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Abstract 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. Highlights • Filtration loss additive for OBDF is synthesized with MMA, BA and LMA. • SPM shows superior performance to asphalt and lignite type products. • Adsorbing base oil, volume swelling and viscosifying lead to filtration loss control. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, the shale hydration inhibitive properties of polyether diamine (PEDA) in drilling fluid system were studied. The inhibition was evaluated by bentonite inhibition test and bulk hardness test. The results indicate that the inhibition properties of PEDA are superior to potassium chloride which is a kind of conventional inhibitor, and can be improved with decrease of pH value. The inhibition mechanism of PEDA was investigated via Fourier transform infrared spectroscopy, X-ray diffraction, zeta potential measurement and surface tension analysis. After being added into the drilling fluid system, the low-molecular-weight PEDA can intercalate into the lattice of clay. The protonated diammonium ions exchange sodium ions and neutralize the negative charges of clay interlayer, which reduces hydration repulsion of diffuse electric double layer. Moreover, hydrogen bonding between PEDA and silica of clay surface can form in the process. The coordination of electrostatic interaction and hydrogen bonding expels water molecules out of the clay gallery and binds the plates together, which leads to the dehydration of clay. Otherwise, monolayer adsorption of PEDA on the interlayer of clay weakens the hydrophilicity of clay particles which further inhibits water ingress. [Copyright &y& Elsevier]

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 testing method and descriptive function of dynamic mud cake properties were developed. • A hydro-mechanical model was developed with the effect of time-dependent mud cake. • The combination of bentonite, polymer and viscoelastic plugging material had the best pressure isolation effect. The addition of plugging material decreases the pore pressure, which indicates that the pressure isolation effect of mud cake can be effectively improved with the packing of rigid particles (Fig. 20). When the content of plugging material is high, the pore pressure decreases slightly, which illustrates that the increase of rigid materials cannot obviously enhance the compactness of the plugging zone. The addition of plugging material increases the effective stress of compression, which shows that the plugging of rigid particles decreases the risk of wellbore instability with higher compressive strength. [ABSTRACT FROM AUTHOR]

The increase in the pressure bearing capacity is critical for mitigation of lost circulation, and to do so the strength of the plugging zone should be increased via picking an effective evaluation method and choosing the proper particle size distribution (PSD). Therefore, in order to successfully investigate this process, the traditional evaluation device of lost circulation should be improved for dynamic variation of fracture width to find the optimized plugging materials. This means, a new testing apparatus device capable of testing dynamic fracture width should be developed. Additionally, commonly used selection criteria of PSD are incapable of providing us with a continuous PSD curve since they only fulfill discrete PSDs. To overcome these two obstacles, a variety of continuous PSD models based on a new testing device and method are examined. Likewise, the impact of governing parameters and an optimized PSD selection criterion were studied with a specific lost circulation material (calcium carbonate) and concentration in a water-based drilling fluid. The experimental results demonstrate that the normal distribution model provides the best selection result while the sensitive analysis indicate that the factors influencing pressure bearing capacity in a sequential order are: minimum particle size, standard deviation, maximum particle size and average value. Besides, the optimized experiments illustrated that the maximum particle size should be 1.3 times of the maximum fracture width and the minimum particle size should be 0.8 times of the average fracture width. Moreover, the average value should be equal to the optimal particle average size and the standard deviation should be 0.3 times of the particle average size. It was found that the optimized selection method results in the highest plugging efficiency compared to conventional selection criteria. Collectively, the new device and optimal assessing approach would improve the prediction accuracy of pressure bearing capacity to select an appropriate PSD with varying of fracture width in field operations. • A lost circulation testing device with dynamic fracture width is developed. • The normal distribution model has the best selection efficiency. • Influencing factors are found to be minimum size, standard deviation, maximum size and mean. [ABSTRACT FROM AUTHOR]

Wellbore strengthening technique is one of the most commonly used treatment to prevent leakage into natural or induced fractures. The accurate prediction of fracture geometry is essential for the particle size distribution (PSD) optimization of wellbore strengthening material (WSM). Due to the existence of weak planes, the rock has the anisotropic characteristic, which results in fracture distortion. Thus, a fully coupled poro-elastic model is solved with Abaqus by using the extended finite element method (XFEM) - based cohesive zone method (CZM) to investigate the geometry of near-wellbore fracture. The fracture trajectory and opening are captured before and after bridging by coupling the rock deformation, seepage flow of pore fluid, fluid flow in fracture and leak-off of drilling fluid. The sensitive analysis results illustrate that the fracture tends to propagate along the principal material direction with the effect of anisotropy. The larger ratio of Young's modulus aggravates fracture distortion in the near-wellbore area. The increase of ratio of horizontal in-situ stress weakens the effect of anisotropy. The anisotropic permeability has minor effects on fracture geometry with the combined influence of anisotropic mechanical properties and horizontal stress. The lower injection rate decreases fracture width after bridging and makes the fracture propagation process more controllable through the influence of filtration. The increase of viscosity widens the fracture aperture and increases cuttings transport capacity of drilling fluid. • A fully coupled wellbore strengthening model is developed by using XFEM-based CZM. • The effect of anisotropy on fracture geometry is investigated without predefining fracture path. • The fracture geometry is mainly determined by rock anisotropy and ratio of horizontal stress. [ABSTRACT FROM AUTHOR]

As an important preventive and remedial application of lost circulation, the optimization of lost circulation materials (LCMs) has become a crucial problem. In order to optimize the design of LCMs, the mechanism of bridging was studied. Based on the theory of solid-liquid flow, a coupled Computational Fluid Dynamics (CFD) - Discrete Element Method (DEM) model was introduced to accurately simulate the dynamic bridging process of plugging particles. The criterion of critical bridging concentration was developed to quantitatively evaluate the effects of Young's modulus, particle density, particle size distribution (PSD), friction coefficient, fracture geometry, lost circulation velocity and fluid yield stress on bridging capacity. The results show that the plugging zone will become more stable with the increase of particle angularity, Young's modulus, density of LCMs, lost circulation velocity and the decrease of fluid yield stress. The friction failure is easier to occur, because the stability of plugging zone is more sensitive to the friction between particle and fracture surface. The critical bridging concentration is more sensitive to the ratio of inlet size to outlet size (R io) with a higher ratio of outlet size to particle size (R o). For a small outlet size (R o ≤ 2.5), the bridging state can be observed obviously because the large particles are dominant. When the outlet size is larger (R o > 3), the stable plugging zone is hard to form because the dense state increases the risk of friction failure. The hindering effect of gravity on the trajectory of particles will be more significant when the fracture extension direction is perpendicular to the gravity direction. • A coupled CFD-DEM model was developed to simulate bridging mechanism. • The criterion of critical bridging concentration was developed for evaluation. • The effect of particle properties, fracture geometry and loss velocity were studied. [ABSTRACT FROM AUTHOR]

Abstract 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. Highlights • 1-Octyl-3-methylimidazolium bromide (OMB) was introduced as a promising shale inhibitor. • OMB exhibited excellent shale swelling and dispersion inhibition properties. • The shale hydration inhibition mechanism of OMB was thoroughly studied. [ABSTRACT FROM AUTHOR]

Abstract 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. Highlights • The damage mechanism of tight sandstone gas reservoir has been explored. • Highly efficient protective agent of microemulsion has been designed and evaluated. • The new protective drilling fluid can protect tight sandstone gas reservoir. [ABSTRACT FROM AUTHOR]

The principle of damage mechanics was adopted in order to solve the lost circulation problem when drilled low bearing capacity formation. According to quadratic nominal stress criterion and B-K criterion, cohesive element with zero thickness was inserted into fracture to simulate fracture initiation and propagation. A 3D fluid-solid coupling finite element model was established to analyze circumferential stress distribution of wellbore, hoop stress distribution of crack and geometry of fracture. Results show that closed fracture surface is still in the stage of complete fracture under the action of crustal stress and crack tip reopens after plugging particle failed. The fracture is difficult to widen for filling more plugging materials in a short time while the lost circulation rate and permeability are low. The wellbore strengthening treatment will makes the fracture become short and narrow under a larger filtration coefficient, but mud cake quality has an insignificant influence on fracture geometry when filtration coefficient increases to a certain value. The simulation results were supported by providing some cases of real wellbore condition. [ABSTRACT FROM AUTHOR]

As one of the most common drilling problems, lost circulation causes severe economic losses. However, the commonly used experimental testing apparatuses and methods are inadequate to evaluate the dynamic sealing capacity of lost circulation process, which neglect the dynamic variation of fracture width. Thus, a lost circulation testing device containing dynamic fracture aperture was developed, which could be used to investigate the influence of lost circulation material (LCM) features, fracture geometry and fluid properties on sealing capacity by evaluating the maximum sealing pressure difference and equivalent plugging position. The wellbore pressure, fracture closure pressure and fracture tip pressure can be controlled and measured independently. The sensitive analysis results show that the sealing pressure has an opposite relationship with the equivalent radius of plugging zone. The particle blends selected by D50 Rule has the best sealing performance by bridging near the fracture mouth with the dynamic variation of fracture width. Properly narrowing the range of particle size distribution (PSD) increases the effective sealing concentration. The increase of LCM concentration improves the extensive adaptability of plugging zone with the same sized particles. The increase of LCM resiliency improves the adaptation of plugging zone to dynamic fracture width. The increase of fracture mouth aperture decreases the sealing capacity by extending the required plugging zone. The fluid viscosity increases the sealing capacity by increasing the carrying capacity of fluid and the flow resistance of lost circulation. The increase of injection rate decreases the plugging capacity by increasing the hydrodynamic disturbance. • A lost circulation testing apparatus with dynamic fracture width is developed. • The new evaluation method of equivalent sealing position is established. • Effect of LCM features, fracture geometry and fluid properties is investigated. • The suggestions for the optimization of LCMs and field operations are provided. [ABSTRACT FROM AUTHOR]

Lost circulation in fractured formations is a major challenge. The addition of conventional lost circulation materials (LCMs) to drilling fluids is the main method to control loss. The seal integrity is affected by different properties of LCMs. Plugging apparatus with slot is extensively used to evaluate sealing performance of the selected LCMs. In general, the fracture in the formation has depths, and the drilling fluids loss far into the formation. A LCM test apparatus with long fracture slot was introduced in this paper. The main objective of this work is to study the effect of LCM type, concentration and particle size distribution on the seal integrity with long fracture slot. The breaking pressure, fluid loss values, position and thickness of plugging zone are the parameters for evaluating LCM performance. Optical microscopy and scanning electron microscopy were used to observe the arrangement and microstructure of the seal. We can better understand the sealing mechanisms in long fracture by analyzing the cell pressure and fluid loss vs. time plot and the seal structure. Results showed that increasing the LCMs stiffness resulted in higher breaking pressure. There is a critical maximum concentration at which LCM will bridging and filling set at the fracture entrance and no tight seal barrier inside the fracture. Loss effect was beneficial to form thicker plugging zone inside the fracture. Suitable wide particle size distribution of LCM formed thick plugging zone inside the fracture. Multi-layer seal barrier can withstand the external load together, resulted in high seal integrities. The addition of fibrous materials in particles resulted in higher breaking pressure and lower fluid loss. • A LCM test apparatus with long fracture slot was designed to investigate the sealing process and the plugging zone. • Loss effect was beneficial to form thicker and impermeable plugging zone inside the fracture. • Multi-layer seal barrier can withstand the external load together, resulted in high seal integrities. • Increasing the LCMs stiffness resulted in higher breaking pressure. • Suitable wide PSD of LCM formed thick plugging zone inside the fracture. [ABSTRACT FROM AUTHOR]