High-temperature geothermal resources are abundant, clean, and non-polluting. Dynamic drilling and fracturing operations are the two main techniques for efficient obtaining of geothermal resources. However, most of the existing studies focus on single-factor analysis, and the dynamic mechanical behavior of deep granite in high-speed dynamic load, high temperature, and high confining pressure is poorly understood, limiting the efficient development of geothermal resources. In this paper, an improved multi-factor coupling rock dynamic mechanics testing device is developed for systematically investigating the dynamic compression characteristics of deep granite under the influence of three major engineering and environmental factors. The results show that granite's uniaxial compressive strength increases with the increase of dynamic load at 25 degrees C. Under constant impact velocity, granite's compressive strength decreases nonlinearly with increasing heat treatment temperature. Especially at 500 degrees C to 600 degrees C, the strength rapidly decreases by 29 %, with maximum fragmentation at 600 degrees C. Under high-velocity impact, compressive strength increases by 8.9 % as confining pressure rises from 5 MPa to 20 MPa. Based on the stress-strain relationship and crack extension mode of granite, three strength evolution modes of granite are identified under three different conditions. Finally, a special nonlinear logarithmic rock constitutive equation is developed by a bold improvement of the Ramberg-Osgood model which is in good agreement with experimental data. This study offers an important theoretical reference for geothermal drilling and development.

With the growing demand for energy, oil and gas exploration and development are progressively moving into deep and ultra-deep formations, where extreme temperatures and pressures create complex challenges for drilling operations. While drilling fluids are critical for controlling bottom-hole pressure, cooling drill bits, and removing cuttings, accurately characterizing their rheological behavior under high-temperature and high-pressure (HTHP) conditions remains a key focus, as existing research has limitations in model applicability and parameter prediction range under extreme downhole environments. To address this, the study aims to determine the optimal rheological model and establish a reliable mathematical prediction model for drilling fluid rheological parameters under HTHP conditions, enhancing the precision of downhole temperature and pressure calculations. Rheological experiments were conducted on eight field-collected samples (4 water-based and four oil-based drilling fluids) using a Chandler 7600 HTHP rheometer, with test conditions up to 247 °C and 140 MPa; nonlinear fitting via a hybrid Levenberg–Marquardt and Universal Global Optimization algorithm and multivariate regression were employed for model development. Results showed that oil-based and water-based drilling fluids exhibited distinct rheological responses to temperature and pressure, with the Herschel–Bulkley model achieving superior fitting accuracy (coefficient of determination > 0.999). The derived prediction model for Herschel–Bulkley parameters, accounting for temperature-pressure coupling, demonstrated high accuracy (R2 > 0.95) in validation. This research provides an optimized rheological modeling approach and a robust prediction tool for HTHP drilling fluids, supporting safer and more efficient deep and ultra-deep drilling operations.

The application of surface rocking-pipe-assisted drilling (SRPAD) technology is largely dependent on the ability driller to transfer an appropriate amount of weight to the bit. However, the impact of surface torque oscillations on the longitudinal drag and frictional torque is not thoroughly understood. For this purpose, in this study, a novel experimental apparatus was developed to study the load transfer characteristics along a drillstring during SRPAD. In the experiment, the forces and torques at both ends of the simulated drillstring were recorded, and the mechanical responses at various positions on the drillstring are monitored using a strain-measuring method. The experimental results suggest that properly rocking the drillstring back and forth can help operators regain most of the friction-reducing performance of conventional rotating drillstrings. Furtherore, we found that, if the pipe-rocking parameters, including the rocking velocity, rocking angle, surface hookload, and holding time, are properly controlled, the load transfer efficiency can be effectively improved. In particular, the rocking angle and surface hookload should be increased to reduce the stick-slip effect and buckling distortion. The experimental results provide important guidance for improving the performance of SRPAD system, though the flowing of drilling fluid and the influence of bent-housing motors are not taken into consideration.

Polycrystalline diamond compact (PDC) bit is one of the most widely used drill bits for improving the rate of penetration in deep oil and gas well and geothermal well. However, the dynamic rock fragmentation mechanics characteristics of PDC bits are still unclearly. A coupled fragmentation mechanics model of PDC cutter-rock interaction is established by combining the mixed fragmentation modes with dynamic strength. The coupling influence laws of cutter angle, cutting depth, dynamic strength ratio, breaking modes on the horizontal force coefficient (HFC), vertical force coefficient (VFC) and specific energy are analyzed. The model of this paper can optimize cutter inclination angle, cutting depth and minimum specific energy. With the increase of the cutter inclination angle, the dynamic VFC changes into two modes. The definition of the dynamic modes depends on the dynamic strength ratio. As the cutting angle increases, the cutting force increases. The cutting force increases nonlinearly with increasing cutting depth. The specific energy of rock fragmentation increases nonlinearly with increasing cutting depth. With the increase of dynamic strength, the specific energy of rock fragmentation increases nonlinearly. When the input-energy increases, the rate of penetration response is divided into three stages. The results have important guiding significance for the PDC bit design and drilling parameters optimization to increase the rate of penetration and the efficiency of exploration and development.

The main goal of this research is to conduct a quantitative assessment of how rocking influences friction during sliding. The kinetic equations were developed, accounting for the friction coefficient's velocity weakening effect. The model's precision and dependability were confirmed by comparing simulation results against experimental data. Furthermore, the study discovered that the Stribeck model presents benefits when compared to alternative models. Investigations were carried out to examine how rocking parameters affect axial friction during rocking motions. The findings indicate that variations in axial friction forces are predominantly observed during the phase of tangential acceleration. Crucially, the research demonstrates that modulating the rocking velocity's waveform can markedly decrease the average axial friction, even when the maximum rocking speeds remain the same.

Multiwell pad drilling (MWPD) can shorten the well construction period, lower drilling costs, and greatly promote the utilization level of resources and equipment. It has become the most effective and efficient solution for shale gas extraction. This chapter first introduces the development status of MWPD worldwide. Then, the key factors that influence the implementation of MWPD are summarized and analyzed. The MWPD mainly comprises standardized predrilling engineering and global optimization of drilling design and operation. The former includes well patterns optimization and platform deployment, and the latter consists of well trajectory design and control, portable drilling rigs, high-performance polycrystalline diamond cutter bits, drilling fluids technology, high-efficiency cementing technology, improving rate of penetration methods, friction-reducing technologies, and specific drilling operation procedures. The relevant data collected from the shale gas fields both in China and North America are used to analyze the achievements and challenges of MWPD are also discussed. Future expectations for MWPD technologies are considered.

For the exploration and exploitation of natural gas resources in deep and unconventional reservoirs, deep and horizontal drilling are the key technologies. This chapter aims to introduce the key deep and horizontal drilling technologies for natural gas. This chapter begins with a classification of the major types of natural gas wells, such as vertical wells, directional wells, horizontal wells, and extended-reach wells, and briefly introduces their main characteristics and application situations. Then, the key technologies for deep and horizontal drilling are presented in detail. These include casing program design, managed pressure drilling, underbalanced drilling, vertical drilling system, high efficiency drill bits, penetration rate increasing technologies, high-temperature and high-pressure drilling fluids, directional technologies, measurement while drilling, geo-steering technologies, and drilling risk monitoring and management. Finally, the deepest and longest wells all over the world are identified from literature to show the application and recent achievements of deep and horizontal drilling technologies in the petroleum industry, and the challenges of deep and horizontal drilling are briefly discussed.

Shale gas in the Sichuan Basin has become an important field and object of large-scale benefit development. The southern Sichuan Basin has become the main battlefield of shale gas exploration and development in China, and its annual shale gas production in 2022 is 223.23×10~8 m~3, revealing great exploration and development potential of shale gas in this area. In the southern Sichuan Basin,however, shale gas long horizontal wells face severe challenges of low rate of penetration（ROP） and long drilling cycle. In order to clarify the shale dynamic crushing behaviors and energy consumption laws in the rock breaking process of shale-gas horizontal well drilling, this paper tests the stress wave propagation process of dynamic rock breaking by virtue of the Hopkinson pressure bar test technology. In addition, the evolution characteristics of shale dynamic strength, the response laws of crushing energy consumption and the dynamic propagation behaviors of rock cracks under dynamic loading are evaluated by using the stress wave balance method. And the following research results are obtained. First, in the process of shale crushing, dynamic strength and energy consumption directly influence the crushing and ROP improvement effect. Shale strength changes dynamically under different loading rates and loading modes. Dynamic loading can activate more microcracks inside the shale and penetrating cracks. Second, as the loading rate increases, the compressive strength and tensile strength of shale increase slowly at first and then rapidly when the critical strain rate is exceeded. Third,there is a critical strain rate interval corresponding to the energy consumption of shale compression crushing. As the strain rate increases,rock breaking energy consumption increases slowly. When it exceeds the critical interval, energy consumption increases rapidly. The energy consumption of shale tensile crushing increases logarithmically, and it also has a critical strain rate interval. Fourth, in the process of bit-rock interaction, excessively high loads can directly increase the contact strain rate, resulting in bit damage and increased contact strength, so as to reduce the rock breaking efficiency. Fifth, the comprehensive evaluation indicators of efficient rock breaking should include static uniaxial compression strength, rock dynamic strength and energy consumption. In conclusion, the research results provide a theoretical support for the ROP improvement and optimization of shale-gas horizontal well, and is of important guiding significance to improve the rock breaking efficiency in deep shale and conducive to the large-scale benefit development of shale gas.

Formation testing while drilling (FTWD) is useful for acquiring dynamic formation parameters in real time. However, the FTWD is inevitably affected by the thermal-hydro-mechanical (T-H-M) coupling effect as the current drilling depth and the formation temperature continue to increase. In order to clarify the mechanisms of the effect of T-H-M coupling on the pressure response of FTWD, a novel T-H-M coupling mathematical model of the pressure response of FTWD was proposed. The parametric studies of the pressure response of FTWD under T-H-M coupling condition were simulated using the finite element method (FEM). The influence of different borehole wall conditions and coupling conditions on the pressure response of FTWD were compared, and the effect of T-H-M coupling on the interpretation of formation parameters was also discussed. The results indicated that the stress perturbation of drilling has little effect on the FTWD, but the change in formation pressure and temperature around the wellbore will significantly affect the pressure response of FTWD. The T-H-M process can create the “coupling skin” effect in the formation, which further leads to the greater pressure drop in the pressure drawdown stage and the lower growth rate in the pressure recovery stage. Formation permeability and temperature differential have a large effect on the pressure response of FTWD, but in situ stress and probe orientation have almost no effect. When the inversion of the original formation parameters is performed using the formation rate analysis (FRA) method, the near-wellbore supercharging can lead to an overestimation of the original formation pressure and the T-H-M coupling can lead to an underestimation of the original formation mobility. The model established in this paper is more in line with the actual test conditions, and the results can provide theoretical support for parameter optimization and data interpretation of FTWD.

Over recent years,building a surface drilling test platform to simulate the actual downhole conditions has become an important approach for verifying and improving new technology,new process and new tools of drilling.The design and development of the multi-functional drilling test platform of a drilling company in China and its hydraulic control system were introduced in this paper,and the test platform was tested in the field.The hydraulic control system of the test platform includes the pitching system,rotation system,tension-compression system,traction system and auxiliary system.It can control and drive the test platform to move smoothly on the track.It can also manually and remotely control the rotating speed and torque of the drill string of the test platform and the tensile load,compressive load and cross beam lifting speed of the test platform.Moreover,it enables the manual and remote control of switching working angles of the portal frame of the drilling test platform at 0°,60° and 90°(the intersection angles with the plumb line),clamping of drill pipes,locking of the portal frame cross beam and anchoring the platform base on the track.The findings of this research provide technical references for the research and development of the hydraulic control system of drilling test platforms.

Indentation provides a powerful tool for measuring the mechanical properties of shale fragments at the nano- or micro-scale, allowing test with shale cuttings and avoiding the difficulties of coring in horizontal wells, but the meso-mechanical properties and their anisotropy are rarely investigated. Therefore, three types of Longmaxi shale fragments were collected to perform micro-indentation tests, and the meso-mechanical properties, such as hardness, elastic modulus, uniaxial compressive strength (UCS), fracture toughness, and brittleness, in both the bedding plane normal (BPN) and bedding plane parallel (BPP) directions were determined. Finally, the anisotropy of different mechanical properties was compared, and the relationships among mechanical properties, clay minerals, and brittle minerals were discussed. The results indicated that the hardness, elastic modulus, UCS, and brittleness in the BPP direction are higher than those in the BPN direction, while the fracture toughness is opposite. All the mechanical properties in the BPP direction are in positive proportion to those in the BPN direction for these three shales. The hardness, elastic modulus, UCS, and brittleness of the Fuling shale are higher or slightly higher than those of the Changning shale, followed by the Weiyuan shale. All the mechanical properties in both the BPP and BPN directions increase with the brittle minerals. The anisotropy in elastic modulus increases with increasing clay content, the anisotropy in brittleness decreases with increasing clay content, because the anisotropy depends on both the anisotropic clay minerals and the oriented geometric factors of the solid phase.

Low-cost, fast and accurate acquisition of multi-scale mechanical properties of shale is essential to realize the complex hydraulic fracture network and the optimization of multi-scale fracture efficiency of bottom hole rock. The macro-scale indentation test is used to study the mechanical parameters of the meso-scale across the scales. The rapid evaluation method of the shale mechanical parameters across the scales is established, which can be used as a method to obtain the mechanical parameters of shale quickly and accurately. The macro-indentation test experiment is carried out through the elastic contact hypothesis, the macro-micro mechanics theory, and the grid indentation experiment method. Considering the coupling effects of peak indentation fluctuations, contact stiffness(S), and the ratio of elastic work to total work (Wu/Wt), the cross-scale distribution characteristics of shale hardness(H) and elastic modulus(E) are studied. The results showed that the elastic modulus and hardness showed a normal distribution on the whole, and the macroscopic indentation process of the rock was accompanied by the meso-scale indentation. For Longmaxi shale, the elastic modulus and hardness have large discreteness and heterogeneity. The kernel density values of elastic modulus and hardness were studied by using kernel density analysis method. Different factors have different effects on the fluctuation amplitude of elastic modulus and hardness. The fluctuation range of the S is the largest, and the fluctuation range of Wu/Wt is the smallest. At the meso-scale, the dispersion of hardness peak value is larger, and the dispersion of elastic modulus peak value is smaller than that of valley value. The S-Wu/Wt-E-H coupling response law presents a "striped" characteristic. The S-Fm-E-H coupling response law presents a "multi-point" distribution. The Fm-Wu/Wt-E-H coupling response law shows a "wave-like" distribution. The hardness of vertical bedding indentation is greater than that of parallel bedding. There is a good linear correlation between the elastic modulus and hardness. With the increase of the elastic modulus, the hardness increases. The research results are helpful to determine the parameters in the actual design of hydraulic fracturing and improving the rock breaking efficiency.

Formation testing while drilling (FTWD) shows a great potential for characterizing reservoir type, estimating reserves, and determining reservoir energy. With the increasing in unconventional reservoirs, however, the utilization of FTWD has encountered some challenges. The near-wellbore formation prior to conducting the formation testing is usually supercharged due to the mud filtrate invasion during drilling. The supercharged pore pressure can directly influence the pressure response of FTWD. If the pressure response curves are misinterpreted, it may bring some mistakes or risks to engineering design. In addition, the formation testing process is a hydro-mechanical (H-M) coupling process, and the variation of pore pressure in formation can change the porosity and permeability of the rock, which will in turn affect the pressure response of formation testing while drilling. To clarify the effects of near-wellbore supercharging and H-M coupling on pressure response of FTWD, a three-dimensional simulation model of FTWD was built andthe pressure responses with different parameters wereanalyzed. The simulated results indicated that: In thesupercharged condition, the initial probe pressure ishigher than the original pore pressure. And during thepressure recovery stage, the probe pressure rises abovethe original formation pressure in the early part of thebuildup and then decreases to reach the equilibrium state.Also, the supercharging effect can result in theoverestimation of the original formation pressure. TheH-M coupling can produce an extra coupling skin on thepressure response of FTWD, and H-M coupling cancontribute to the underestimation of the originalformation mobility. The results of this paper can help usto understand the pressure response behavior andimprove the formation parameter interpretation accuracyof FTWD.

Aiming at efficiently and reliably controlling the intelligent sliding sleeve to open and close from the surface, a bypass downlink system is proposed without influencing the current stimulation equipment and process. A mathematics model based on the fluid hammer model and the method of characteristics is established to analyze the characteristics of the downlink system and validated by the experimental and field results. According to simulation results, the pressure change of the wellbore due to valve opening increasing or decreasing is coincident with the exponential expression. The expression contains three coefficients, which are the high pressure, the low pressure, and the system constant. The system constant is relative to the volume of the wellbore and the steady-state pressure. According to the characteristics of the pressure wave in the cased wellbore, digital base-band transmission is adopted for sending the pressure signal. A float matching decode method is proposed based on these characteristics and validated by the simulation results. The simulation results also indicate that the float matching decode method has high transmission efficiency and a low bit error rate. It can adapt to complex conditions of stimulation treatment. The work of this article can improve the efficiency and reliability of controlling the intelligent down-hole sliding sleeves.

The widely accepted hydraulic fracturing technology in the unconventional oil/gas exploitation needs an effective way to open and close the casing sliding sleeve in order to save cost and avoid the unnecessary risk of the downhole operations. We propose a novel downlink system using downhole pressure waves to remotely control the opening and close of the casing sliding sleeve from the surface. We develop a laboratory device to understand the characteristics of the downhole pressure waves. According to the laboratory experiments, we find that the changing of the wellbore pressure can be described by an exponential expression when the cased wellbore is either closed or leaked. There are three parameters in the expression, including the steady-state high pressure (P-H), the steady-state low pressure (P-L), and the system constant (tau(0)) of pressure increasing and decreasing. Both P-H and P-L are a function of the wellbore leakage volume and the tau(0) is a function of both the wellbore volume and the leakage volume. In addition, due to the strong attenuation of the high-frequency component (>10 Hz) of the pressure wave resulting from the long distance propagation in the wellbore channel, we use the baseband transmission to send the codes from the surface to the casing sliding sleeves by opening and closing the bypass valve. We also proposed a float matching decoding method to overcome the inaccuracy and inefficiency of the existing baseband transmission method. The novel codec method is validated by the laboratory experiments and the results indicate that the float matching decode can effectively improve the accuracy and efficiency of baseband transmission with a relatively short length of code under the situation of the strong noise and well leakage.

Formation Testing While Drilling (FTWD) to evaluate pore pressure has significantly progressed in geo-energy exploitation. However, with the increasing number of unconventional reservoirs, this technology is facing some challenges owing to the common occurrence of supercharging in the vicinity of boreholes. The pressure response of FTWD under supercharged conditions is different from that of non-supercharged; thus, conventional methods — such as Pressure DrawDown (PDD), Pressure BuildUp (PBU), and Formation Rate Analysis (FRA) — cannot be used to determine the original pore pressure. In this paper, a new mathematical model of the FTWD pressure response under supercharged conditions is proposed by considering the time-dependent filter cake, supercharging, skin, geometry, and storage effects. The calculated results indicated that when the formation permeability was lower than 5 mD, the supercharging effect on the pressure response of FTWD cannot be neglected. The pressure response curve of FTWD under supercharged conditions only includes three segments: the fast pressure drawdown stage, pressure buildup stage, and recovered pressure decreasing stage. It is also found that the pressure response in the later stage is independent of the withdraw withdrawal pumping modes and the pressure response curves are nearly the same. Based on the pressure response characteristics, a novel method to evaluate the original pore pressure was proposed. Case studies show that the proposed method can obtain the correct virgin pore pressure after a few iterations and the relative error is lower than 0.2%. Compared with conventional methods, this method is more accurate and saves time; thus, it can be used to evaluate the original pore pressure of FTWD under supercharged conditions.

The rock mechanical behavior and damage characteristic is of great importance for in situ stress evaluation, wellbore stability analysis and hydraulic fracturing design. The velocities of elastic waves are usually reduced in the presence of rock damage, it may be used for determining the progressive damage of the rock. Therefore, this paper aims to investigate the damage characteristics of transversely isotropic tight sand formation, the rock mechanical and damage parameters in the vicinity of the wellbore were calculated using acoustic logging data. The results indicated that the Poisson's ratio and damage parameters decrease with increasing in radial distance, while the elastic modulus and Thomsen’s coefficients increase. At the same radial position, the vertical elastic modulus is smaller than that of the horizontal, the degree of anisotropy for P-wave is greater than that of S-wave, and the horizontal damage parameter is greater than that of the vertical, which indicated that the micro-cracks near the wellbore mainly occur in the horizontal direction. The changes in mechanical parameters, Thomsen’s coefficients and damage parameters rapidly changed in the range of 1.0–1.8 times of borehole radius. The variations of Thomsen’s coefficients and damage parameters in mudstone are obviously greater than that of sandstone, which may be due to the induced damage between rocks and drilling fluid of mudstone is much higher than sandstone.

The excessive drag/torque and the backing pressure is an important factor that restricts the improvement of the penetration rate and the extension of the drilling in the sliding drilling process of extended-reach wells and horizontal wells. To deal with this problem, this paper developed a novel controllable hybrid steering drilling system (CHSDS) based on the friction-reducing principle of a rotating drill string. The CHSDS is composed of a gear clutch, hydraulic system, and measurement and control system. By controlling the meshing and separation of the clutch with the mud pulse signal, the CHSDS has two working states, which leads to two boundary conditions. Combined with the stiff-string drag torque model, the effects of the drilling parameters on the friction-reducing performance of the CHSDS are analyzed systematically. The results show that the friction reduction effect in the inclined section is the most significant, followed by that in the horizontal section, whereas there is almost no impact in the vertical section. Friction reduction increases with the rotary speed and the drilling fluid density, whereas it decreases with the increase in the surface weight-on-bit and the bit reaction torque. Field tests confirm the separation and meshing function of the CHSDS. The developed controllable hybrid steering and friction-reducing technology provides an alternative approach for the safe and high-efficiency drilling of horizontal wells.

由于地质环境的隐蔽性和复杂性,地质力学参数和岩石力学参数具有很大的不确定性,井壁稳定分析中忽略不确定的影响可能导致钻井液密度设计不合理,进而直接影响钻井井壁稳定.国内外学者对确定参数条件下的井壁稳定开展了深入的研究,但对不确定参数条件下井壁稳定影响的研究并不深入,尤其是对于任意斜井失稳风险的评价.为此,在井壁稳定孔弹性力学模型和崩落宽度模型基础上,建立了基于可靠度理论的井壁失稳风险评估方法,采用Monte-Carlo随机方法模拟了四川盆地CW气田直井、斜井和水平井的井壁失稳风险,并系统分析了参数均值和方差对井壁失稳的影响规律.研究结果表明:①随着井斜角增加,井壁坍塌当量密度显著增加,井壁破裂当量密度显著降低,不同井型失稳风险与常规井壁稳定规律基本一致.②考虑参数不确定影响后,坍塌当量密度增加、破裂当量密度降低,安全密度窗口逐渐变窄,说明参数不确定影响不可忽略.③崩落宽度对井壁坍塌影响显著,崩落宽度越大则井壁坍塌可靠度越高,在不发生井壁失稳事故的前提下,允许适当的井壁崩落有助于提高成功钻井概率.④参数均值和变异系数敏感性分析结果表明,影响井壁稳定最为显著的因素为地应力,其次为孔隙压力和岩石强度;随着变异系数的增加,坍塌当量密度逐渐增加、破裂当量密度逐渐降低,安全密度窗口逐渐变窄,井壁失稳的风险越高.