High Pressure And High Temperature Research Articles

Small-scale Experimental Research on the Key Parameters Affecting Metal-to-metal Sealability

The harsh service environment of high pressure and high temperature (HPHT) gas well have posed great challenge to the seal integrity of the casing string thread connections. All the time, due to the complexity of metal-to-metal sealing mechanism, there is no uniform reliable sealability criterion in the Finite Element Analysis process of the casing premium thread, so a systematic research on the sealing mechanism of the casing premium thread metal-to-metal seals. In this paper, a series small-scale metal-to-metal seal tests considering different influence factors were conducted, the results showed that the average contact stress at the leakage threshold increases with gas pressure, thread compound and metal roughness has significant influence on the metal sealing property, tests with fine surface roughness (<0.05um) and without thread compound show the lowest critical average contact stress value. Based on the test results, the key parameters for calculating the sealability criterion model were calibrated, and the sealability criterion is recommended for use in high-pressure applications.

Mechanical properties and microstructures of HPHT sintered polycrystalline diamond compacts reinforced with SiC whiskers

Polycrystalline diamond compact (PDC) cutters, the most widely used cutting tool in oil and gas drilling, often fail very quickly because of excessive wear and poor impact toughness in hard and complex formations. In this study, dense PDC samples doped with SiC whiskers (SiCw) were successfully synthesized under high pressure and high temperature (HPHT) conditions. The microstructures, elemental distributions, and phase compositions of the PDC samples were systematically investigated. The mechanical properties including the Vickers hardness, wear resistance, impact toughness and thermal stability, were tested. The introduction of SiCw reduced the Co content in the polycrystalline diamond layer. The testing results showed that the hardness of PDC doped with 0.5 wt% SiCw was 75.25 GPa, wear ratio was 145.6 × 104 and impact toughness reached 4020 J, which were 12.2%, 30.9%, and 142.2% higher than those of the control group (CG), respectively. The PDC-0.5 wt% SiCw had the lowest thermal expansion value, with a 57.5% reduction compared with the CG. In addition, the strengthening mechanism of SiCw was explored. SiCw partially replaced the weakening cobalt phase in the binder phase, thus improving the hardness, wear resistance, and thermal stability of the PDC. The introduction of SiCw inhibited the abnormal growth of WC at the interface of the cemented carbide substrate and changed the fracture mode from intergranular to transgranular, which were the main mechanisms responsible for the significant increase in the impact toughness of PDC.

Multifunctional tantalum disilicide ceramics sintered at high temperature and high pressure

AbstractSilicide ceramics are attracted significant attention due to their exceptional physical properties. However, their inherent brittleness at room temperature limits their practical applications. Here, we aim to improve the mechanical properties of tantalum disilicide (TaSi2) ceramics by modifying the preparation methods and sintering conditions. We fabricate the dense TaSi2 ceramics using high‐pressure and high‐temperature (HPHT) sintering technology at 5.5 GPa and 800–1300°C. We characterize the microstructures, mechanical, and electronic properties of bulk TaSi2 ceramics at HPHT conditions. Our results indicate an enhancement in the fracture toughness of 11.97 MPa m1/2 for TaSi2 ceramics. More importantly, the electrical conductivity (47.13 × 105 S/m) of TaSi2 ceramic is increased and verified by theoretical calculations. These findings provide crucial insights for future applications of TaSi2 and offer valuable avenues for the design and synthesis of advanced ceramic materials.

Synthesis of electronegative S-filled and Sn–Te double substituted skutterudite compounds by HPHT and its thermoelectric properties

Electronegative S-filled skutterudite compounds have attracted considerable attention due to scattering of low-frequency phonons (35–50[Formula: see text]cm[Formula: see text]), but there are few reports on S-filled double-substituted skutterudite. In this work, a series of S-filled Te-Sn double-substituted skutterudite compounds were synthesized using high pressure and high temperature (HPHT) method. The phase composition, microstructure, and thermoelectric properties of SxCo4Sb[Formula: see text]Te[Formula: see text]Sn[Formula: see text] ([Formula: see text], 0.05, 0.10, 0.20) samples were systematically determined. The results show that the introduction of S increases the substitution limit of Te in the Sb site. Under the combined effect of the sample synthesis pressure and S filling, the nucleation density of the sample grains increases and the grain size decreases. At the same time, there are numerous special microstructures in the samples synthesized in a high-pressure environment. Thermoelectric performance studies show that a small amount of S filling optimizes the Seebeck coefficient of the sample and increases the power factor (PF). With an increase of the amount of S introduced, the thermal conductivity of the samples decreases significantly. Therefore, the maximum zT value of an S[Formula: see text]Co4Sb[Formula: see text]Te[Formula: see text]Sn[Formula: see text] sample synthesized at 2.5[Formula: see text]GPa pressure is 0.17 at room temperature. The maximum zT value of the sample is 1.26 at 773[Formula: see text]K.

Synthesis and investigation of structural properties of high-quality niobium disilicon ceramics at high pressure and high temperature

The synthesis of high-quality silicide ceramic has been challenging mainly due to the high melting point of the silicide and the difficulty in achieving a uniform composition. This study reports the synthesis of dense and homogeneous niobium disilicon (NbSi2) ceramics, accomplished with high-temperature and high-pressure (HTHP) technology. The synthesized bulk NbSi2 ceramics were extensively characterized for their microstructures, phase stabilities, and oxidation resistance, along with the effects of pressure on their physical properties, and the results were then compared with first-principles calculations. The microstructures and relative densities of the sample were regulated by the slow-modulated HTHP sintering method, thus significantly influencing the physical properties and thermal stabilities of the NbSi2 ceramics. The findings of the study provide valuable insights into the physical properties of silicide ceramics, th potentially paving the way for the further design and synthesis of multifunctional ceramic materials.

A high-temperature and high-pressure cell for in situ visualization of reaction processes by neutron imaging

This study developed a high-temperature and high-pressure (HTHP) cell for in situ neutron imaging of hydrothermal reactions. The cell’s maximum temperature and pressure were 500 °C and 50 MPa, respectively, and its vessel for observing reactions comprised SUS316 stainless steel. Neutron transmission images were obtained to observe the behavior of sub- and supercritical water and the decomposition of two plastics (polypropylene and polyethylene) at HTHP. The images showed that water’s density and phase changed with temperature and pressure, affecting neutron transmission (and thus image brightness). The plastics began to melt and change shape at 150–200 °C, and they decomposed at 500 °C and 20 MPa. This study provides a basis for future research using the HTHP cell to examine various reactions such as the decomposition of biomass samples, the reforming of heavy oil, and the synthesis of nano-materials using sub- and supercritical water.

Influence of Ni doping on the thermoelectric properties of Bi2S3 via high pressure and high temperature

Bi2S3-based materials without containing expensive, toxic, and rare components are considered more attractive for the realization of large-scale practical applications. Under this perspective, in this work, nickel was employed to enhance the thermoelectric properties of polycrystalline Bi2S3, which was fabricated by using the mechanical alloying (MA) method combined with the high pressure and high temperature (HPHT) technology. By increasing the Ni doping content, the electrical conductivity was significantly enhanced, whereas the absolute value of the Seebeck coefficient was gradually reduced, which was mainly derived from the increased carrier concentration. Moreover, numerous lattice and distortions were introduced in the stucture of the Bi2S3 samples that were synthesized by the HPHT technology. At the same time, Ni doping contributed to the strong impedance for phonon propagation and suppressed thermal conductivity. As a consequence, a peak zT value of 0.23 at 543 K was achieved in the Ni0.005Bi2S3 sample, being about three times than that of pure Bi2S3.

Effect of H2S on CO2 Corrosion of Mild Steel in High-Temperature Supercritical CO2 Conditions

The effect of H2S on the aqueous corrosion behavior of mild steel was evaluated at high-pressure and high-temperature (HPHT) conditions at a partial pressure of CO2 of 12 MPa and a temperature of 160°C. The corrosion rate of steel samples was determined by electrochemical and weight loss measurements. The surface and cross-sectional morphology and the composition of the corrosion product layers were analyzed by using surface analytical techniques (scanning electron microscopy, energy-dispersive x-ray spectroscopy, and x-ray diffraction). Results showed that the corrosion rate decreased with time and no significant difference was observed in the presence of 1,000 ppm and 2,000 ppm of H2S at HPHT CO2 conditions. Surface and cross-sectional analyses revealed that the corrosion process was governed by the formation of FeCO3 regardless of the presence of H2S. Furthermore, the corrosion behavior of mild steel in these conditions did not depend significantly on flow velocity.

Pore pressure prediction assisted by machine learning models combined with interpretations: A case study of an HTHP gas field, Yinggehai Basin

Accurately estimating pore pressure in high-temperature and high-pressure (HTHP) formations is crucial to prevent drilling problems and engineering disasters. In this study, four machine learning models were developed, namely decision tree, random forest, gradient boosting decision tree, and extreme boosting decision tree models, along with the classical Eaton's method, to rapidly predict pore pressure in an HTHP gas field in the Yinggehai Basin using seven variables, including ‘depth’ and six common well-logging series. The results demonstrate that the machine learning models outperform the classical Eaton's method in pore pressure prediction. Among the models, the decision tree model exhibits the best performance, with the highest correlation coefficient (R2 = 0.98) and the lowest root mean squared error (0.61). Additionally, the Graphviz tool and Shapley additive explanations method were used for the visualization and interpretation of the decision tree model to gain a deeper understanding of the model's working mechanism and the reasons behind the obtained results. The interpretation revealed that the variable of ‘depth’ was the most significant input parameter in the decision-making process of the model. This study provides insights into the ‘black-box’ interpretation of machine learning models and demonstrates that tree-based machine learning models can accurately predict pore pressure in undersurface HTHP formations.

From h‐BN to Graphene: Characterizations of Hybrid Carbon‐Doped h‐BN for Applications in Electronic and Optoelectronic Devices

AbstractHybrid two‐dimensional materials consisting of graphene and hexagonal boron nitride (h‐BN) have drawn significant interest due to their tunable bandgap and electrical properties. Considering their composition‐dependent properties, ohmic current injection and the development of h‐BN‐based optoelectronic and high‐power electronic devices should be achievable by controlling the C concentration. In this study, electrical and optical characterizations of single‐crystal h‐BN synthesized under high‐pressure and high‐temperature (HPHT) are conducted by varying C concentrations via post‐growth diffusion. Low C‐doped h‐BN (h‐BN:C) with ≈0.1 at% C exhibits nonohmic conduction within a voltage range of ±100 V at all temperatures. In contrast, high h‐BN:C (≈10 at% C) containing C domains and graphite/graphene layers shows additional luminescence peaks and initially exhibits nonohmic conduction at 298 K, which then transforms to ohmic conduction after breakdown‐like behavior at 598 K. This phenomenon, observed only in the high h‐BN:C devices, is attributed to the C‐containing conductive path formed on the channel surface through C drift and local dielectric breakdown of h‐BN mother phase, indicating that ohmic conduction itself does not guarantee the current flow in the conduction/valence bands in h‐BN:C. With these findings, the present thorough and fruitful characterizations are beneficial for the development of h‐BN:C‐based devices.

Optimization of thermoelectric properties in elemental tellurium via high pressure

High pressure and high temperature (HPHT) technology, as an extreme physical condition, plays an important role in regulating the properties of materials, having the advantages of enhancing doping efficiency, refining grain size, and manufacturing defects, therefore it is quite necessary to study the effectiveness on tuning thermoelectric properties. Elemental telluride, a potential candidate for thermoelectric materials, has the poor doping efficiency and high resistivity, which become an obstacle for practical applications. Here, we report the realization of a dual optimization of electrical behaviors and thermal conductivity through HPHT method combining with the introduction of black phosphorus. The results show the maximum zT of 0.65 and an average zT of 0.42 (300 K–610 K), which are increased by 55% and 68% in the synthesis pressure regulation system, respectively. This study clarifies that the HPHT method has significant advantages in modulating the thermoelectric parameters, providing a reference for seeking high performance thermoelectric materials.

Application of microgel latex in water-based drilling fluid to stabilize the fractured formation

Fractured formations are complex formations often encountered during oil and gas drilling. Conventional methods such as plugging formation fractures and pores, improving cutting-carrying efficiency, and inhibiting hydration of cements cannot effectively solve the problem of wellbore instability in fractured formations. In this paper, an adhesive microgel latex (MGL) was prepared to improve the cementing strength of fractured formations. The morphology and thermal property were analyzed by differential scanning calorimetry (DSC), scanning electron microscopy (SEM), etc. The plugging effect of MGL was then studied by visualized plugging experiments and high temperature and high pressure (HTHP) plugging experiments. The evaluation methods for strengthening fractured formation were established, including immersion test, single lap test and compressive strength test, and applied to evaluate MGL. The experimental results show that MGL were spherical particles with a median particle size of 38.9 μm and a glass transition temperature of 61.98 °C. MGL could effectively decrease drilling fluid intrusion and plug fractured formations. Wellbore strengthening experiment showed that MGL could enhance the cementing strength of fractured formations. MGL greatly improved the tensile shear strength between core pieces and cementation strength of the fractured formation. After treatment of MGL, the tensile shear strength between core pieces increased by 81.2% compared to asphalt, and the compressive strength substantially increased for both wet and dry sand cores. The mechanism analysis suggests that the adsorption between MGL and rock matrix and the self-adhesion among MGL particles are responsible for wellbore strengthening. This paper innovatively proposes the use of adhesive particles to enhance wellbore stability in fractured formations.

Modelling upheaval buckling in marine buried pipelines by coupling the shear stresses and soft soil stiffness

Buried marine pipelines employed in the Oil & Gas industry are subjected to pressure and temperature gradients, which cand produce local high compression loads leading to the onset of upheaval buckling failure. Upheaval buckling occurs when the localized stresses across the pipeline are high enough to induce constant deformation due to the low soil restriction in the upward direction. Therefore, models to predict upheaval buckling in buried marine pipes caused by high pressure and high temperature (HP/HT) and soil stiffness have been developed based on Euler-Bernoulli beam theory (EBT). However, this theory does not consider stresses and strains due to shear stresses which can play an important role in upheaval buckling failure. Therefore, in this work an analytical model that takes into account Engesser-Timoshenko beam theory (TBT) and considers the shear effects on pipelines was developed to predict upheaval buckling in buried marine pipelines. Furthermore, equations that govern vertical buckling of buried pipelines considering a plastic soil with initial imperfection were considered. Analytical results were compared with finite element models of buried pipeline and other models reported in the literature, and it was observed that analytical results fall in the range of those reposted in the literature. It was also observed that the incorporation of shear stresses in buried marine pipelines has low effect on upheaval buckling onset and propagation, but the soil stiffness has a strong influence on upheaval failure in buried marine pipelines.

High-pressure and high-temperature induced densely discontinuous nanoprecipitates in multi-principle element alloy

Knowledge of pressure-related microstructural evolution could be beneficial for directionally tuning mechanical performance of materials. Therefore, in this work, a comparison on the precipitates characteristics and mechanical performance of Ni1.5Co1.5CrAl0.2Ti0.1 multi-principle element alloy (MPEA) aged at 800 °C under high-pressure (HP800) or ambient-pressure (AP800) was systematically studied. Results demonstrate both HP800 and AP800 samples exhibit a heterogeneous microstructure consisting of discontinuous precipitate (DP) regions near grain boundaries and continuous precipitates regions in grain interior. However, the volume fraction of DP regions in the HP800 is ∼25.83% higher than that in the AP800. For revealing the intrinsic mechanism, the fully recrystallized matrix Ni1.5Co1.5Cr MPEA was treated by high-pressure. It is found that the plastic deformation occurs, as well as Ni1.5Co1.5Cr MPEA achieves grain refinement via high-pressure and high-temperature (HPHT) because pressure inhibits grain growth and dynamic recrystallization (DRX) process forms the new fine grains. Similarly, in the Ni1.5Co1.5CrAl0.2Ti0.1 MPEA, the grain refinement is also induced by DRX process during HPHT, so as to promote the formation of DP regions. Besides, HP800 reveals a good combination of a yield strength of 801 ± 33 MPa and an ultimate tensile strength of 1249 ± 53 MPa at a good uniform elongation of 41.85 ± 4.03%, owing to high-density stacking faults and Lomer-Cottrell locks. The present finding might provide a fundamental understanding of DRX process under HPHT and a significant guidance for optimizing nano-precipitates via pressure to develop high-performance strengthened MPEAs.

Simulation of Rock Electrical Properties in Deep Reservoirs Based on Digital Rock Technology

Deep reservoirs are in a high-pressure and high-temperature (HPHT) environment, while the experimental conditions for rock electrical properties that meet the deep reservoir conditions are harsh and costly. Although digital rock technology can simulate the electrical properties of rocks, it is limited to electrical simulation studies under normal temperature and pressure conditions (NPT), which limits their ability to capture the electrical characteristics of deep hydrocarbon reservoirs. This limitation affects the accuracy of saturation prediction based on resistivity logging. To simulate the rock electrical properties under HPHT conditions, we proposed a low-cost and high-efficiency HPHT digital rock electrical simulation workflow. Firstly, samples from deep formations were CT-scanned and used to construct multi-component digital rocks that reflect the real microstructure of the samples. Then, mathematical morphology was used to simulate the overburden correction under high-pressure conditions, and the changes in the conductivity of formation water and clay minerals at different temperatures were used to simulate the conductivity changes of rock components under high-temperature conditions. To carry out the electrical simulation of digital rock in deep reservoirs, a numerical simulation condition for HPHT in deep layers was established, and the finite element method (FEM) was used. Finally, based on the equivalent changes in the conductivity of different components, the effects of clay minerals and formation water under HPHT conditions on rock electrical properties were studied and applied to predict the water saturation based on well logging data. We found that considering the influence of temperature, salinity, and clay type, the saturation index (n) of the rock depends on the ratio of the clay conductivity to the formation water conductivity. The larger the ratio is, the smaller the value of n. In addition, the average relative error between the predicted water saturation under HPHT conditions and the sealed coring analysis was 6.8%, which proved the accuracy of the proposed method. Overall, this method can effectively simulate the pressure and temperature environment of deep formations, reveal the electrical conductivity mechanisms of rocks under formation pressure and temperature conditions, and has promising prospects for the study of rock physical properties and reservoir evaluation in deep formations.

Mechanism of phase transition from OLCs with different structures to nPCD at high temperature and high pressure

The molecular dynamics (MD) calculation and experimental research methods were used to study the phase transition from onion-like carbon (OLC) with and without a diamond crystal core to nano-polycrystalline diamond (nPCD) under high temperature and high pressure (HTHP) conditions. The results showed that the transformation from OLC to nPCD was mainly a diffusionless transformation at the heating stage and a diffusional transformation at the cooling stage. There were two main paths for diffusionless transformation from the OLC phase to nPCD. The path (PATH1) was that the diamond grew along the graphite crystal direction [002] with the coherent interface formed by the cubic diamond crystal plane (111) parallel to the graphite crystal plane (002). Another path (PATH2) was that the diamond grew along the graphite crystal direction [120] with the coherent interface formed by the cubic diamond crystal plane (11–1) intersecting the graphite crystal plane (002). The initial path of diffusionless transformation of OLC with and without a diamond core was PATH1 and PATH2, respectively. Increasing the PATH1 phase variable can reduce the sintering conditions of the OLC phase to nPCD. The twin boundary content of nPCD can be improved by increasing the grain size consistency of the OLC and the stacking faults density of the OLC structure.

Effects of B-NiS co-doping on diamonds growing along {1 1 1}-orientation under high pressure and high temperature

Large single diamonds were successfully synthesized from NiS or B-NiS co-doping FeNi-C system under high pressure and high temperature (HPHT) in this paper. In this study, the effects of different additive amounts of NiS doping alone or B-NiS co-doping on the crystal shape, color and quality of diamond were investigated. The color of NiS doped diamonds change from bright yellow to bright green and then to grayish yellow. The color of B-NiS co-doped diamonds change from yellowish green to grayish green and black. Besides, the shape of doped crystals gradually changes from plate to tower. Fourier transform infrared (FTIR) spectra shows that with the increase of additive NiS content, the entry of nitrogen is suppressed to some extent by the entry of S into the diamond lattice, while with B-NiS co-doping, the content of nitrogen impurities increase with the increase of B content. It can be found from the x-ray photoelectron spectroscopy (XPS) results that the obtained diamonds contain B/S/N elements, with S present in the diamond lattice in the form of C–S–O. Raman measurements show that with increasing B content, the peak positions of co-doped diamonds are shifted and the full width at half maximum (FWHM) become slightly larger, but still have a high quality sp3 structure. This study not only contributes to the investigation of the formation mechanism of natural diamond, but also has important implications for the enrichment of diamond species and the expansion of synthetic diamond applications.

Effect of temperature on the fretting wear behavior of Cr-coated Zircaloy cladding in high-temperature pressurized water

Cr-based coatings are promising candidates to protect the claddings on accident-tolerant fuel (ATF) rods in pressurized water reactors (PWRs). In this study, the effect of water temperature on the fretting wear behavior of an N36 Zirconium alloy cladding was investigated. In addition, the effect of applying a protective Cr coating to the cladding was also analyzed. Fretting wear tests were carried out using a unique apparatus that simulated the high-temperature and high-pressure (HTHP) water environment in a real PWR. Results show that changes in temperature alter the wear mechanism of cladding tubes. Furthermore, a Cr-coating improves the wear resistance of an N36 cladding significantly. The main wear mechanisms of Cr-coated cladding in HTHP water were observed to be abrasive and adhesive wear. The protective effect of a Cr coating on the cladding in a low-temperature water environment (25 °C, 90 °C) was less than it was in a high-temperature water environment (260 °C, 315 °C), indicating that Cr coatings can improve wear resistance in HTHP water.

High‐Pressure Induction and Quantitative Regulation of Oxygen Vacancy Defects in Lithium Titanate

AbstractInspired by the functional properties of ion defect induction and charge compensation in defect engineering, these methods are expected to be an effective strategy to solve the constraints of Li4Ti5O12 (LTO) inherent conductivity and diffusion dynamics, and further improve battery rate performance. The oxygen vacancy (OV) content in LTO can be controlled quantitatively by high‐pressure induction using the high‐pressure and high‐temperature (HPHT) method. In addition, the relationship between the electrochemical properties and OV is further explored. The theoretical calculations indicate that the OV defects cause the electrons to delocalize into the conduction band of the LTO, and thus fundamentally improve the intrinsic conductivity. In particular, the high‐pressure quenching strategy of HPHT causes LTO to instantly produce crack holes with massive crystalline layers, which can be regarded as storage for the electrolyte to facilitate ion diffusion. The fabricated LTO anodes containing OVs compensate for the limitation of the poor rate performance with a capacity of 176 mAh g−1 at 20 C. Pressure‐induced OV defects not only open up a new perspective in the field of lithium‐ion batteries (LIBs), but also provide a certain degree of freedom for the functional design characteristics of defect engineering.

Synthesis and Plugging Performance of Nano-Micron Polymeric Gel Microsphere Plugging Agents for Oil-Based Drilling Fluids

As shale gas recovery progresses to deep layers, the wellbore instability during drilling in applications of oil-based drilling fluids (OBFs) becomes increasingly severe. This research developed a plugging agent of nano-micron polymeric microspheres based on inverse emulsion polymerization. Through the single-factor analysis with respect to the permeability plugging apparatus (PPA) fluid loss of drilling fluids, the optimal synthesis conditions of polymeric microspheres (AMN) were determined. Specifically, the optimal synthesis conditions are as follows: the monomer ratio of 2-acrylamido-2-methylpropanesulfonic acid (AMPS): Acrylamide (AM): N-vinylpyrrolidone (NVP) were 2:3:5; the total monomer concentration was 30%; the concentrations and HLB values of emulsifier (Span 80: Tween 60) were 10% and 5.1, respectively; the oil-water ratio of the reaction system was 1:1; the cross-linker concentration was 0.4%. The polymeric microsphere (AMN) produced via the optimal synthesis formula had the corresponding functional groups and good thermal stability. The size distribution of AMN ranged mainly from 0.5 to 10 μm. The introduction of AMND in OBFs can increase the viscosity and yield point of oil-based drilling fluids and slightly decrease the demulsification voltage but significantly reduce high temperature and high pressure (HTHP) fluid loss and permeability plugging apparatus (PPA) fluid loss. The OBFs with 3% polymeric microsphere dispersion (AMND) reduced the HTHP and PPA fluid loss by 42% and 50% at 130 °C, respectively. In addition, The AMND maintained good plugging performance at 180 °C. The AMN particles can block leakoff channels of artificial cores, effectively prevent the invasion of oil-based drilling fluids into formations and suppress pressure transfer. OBFs with 3% AMND enabled the corresponding equilibrium pressure to decrease by 69%, compared with that of the OBFs. The polymeric microspheres had a wide particle size distribution. Thus, they can well match leakage channels at various scales and form plugging layers via compression-deformation and packed accumulation, so as to prevent oil-based drilling fluid from invading formations and improve wellbore stability.