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Supercharging in the vicinity of a borehole is an important factor that affects formation damage and drilling safety, and the filter cake growth process has a significant impact on supercharging in the vicinity of the borehole. However, existing models that predict pore pressure distribution overlook dynamic filter cake growth. Thus, an analytical supercharging model was developed that considers time-dependent filter cake effects, and this model was verified using a two-dimensional numerical model. The influences of filter cake, formation, and filtrate properties on supercharging were investigated systematically. The results indicate that time-dependent filter cake effects have significant influence on supercharging. Supercharging increases in the early stage but decreases over time because of the dynamic growth of filter cake, and the supercharging magnitude decreases along the radial direction. Because of filter cake growth, the magnitude of supercharging falls quickly across the filter cake, and the decreased magnitude of pore pressure caused by the filter cake increases. Supercharging in low-permeability formations is more obvious and the faster rate of filter cake growth, a lower filtrate viscosity and faster reduction rate of filter cake permeability can help to weaken supercharging. The order of importance of influencing factors on supercharging is overbalance pressure > formation permeability > formation porosity ≈ filtrate viscosity > filter cake permeability attenuation coefficient > initial filter cake permeability control ratio > filter cake growth coefficient > filter cake porosity. To alleviate supercharging in the vicinity of the borehole, adopting drilling fluids that allow a filter cake to form quickly, optimizing drilling fluid with a lower filtrate viscosity, keeping a smaller overbalance pressure, and precise operation at the rig site are suggested for low-permeability formations during drilling.

Pore pressure is a significant parameter in drilling engineering. A better understanding of near-wellbore pore pressure propagation by taking into account the thermal-hydro-mechanical coupling process and dynamic mudcake growth is beneficial for wellbore stability analysis, formation testing while drilling and well logging while drilling. To reveal the pore pressure evolution mechanism, a new fully coupled thermal-hydro-mechanical model of pore pressure propagation around a borehole with dynamic mudcake growth is proposed. A comparison with the conventional approach suggests that neglecting the dynamic mudcake effect or the coupling effect can overestimate the pore pressure in the vicinity of borehole. The variation process of pore pressure can be divided into four stages, and the coupling effect weakens as time progresses. Compared with the Young's modulus of the rock, formation porosity, and temperature difference, the near-wellbore pore pressure is more sensitive to the overbalanced pressure, in situ stresses, formation permeability, and the Poisson's ratio of the rock. The near-wellbore pore pressure disturbance in formations with a low permeability and high in situ stress is more significant; therefore, to consider the effects of thermo-hydro-mechanical coupling and dynamic mudcake growth in deep and ultra-deep well drilling is required.

In the filter cake formation process (FCFP), existing models have assumed that the particles in drilling fluids are monosized, which is completely inconsistent with actual drilling fluids. Therefore, a novel FCFP prediction model was proposed by considering the solid particle size distribution (SPSD) and was verified using Hashemzadeh's model. The influences of SPSD, formation properties, drilling fluid properties, and engineering parameters on the FCFP were investigated. The results indicated that when the influence of SPSD is involved, the filter cake thickness is 2 times thicker than that of mean particle size; this means that the influence of SPSD on the FCFP cannot be ignored. The solid particle distribution type and mode in drilling fluids directly influence the FCFP. Adding the appropriate content of small-sized particles in drilling fluids will help form a high-quality filter cake. The filter cake growth rate increased with increasing in formation permeability, drilling fluid consistency coefficient, drilling fluid mobility index, pressure difference between the wellbore and formation; while it decreased with rising filtrate viscosity and drill-string rotational speed. The order of importance of influencing factors is mobility index > particle mean size > filtrate viscosity > particle size standard deviation > pressure difference > consistency coefficient > formation permeability > drill-string rotational speed. The present model can provide theoretical guidance for drilling fluid optimization, reservoir protection and well construction safety, thereby to facilitate safe, high-quality, efficient and economical drilling.

The significant decreased wellbore temperature and increased casing pressure during fracturing fluid injection present a big challenge for the mechanical integrity of cement sheath in fracturing wells. Based on the theories of elastic mechanics, thermodynamics, and a multi-layer composed thick-wall cylinder, this paper proposed a new mechanical model of cement sheath for fracturing wells, coupling pressure, and thermal loads, which consider the failure modes of de-bonding, radial cracking, disking, and shear failure. The radial nonuniform temperature change and the continuous radial stress and radial displacement at two interfaces have been considered. With the proposed model, the radial distributions of failure stress and the corresponding safety factor for cement sheath during fracturing fluid injection have been analyzed and compared under four failure modes. Results show that the decreased wellbore temperature will produce significant tri-axial tensile stress and induce cement failure of de-bonding, radial cracking, and disking. The increased casing pressure will significantly lower the risk of de-bonding but also aggravate radial cracking and shear failure. For integrity protection of cement sheath, increasing the injected fluid temperature, maintaining higher circulation pumping pressures, and adopting cement sheath with a low elasticity modulus have been suggested for fracturing wells.

The state of health (SOH) and remaining useful lifetime (RUL) estimation are important parameters for battery health forecasting as they reflect the health condition of battery and provide a basis for battery replacement. This study proposes a novel on-line synthesis method based on the fusion of partial incremental capacity and artificial neural network (ANN) to estimate SOH and RUL under constant current discharge. Firstly, the advanced filter methods are applied to smooth the initial incremental capacity curves. Then the strong correlation feature values are extracted from the partial incremental curves by using correlation analysis methods. Finally, two ANN models aiming at estimating SOH and RUL are established to estimate the SOH and RUL simultaneously. The training and verification results indicate that the proposed method has highly reliability and accuracy for SOH and RUL estimation.

Geothermal energy development has increasingly been studied in recently decades because of its renewable and sustainable features. It can be divided into two categories: traditional geothermal (hydrothermal) systems and enhanced geothermal systems (EGS) based on the type of exploitation. The hot dry rock (HDR) in the EGS incorporates about 80% of all thermal energy, and its value is about 100&ndash, 1000 times that of fossil energy. It is pivotal for geothermal wells to improve the flow conductivity of the HDR mass, enhance the communication area of natural fractures, and constitute the fracture network between injection and production wells by hydraulic treatments. While the wellbore temperature significantly decreases because of fracturing, fluid injection will induce additional thermal stresses in the cement sheath, which will aggravate its failure. Considering the radial nonuniform temperature change, this paper proposes a new thermal stress model for a casing-cement sheath-formation combined system for geothermal wells during fracturing based on elastic mechanics and thermodynamics theory. This model is solved by the Gaussian main elimination method. Based on the analytical model, the thermal stresses of cement sheath have been analyzed. The effects of the main influencing parameters on thermal stresses have also been investigated. Results show that the radial and axial tensile thermal stresses are both obviously larger than tangential tensile thermal stress. The maximum radial and axial thermal stresses always occur at the casing interface while the location of the maximum tangential thermal stress varies. Generally, thermal stresses are more likely to induce radial and axial micro cracks in the cement sheath, and the cement sheath will fail more easily at the casing interface in fracturing geothermal wells. For integrity protection of the cement sheath, a proper decrease of casing wall thickness, casing linear thermal expansion coefficient, cement sheath elasticity modulus, and an increase of the fracturing fluid temperature has been suggested.

Strength anisotropy is one of the most distinct features of anisotropic rocks, and it also normally reveals strong anisotropy in Brazilian test Strength (“BtS”). Theoretical research on the “BtS” of anisotropic rocks is seldom performed, and in particular some significant factors, such as the anisotropic tensile strength of anisotropic rocks, the initial Brazilian disc fracture points, and the stress distribution on the Brazilian disc, are often ignored. The aim of the present paper is to review the state of the art in the experimental studies on the “BtS” of anisotropic rocks since the pioneering work was introduced in 1964, and to propose a novel theoretical method to underpin the failure mechanisms and predict the “BtS” of anisotropic rocks under Brazilian test conditions. The experimental data of Longmaxi Shale-I and Jixi Coal were utilized to verify the proposed method. The results show the predicted “BtS” results show strong agreement with experimental data, the maximum error is only ~6.55% for Longmaxi Shale-I and ~7.50% for Jixi Coal, and the simulated failure patterns of the Longmaxi Shale-I are also consistent with the test results. For the Longmaxi Shale-I, the Brazilian disc experiences tensile failure of the intact rock when 0° ≤ βw ≤ 24°, shear failure along the weakness planes when 24° ≤ βw ≤ 76°, and tensile failure along the weakness planes when 76° ≤ βw ≤ 90°. For the Jixi Coal, the Brazilian disc experiences tensile failure when 0° ≤ βw ≤ 23° or 76° ≤ βw ≤ 90°, shear failure along the butt cleats when 23° ≤ βw ≤ 32°, and shear failure along the face cleats when 32° ≤ βw ≤ 76°. The proposed method can not only be used to predict the “BtS” and underpin the failure mechanisms of anisotropic rocks containing a single group of weakness planes, but can also be generalized for fractured rocks containing multi-groups of weakness planes.