sag control. The polarizing microscope observation showed that the nano carbon spheres accumulated at the water-oil interface and formed a steric barrier which probably explained the reason of the above enhanced performance. The green synthetic routes and environmental friendly characteristics of the nano carbon spheres, in combination with the excellent properties suggested that the nano carbon spheres hold potential as multi-functional additives for formulating oil-based drilling fluids for HTHP drilling operations.

The application of wellbore strengthening treatment has less effect on shale formations. Several numerical studies were developed to describe the mechanism, which promoted the development of wellbore strengthening theory. Previous studies explored the mechanism mainly by considering the seepage flow. Therefore, multi-field coupled models were established to analyze the solute transmission, thermal convection, and heat conduction on wellbore strengthening by introducing the theory of multi-field coupling into physical model. First, the fracture width distribution and wellbore tangential stress were investigated to research the interaction of thermal and chemical effects with different gradients. Then, the concrete mechanism of temperature and solute concentration gradient was analyzed based on the distribution of pore pressure and stress field. Results show that the prediction of hoop stress and fracture aperture may not be accurate without considering the influence of solute transfer, thermal convection, and heat conduction, because stress state is mainly affected by temperature field and the pore pressure varies greatly under different chemical gradients. Additionally, the lower temperature and larger solute concentration improve the wellbore strengthening effect of drilling fluid.

Solvent extraction is commonly used to separate mixtures of hydrocarbons and their derivatives, and solvent choice is strongly influenced by the affinity to the target component, cost and safety. Nitrogen-containing switchable solvents can switch from water-immiscible form to water-miscible bicarbonate salts when CO2 is injected and back to water-immiscible form when N2 is injected. Switchable solvents, as a type of recyclable green solvent, can be used not only to separate such mixtures but also to reduce process costs. Herein, four switchable solvents, namely, dipropylamine, di-sec-butylamine, N,N-dimethylcyclohexylamine (CyNMe2), and N,N,N′,N′-tetraethyl-1,3-propanediamine (TEPDA), were employed to separate benzene/cyclohexane, ethyl acetate/acetonitrile, and ethyl acetate/n-heptane mixtures, and the corresponding partition and selectivity coefficients were determined. In all cases, separation selectivity increased in the order of dipropylamine < di-sec-butylamine < CyNMe2 < TEPDA, i.e., TEPDA was best suited for the separation of hydrocarbons and their derivatives. In addition, cycling experiments revealed that the TEPDA can be re-used at least 15 times and was well suited for industrial applications. In the end, the mechanism of the separation was put forward.

This paper is concerned with the formation of bitumen during the drilling of the H oilfield in Iraq. The high viscosity and strong adhesion properties of bitumen can influence the drilling operations. Some complex problems include paste screening, and drill pipe sticking, which cause huge economic losses. Therefore, it is necessary to effectively reduce the bitumen viscosity. The contribution of a single subcomponent of bitumen to the viscosity can vary, and the combined effect of different components of bitumen on the viscosity remains unclear. Furthermore, the mechanism of viscosity reduction remains unclear. In this study, the effects of organic solvents on the viscosity of bitumen were studied, and toluene was selected as the best organic solvent. The results showed that aromatics/resins, aromatics/asphaltenes, and resin/asphaltenes can help increase the bitumen viscosity. Novel methods, including the use of nanoparticles, ethyl cellulose, and the quaternary ammonium salt of heptadecenyl hydroxyethyl imidazoline (QASHI), were proposed to decrease the viscosity. TiO2 and CuO nanoparticles were chosen, and the main factors influencing the viscosity, such as the particle type, concentration, particle size, temperature, and shear rate, were analysed. The results show that the bitumen viscosity decreases with the increase in the concentrations of ethyl cellulose and QASHI. A synergistic effect between ethyl cellulose and QASHI was found with an optimal concentrations of ethyl cellulose and QASHI (1000 and 1600 mg L−1). A synergistic effect was also observed when nanoparticles, ethyl cellulose, and QASHI were used in combination. This paper reports the micro-mechanism whereby the viscosity of bitumen is decreased.

During the process of hydrate reservoir drilling, the heat generated by friction in a horizontal well section leads to hydrate decomposition and affects the wellbore stability. Consequently, the low-temperature environment of subsea wellbores can cause hydrate regeneration and flow obstacles. In this study, modified n-alkanes (MNA) with an appropriate phase transition temperature and acceptable latent heat were synthesised, and the thermal conductivity was improved by adding nano-Cu. A composite phase change material (PCM) was prepared using expanded graphite (EG) with adsorbing MNA, and nano-silica modification to improve its hydrophilicity and stability. Structural properties of the composite PCM were characterised via scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, and nitrogen sorption. The DSC curves revealed an acceptable latent heat capacity and reliability of the prepared composite PCM. In addition, by simulating the temperature heating and cooling during the drilling process, the composite PCM exhibited a reasonable thermoregulating and hydrate decomposition inhibition performance, which can effectively inhibit hydrate decomposition in reservoirs and hydrate regeneration in wellbores. Moreover, the composite PCM exhibited a reasonable compatibility with the drilling fluid. The study results can be applied to realise intelligent temperature control and reduce the risk of drilling in hydrate reservoirs.

The challenge of wellbore stability is getting harder in complex formations. A better understanding of mud cake and its effect on stress state around the wellbore is beneficial for the solution of wellbore stability problems. Thus, the testing method and describing function of dynamic mud cake properties were developed with different drilling fluid additives. The microstructure of mud cake was investigated to further illustrate the mechanism of drilling fluid additives. Based on the poroelastic model, the effect of time-dependent mud cake properties on the distribution of pore pressure, effective stress state around the wellbore and safe mud weight window was analyzed. The results indicate that the buildup of mud cake slows down the increase of pore pressure and broadens the safe mud weight window by increasing the effective stress of compression. The significant increase in fracturing pressure illustrates the potential effect of mud cake in wellbore strengthening. Although the pressure isolation effect increases with the addition of bentonite, it has a limited influence on the safe mud weight window. The addition of Xanthan (XC) enhances the pressure isolation effect of mud cake by improving the coalescence stability of fine-grained particles and plugging the pore with hydration membrane. The viscoelastic granular plugging material has the best sealing efficiency with the deformation property. Although the fine-grained walnut shell has a relatively good pressure isolation effect, its effect on borehole cleaning is far greater than that on wellbore stability.

The injection of CO2 into deep coal seams can not only increase the recovery of CH4 but also contribute to the geological sequestration of CO2. In deep coal seams, CO2 can easily become the supercritical state when the pressure is over 7.38 MPa, and the temperature is over 31.04 °C. It can influence both the physical and chemical properties of coals, especially weakening the mechanical strength, which could compromise the long-term integrity and stability of the deep coal seams. Through acoustic emission experiment and triaxial compression experiment, the results show that after the treatment of supercritical CO2, the mechanical parameters of coals, including dynamic Young’s modulus, static Young’s modulus, rock cohesion, and peak strength, decrease significantly. It demonstrates that supercritical CO2 can reduce the mechanical strength of coals. This macroscopic phenomenon can be explained by the mechanism of the enlargement of microscopic pore spaces of coals, and this mechanism has not been studied thoroughly yet. Therefore, several microscopic quantitative experiments are comprehensively conducted, including scanning electron microscope (SEM), mercury injection porosimetry (MIP) and nuclear magnetic resonance (NMR). The results of these three tests are relatively consistent, and they show that after treatment of supercritical CO2, not only the diffusion space consisting of micropore and transitional pore but also the percolation space involving large pores and even cracks has been enlarged and expanded. This can be a significant underlying microscopic mechanism to effectively explain the weakening of the mechanical strength of coals. The pore space enlargement, together with the swelling of coal matrix, and the theoretical explanation of fracture mechanics and thermodynamic theory are all underlying mechanisms to explain the weakening behavior of coals influenced by the supercritical CO2.

In order to solve wellbore instability problems in the drilling process, high-performance shale stabilizers in water-based drilling fluids are essential. In this paper, a novel shale stabilizer based on polyethylene glycol grafted nano-silica composite (PEG-NS) was successfully synthesized. Conventional techniques were used to characterize PEG-NS including Flourier transform infrared spectroscopy (FT-IR), scanning electron microscope (SEM), transmission electron microscopy (TEM), particle size distribution (PSD) and thermogravimetric analysis (TGA). The results showed that the structure of PEG-NS was in agreement with the design objective. The particle size of PEG-NS ranged from 110 to 434 nm, and PEG-NS had good thermal stability. Pressure transmission test, pore structure characterization of shale, rolling recovery test were applied to evaluate the comprehensive performance of PEG-NS as a novel shale stabilizer. The experimental data indicated that PEG-NS could effectively retard the pore pressure transmission and reduce the permeability of shale sample. PEG-NS could adsorb onto the shale surface and a dense plugging film could finally coat on it. Moreover, PEG-NS also exhibited better inhibition performance compared with that of potassium chloride (KCl) and polymeric alcohol (JHC) at the same concentration. Hence, PEG-NS could be a good shale stabilizer in water-based drilling fluids for drilling reactive shale formations.

To convert the hazardous and waste oil sludge into shale stabilizer using in drilling fluid, a novel preparing method has been proposed in the current study. According to the characteristics of oil sludge, the preparation procedure was designed learning from conventional asphaltic-type shale stabilizer preparation method. Several additives such as gilsonite, calcium carbonate, surfactants (hexadecyltrimethylammonium chloride (CTAC) and polyoxyethylene octylphenol ether (OP-10)), polymers (polyethylene glycol (PEG-2000) and cationic polyacrylamide (CPAM)), and fluid loss reducers (sulfonated lignite and potassium humate) were incorporated into oil sludge with the assistance of melting, stirring, kneading, and drying. After the sequential steps of asphaltization, emulsification, and stabilization, a powdered shale stabilizer product (SS-OS) was obtained. The effect of SS-OS on rheology, filtration, and lubricity of drilling fluid bentonite dispersions was tested. Furthermore, the properties of stabilizing shale were evaluated. The results indicated that SS-OS could effectively inhibit shale swelling, dispersion, and exhibit excellent sealing performance. Moreover, it helped to reduce the fluid loss and significantly improve the lubricity of the drilling fluid. From the preliminary study, it was concluded that the strategy of preparing shale stabilizer with oil sludge could be employed to overcome waste, energy and environmental problems simultaneously and show promising application.

Interest in using nanomaterials to improve shale stability during drilling operations has been increasing. Herein, a polymer microsphere emulsion (PME) as a high-performance shale stabilizer for water-based drilling fluids (WDFs) was prepared via emulsion polymerization. The particle sizes in PME in aqueous solution ranged from 90 to 320 nm. PME was found to exhibit excellent salt tolerance and temperature resistance. The plugging performance of PME was tested through pressure transmission tests. The results indicated that the polymer microspheres in PME could effectively plug shale pores and reduce shale permeability. In addition, rolling recovery tests were used to evaluate the shale hydration inhibition performance of PME. It was found that PME showed great performance for decreasing shale hydration potential. These factors make PME a promising shale stabilizer for WDFs used to drill shale formations.

Effective filtration loss control is a challenge for water-based drilling fluid when drilling high temperature and high pressure (HTHP) formations. After thermal aging at 200 °C for varying times, the static filtration loss controlling capabilities of β-cyclodextrin polymer microspheres (β-CDPs) and synthetic polymer Driscal-D in bentonite suspensions were compared in this study. A comprehensive physicochemical characterization was presented using Fourier transform infrared (FT-IR), X-ray diffraction (XRD), particle size distribution, zeta potential, electrical conductivity and scanning electron microscope (SEM). The results indicated that when exposure to thermal aging of 200 °C, the filtration loss of bentonite suspension increased with the initial 6 h of aging, after that it increased slightly. While for synthetic polymer Driscal-D, the filtration loss increased gradually with thermal aging time until to 36 h. In contrast for β-CDPs, the filtration loss increased firstly and then decreased and maintained relatively stable after thermal treatment for 6 h. High temperature destroyed the hydrated structure of bentonite particles and induced the increase of filtration loss volume. In viewing of Driscal-D, degradation of the polymer in combination with dehydration of bentonite particles led to the gradual losing of suspensions stability. For β-CDPs, after aging of 6 h, the hydrothermal carbonization reaction generated numerous nano and micro carbon spheres and bentonite-carbon sphere composited structures, which fill in the gaps between the larger particles and contribute to reducing the filter cake permeability. This result identified that β-CDPs are superior to synthetic polymer Driscal-D in filtration control when subjected to long term high temperature environments.