The Influence Law of Temperature-Pressure Coupling Effect on the Mechanical Properties of Cement Stone
To address the unclear mechanical response mechanisms of the cement sheath in deep oil and gas well cementing under complex temperature-pressure conditions，this study systematically investigated the coupled effects of curing temperature，curing pressure，testing temperature，and confining pressure on the mechanical properties of Class G oil-well cement. Specimens were prepared at different curing temperatures ranging from 25°C to 140°C and curing pressures of the atmospheric pressure and 21.7 MPa. Triaxial compression tests were then conducted at different testing temperatures from 25°C to 140°C and confining pressures from 0 to 20 MPa to analyze the evolution of stress-strain curves，peak stresses，and elastic moduli. The results show that increasing curing temperature reduces the peak stress and elastic modulus of the set cement，while high-pressure curing significantly inhibits pore development and improves structural compactness. Increasing testing temperature helps enhance the mechanical performance of cement at the testing temperature close to its curing temperature. Increasing confining pressure improves the plasticity and residual strength of cement，and the cement exhibits staged evolutionary characteristics under different coupled conditions. The findings of this study provide a theoretical basis and experimental support for the design of cement slurry systems and the integrity evaluation of cement sheaths in deep formations.
To address the challenges of high compressive strength，low drillability，and extended drilling time in the deep tight shale of the Mabei Fengcheng Formation，an experimental study on drilling parameter optimization for axial-torsional coupled percussive drilling was conducted. This study intended to identify the dominant factors affecting rock-breaking efficiency and to establish a parameter matching method suitable for this layer. Using a composite percussive rock-breaking efficiency testing device，drilling experiments were performed on core samples from the target layer under fixed conditions of weight-on-bit and rotational speeds with different combinations of axial and torsional impact frequencies. The effects of impact frequency on rate of penetration （ROP），torque，and mechanical specific energy （MSE） were evaluated. The results show that axial-torsional coupled impact can significantly increase the ROP while reducing torque and MSE. High-frequency torsional impact plays a dominant role in torque reduction and energy saving，and a synergistic rock-breaking mechanism was identified in which axial impact promotes crack propagation，whereas torsional impact alleviates bit sticking. This study confirms that axial-torsional coupled impact technology provides significant benefits in improving drilling efficiency，reducing torque，and lowering energy consumption in this shale formation. The obtained parameter optimization method provides a theoretical basis for field drilling design and offers important engineering guidance for accelerating drilling in deep hard strata.
To address the limitation of conventional smooth parallel-plate models，namely neglecting the influence of roughness on proppant transport，this study establishes a fracture geometry model based on fractal theory to embody actual reservoir roughness characteristics，where the Weierstrass-Mandelbrot function is modified using measured rock-wall profile data. The coupled computational fluid dynamics and discrete element method （CFD-DEM） is employed to simulate proppant migration and post-fracturing flowback，with a focus on the effects of the injection port number，proppant concentration，and particle size combination. The results indicate that in the case of single-port injection，fracturing fluid energy is concentrated to deliver a maximum equilibrium proppant bank height of 3.25 cm and the lowest flowback rate. When the proppant concentration increases from 8% to 11%，the proppant bank height rises from 2.60 cm to 3.28 cm，accompanied by a corresponding decrease in the flowback rate of proppants. For a particle size combination of 70/140 mesh，40/70 mesh，and 20/40 mesh at a mass ratio of 1：6：3，the equilibrium proppant bank height reaches 3.28 cm，and the proppant retention rate reaches 97.64%. It is revealed that rough-wall undulations induce localized vortex zones，increase the resistance to particle settling，and promote the formation of inter-particle force chains，thereby significantly enhancing proppant bank stability and near-wellbore placement efficiency. The proposed fractal-modified fracture model and the CFD-DEM coupling approach effectively capture the nonlinear effects of wall undulations on particle transport，providing theoretical support for optimizing proppant placement strategies in shale oil hydraulic fracturing.
The deep shale oil reservoir of the Fengcheng Formation in Mabei area features complex geological conditions and its fracturing stimulation is faced with multiple challenges，including high in-situ stress，strong heterogeneity，and densely laminated rock texture. Taking the C2-C4 layers as the research object，this study systematically investigates the mineral composition，laminar texture，rock mechanical properties，and in-situ stress distribution of the reservoir to reveal their control mechanisms on hydraulic fracture initiation and propagation，as well as proppant transport. The results indicate that the reservoir is predominantly composed of high-brittleness minerals，with the combined content of quartz and carbonate rocks exceeding 70%. However，the mineral composition varies dramatically at the centimeter to millimeter scale，forming dense rigid-brittle laminae and lithological interlayers which significantly enhance mechanical anisotropy and interface strength of rocks. The overall in-situ stress is high，with the minimum horizontal principal stress exceeding 80 MPa，while the horizontal stress difference is only 7.0-9.8 MPa. Theoretically，this low stress difference favors the formation of complex fracture networks. Yet，due to the dual effects of high stress magnitudes and laminar interfaces，vertical fracture propagation suffers from step-like restrictions，with fracture heights generally less than 20 m，and thus，it is difficult to achieve thorough stimulation of multiple layers. Natural fractures intersecting hydraulic fractures at small intersection angles may induce branch fractures to increase fracture network complexity；at high intersection angles，they primarily act as conduits for fluid filtration loss. Under the “composite layer effect，” the effectiveness of layer penetration of hydraulic fractures is low，with width reduction at points of fracture deflection exceeding 60%. Proppant can only pass through fractures with widths 2.7 times larger than proppant particle sizes，and accordingly，proppants of conventional sizes are found hard to enter secondary fractures. This study reveals the control mechanism of the “composite layer effect” on hydraulic fracture propagation and proppant transport in the Mabei Fengcheng Formation，provides a theoretical basis for volume fracturing parameter optimization and fracture network regulation in deep complex shale oil reservoirs，and offers direct technical support for the efficient development of shale oil in Mabei area.
Progress and Research Directions of Shale Oil and Gas Drilling and Completion Technology in China and Abroad
The vital technological breakthroughs in horizontal drilling and multi-stage hydraulic fracturing have promoted a great leap in the development of the shale oil and gas industry and profoundly reshaped the global energy landscape. This study reviewed the latest technological advances in horizontal drilling and well completion for shale oil and gas both in China and internationally. A comprehensive comparative analysis was conducted across the casing program，rotary steering，automated drilling rigs，intelligent drilling，high-performance drilling fluids and lost circulation prevention and control，as well as fracturing equipment and monitoring technologies. The drilling time，horizontal wellbore length，and "one-trip" ratio in China showed considerable room for improvement，compared to those abroad. Therefore，efforts of research and development shall be strengthened for automated drilling rigs，next-generation rotary steering tools，high-efficiency polycrystalline diamond compact（PDC） bits，high-torque screws，and multi-stage fracturing and monitoring technologies，to enhance the "one-trip" ratio of long horizontal wellbores and boost production per well，promote the digital and intelligent transformation of drilling and completion technologies in China，and provide strong technical support for China's shale oil and gas revolution.
The Mahu Fengcheng Formation shale oil reservoir features deep burial depths，complex pressure systems and strong formation heterogeneity，and such complex geological conditions severely restrain the drilling efficiency of horizontal wells，highlighting the urgency of research on technologies enhancing rate of penetration（ROP）. Based on rock mechanical tests，whole-rock mineral composition analyses and geological characteristics of the Mahu Fengcheng Formation，the technical challenges of drilling were systematically investigated，and two technical routes for ROP enhancement were proposed，namely the optimized casing program and high-efficiency rock breaking with high weight on bit （WOB）. The proposed technologies were applied to six horizontal wells drilled in 2025 and delivered remarkable ROP enhancement performance. Specifically，based on characterization of the four pressure profiles （pore pressure，breakdown pressure，collapse pressure，and leak-off pressure），the surface casing set depth was optimized to precisely isolate low-pressure and fluid-loss-prone zones. A combined borehole profile and a potassium-calcium-based polyamine organic salt drilling fluid system were adopted to maintain a borehole enlargement ratio within 5%. Rock mechanics tests determined that efficient rock breaking in the Fengcheng Formation requires a WOB greater than 111.1 kN. A differentiated polycrystalline diamond compact （PDC） bit matching technology based on high WOB and an optimization technology for Ф139.7 mm high-stiffness bottom hole assembly （BHA） were developed to perform layer-specific optimization of drill bit parameters and operational parameters. Laboratory analyses and field applications demonstrated that this technology package can effectively resolve the ROP improvement challenges of the Mahu Fengcheng Formation shale oil and provide technical support and references for the efficient development of deep and ultra-deep hydrocarbon resources.
Given the accurate prediction and evaluation of reservoir sweet spots，a core demand for the large-scale and cost-effective development of shale oil and gas against the backdrop of China’s high dependence of imported oil and gas，this study systematically reviews the research progress of prediction and evaluation methods for shale reservoir sweet spots，clarifies the applicability and limitations of various methods，and provides theoretical reference and technical support for the efficient deployment of shale oil and gas exploration and development. Based on relevant domestic and international research achievements，this study focuses on two core dimensions，namely the geological and engineering sweet spots，and systematically analyzes the core control factors，index system and technical characteristics of sweet spot evaluation with respect to three mainstream technical routes：experimental core tests，well logging and seismic data analysis，and numerical simulation. It is demonstrated that the evaluation concept of the geological-engineering "dual sweet spot" has become a consensus across the industry. Moreover，the core advantages of the three types of evaluation methods are clarified，and the key limitations are presented，including the uncertainty of scale conversion in core experiments，the multiplicity of well logging and seismic interpretation，and the accuracy of numerical simulation restricted by simplified assumptions. Finally，it is pointed out that the future evaluation of shale reservoir sweet spots needs to develop towards multi-dimensional integration，intelligent multi-source data fusion，and multi-physical field coupling dynamic evaluation，so as to comprehensively improve the prediction accuracy and engineering applicability.
To address the technical challenge that conventional polyacrylamide fracturing fluids cannot simultaneously deliver desired performance of friction reduction and proppant carrying in deep tight/shale oil reservoir，an associative friction reducer SMAP-2000 was prepared and compared with conventional ultra-high molecular weight polyacrylamide friction reducer HPAM-3000. The thickening，friction reduction，dynamic proppant carrying and rheological properties of the two friction reducers were systematically evaluated，and a low friction high proppant carrying supramolecular fracturing fluid was developed. Results show that SMAP-2000 exhibits better thickening efficiency than HPAM-3000 in both fresh water and simulated brine，with significantly lower dosage required to achieve the same apparent viscosity. Within the tested viscosity range，the friction reduction rate of the low viscosity slickwater of SMAP-2000 is similar to that of HPAM-3000，but the friction reduction rates of SMAP-2000 are significantly higher for medium and high viscosity slickwater and linear gel. With the same apparent viscosity，the dynamic proppant carrying capacities of low，medium and high viscosity slickwater of SMAP-2000 is 10%-16% higher than those of HPAM-3000. SMAP-2000 exhibits shear thickening at low shear rates，with a multi-stage rheological structure of “shear thickening-shear thinning-second plateau”. The elastic modulus of 30 mPa·s linear gel of SMAP-2000 is 60%-80% higher than that of HPAM-3000，indicating the characteristics of supramolecular solutions. Field test shows that the wellbore friction of the test well using SMAP-2000 fracturing fluid is reduced to 1.8 MPa/1 000 m，with a friction reduction rate of 85%，the maximum proppant concentration increased to 400 kg/m³，and the daily oil production per well increased by 40%. The developed fracturing fluid system balances friction reduction and proppant carrying to provide technical support for cost effective development of deep tight/shale oil reservoirs.
Given the characteristics of unconventional oil and gas reservoirs，such as low porosity，low permeability，and well-developed natural fractures，this study systematically reviews the theoretical and application progress of the hydraulic fracturing technology. The full text revolves four core domains：First，it reviews the development history of hydraulic fracturing numerical simulation methods and explores the control mechanisms of cross-layer behaviors of fractures in vertical wells and balanced multi-cluster propagation of fractures in horizontal wells. Second，it analyzes the characteristics of fracturing operation curves and the formation mechanisms of screen-out，while highlighting real-time intelligent early warning technologies based on artificial intelligence. Third，it summarizes the evaluation methods for stimulated reservoir volume （SRV） and post-fracturing effectiveness based on instantaneous shut-in pressure decline analysis （G-function），micro-seismic data，and dynamic production data. Finally，it compares the application advantages of traditional empirical production decline models，modern numerical simulation，and machine learning in predicting production of oil and gas wells with complex fracture networks. This study intends to provide a solid theoretical foundation and reference for exploring fracturing mechanisms in complex reservoirs，optimizing operational designs，and improving the development efficiency of unconventional oil and gas. It also points out future development directions，including refined simulation of multi-field coupling，engineering adaptability of intelligent early warning，and precise dynamic characterization of SRV.
To address the challenges of low signal-to-noise ratio （SNR） and the lack of clean ground truth labels for distributed acoustic sensing （DAS） and distributed temperature sensing （DTS） data during hydraulic fracturing，a physics-informed self-supervised blind denoising method is proposed. Instead of relying on clean reference data，this method constructs self-supervised training samples using a checkerboard alternating sampling strategy on time-depth sub-blocks and enables the denoising network to learn noise distributions adaptively. Furthermore，physical constraints such as thermal conduction smoothness，edge preservation，and wavefield coherence are introduced for DTS and DAS characteristics，respectively. A spatially weighted loss function based on perforation cluster positions is also established to prioritize the preservation of key response features near clusters. It is demonstrated that the proposed method outperforms traditional filtering methods in terms of SNR and structural similarity （SSIM）. The denoised data significantly enhances the clarity of cooling fronts and acoustic energy bands at fluid entry points，which effectively improves the accuracy of fracturing event recognition and cluster efficiency evaluation.
The produced fluids from the Mabei shale oil reservoir of the Xinjiang oilfield is characterized by high salinity and strong emulsion stability，leading to a series of challenges in thermal-chemical sedimentation and electrical dehydration processes，such as high demulsification temperature，low dehydration efficiency，high energy consumption，and unstable operations of electrical dehydration. Accordingly，this study focuses on the produced fluids from the Mabei shale oil reservoir and performs molecular dynamics simulations and laboratory-scale hydrocyclone-electrostatic coupled dehydration experiments to elucidate the mechanisms underlying their high emulsion stability and to explore effective demulsification methods. Molecular dynamics simulation results indicate that resins，asphaltenes，silicon compounds，and high-salinity salts form a rigid interfacial film at the oil-water interface through hydrogen bonding and polar interactions. This film，exhibiting high mechanical strength and significantly hindering droplet coalescence，is fundamental driver for the high stability of produced fluids. Based on these findings，laboratory experiments integrating hydrocyclone pretreatment with electrochemical dehydration are conducted. This coupling process can fully utilize the pre-demulsification effect of the swirling flow field. The results demonstrate that at 70 ℃ and a demulsifier concentration of 200 mg/L，the water content in the produced fluid is reduced to below 0.1% after hydrocyclone pretreatment and electrochemical dehydration，meeting the requirements for deep dehydration. This study provides a theoretical foundation and technical reference for the development of low-energy-consumption，high-efficiency dehydration technologies for Mabei shale oil.
High-salinity produced water is prone to salt precipitation in the gathering and transportation process of oilfields，which affects pipeline transportation safety and system stability. To address salt precipitation in produced water from a high-salinity and high-alkali reservoir in Mabei Oilfield，this study conducted experiments on salt precipitation factors，single-salt critical salinity，and mixed-salt cooperative precipitation experiments，based on produced-water property testing and water-chemistry analysis. The results showed that the salinity of the aqueous phase from Mabei Oilfield is approximately （2.5-3.0）×104 mg/L. The produced water is of the sodium bicarbonate type，and the main ions are Na+，CO2-3 ，HCO-3，and Cl-. The system basically reaches salt precipitation equilibrium after 20 min of cooling. Sand content of 1%-5% and oil content of 0%-40% have little effects on the critical salt precipitation salinity of the free water phase. The critical salinity of NaCl changes only slightly with temperature，whereas Na2CO3 is the most temperature-sensitive salt and is an important driver for salt precipitation. The critical salinity of the NaCl-Na2CO3-NaHCO3 mixed-salt system generally grows with increasing temperature and is significantly affected by salt proportions. A cooperative mixed-salt precipitation model was further established. After interactive correction，the mean absolute error for the D-M mixed-salt system decreased from 10.68% to 6.76%，indicating that the critical salt precipitation salinity of the mixed-salt system is controlled by the Na+ common-ion effect，carbonate equilibrium transformation，and high-ionic-strength coupling. The findings of this study provide a theoretical basis for salt precipitation risk prediction and process optimization for the gathering and transportation of high-salinity and high-alkali produced water in Mabei Oilfield.
To address the challenges of hydraulic fracturing in unconventional oil and gas reservoirs in terms of water resource consumption，formation damage of water sensitivity，and environmental pollution，this study systematically investigates the fracturing mechanism，application status，and engineering challenges of the supercritical carbon dioxide （SC-CO2） fracturing technology. Compared with water-based fracturing fluids，SC-CO2 is more likely to generate complex fracture networks because of its low viscosity，high diffusivity and near-zero interfacial tension. The mechanism of “modification-permeability enhancement” is clarified：SC-CO2 reconstructs pore-fracture systems through mineral dissolution and organic matter extraction，while also degrading rock strength and increasing brittleness，which jointly facilitates fracture initiation and propagation. Recent advances in fluorine-/silicon-based thickeners，non-fluorinated thickening systems，and SC-CO2 jet and compound fracturing technologies are summarized. Although challenges remain in proppant transport，equipment compatibility，cost control，and quantification of storage efficiency and leakage risks，SC-CO2 fracturing shows strong potential for the integrated development pathway of fracturing，enhanced recovery and carbon sequestration in unconventional reservoirs.
