Multi-Field Coupling Analysis of Wellbore Instability in Hutubi Anticline While Using Water-Based Drilling Fluid
Complex problems，such as HTHP，wellbore collapse，and lost circulation were encountered during drilling Hutubi Anticline. The fabric，mechanical properties，and in-situ stress state of the troublesome medium-deep shale formation were analyzed to determine the countermeasures necessary to stabilize the borehole and reduce risk. A multi-field calculation model for collapse pressure was established linking mechanical factors，drilling fluid chemistry，and fluid seepage. The mechanism of wellbore instability when using water-based drilling fluid in the middle-deep shale formation of Hutubi anticline was identified. The results indicate that the montmorillonite content of the illite-smectite mixed layers in the mudstone in Paleogene Anjihaihe-Ziniquanzi Formation （which collapses readily） exceeds 35%. The formation exhibits moderate expansion and high dispersion，with water expansion rate and recovery rate both being less than 10%. The formation strength also shows anisotropic properties. The mudstone in Cretaceous Hutubihe-Qingshuihe Formation shows reduced swelling and increased hardness and brittleness. Hutubi Anticline is subject to significant tectonic stress，with the maximum horizontal principal stress being equivalent to nearly 2.50 g/cm3，which is higher than the overburden pressure. Applying the multi-field coupling model indicates that the main causes of wellbore instability are seepage of drilling fluid along cracks，hydration of mudstone after contact with drilling fluid，and insufficient effective mechanical support of drilling fluid for the wellbore. The collapse pressure calculated using the multi-field coupling model is 0.05-0.25 g/cm3 higher than when only considering mechanical factors. It was established that the wellbore can be kept stable by maintaining the sealing ability of the drilling fluid to the fractured formation，preventing contact with easily hydrated mudstone during drilling，and increasing the density of the drilling fluid to slightly higher than the collapse pressure.
The Lost Circulation Mechanism in Formations Prone to Lost Circulation at Misui Block in Changqing Oilfield
Rich in natural gas resources，Misui Block is a key strategic area for accelerating the production of Changqing Oilfield. However，the frequent occurrence of lost circulation in recent years has seriously restricted improvement of cost-efficiency. Therefore，to curb the occurrence of well lost circulation，the lost circulation mechanism in this area has been studied through multi-scale quantitative experiment on rock physical property，microelectrode imaging logging observation and geo-mechanical profile analysis of wells. The results show that the thief zones in Misui Block are mainly in Liujiagou and Shiqianfeng Formations，which have large numbers of micro- and nano-scale fractures. The average calculated dynamic Yang's modulus of the cores from these formations are 17.94 GPa and 8.47 GPa，respectively，and the average dynamic Poisson's ratios are 0.394 and 0.064，respectively. The clay mineral content of Liujiagou Formation is more than 50%，and the brittle mineral content of Shiqianfeng Formation is as high as 72%. The imaging logging map shows that the area contains tensile fractures. The single well geo-mechanical profile reveals a low-pressure area in the thief zone，where the density of drilling fluid is 1.18 g/cm3. Finally，analysis shows that the high contents of clay and brittle minerals in the formation caused both hydration fractures and structural fractures，with low formation pressure causing drilling fluid to flow into natural fractures，eventually resulting in lost circulation. The research results not only reveal the lost circulation mechanism at Misui Block，but also provide technical guidance for selecting plugging methods and reducing well lost circulations at Misui Block.
Fracture Propagation Law of Hydraulic Fracturing in High-Salinity Reservoir of Fengcheng Formation in Mahu #br#
The high salt content of Fengcheng Formation in Mahu leads to high operating pressures, difficulty in fracture propagation when penetrating salt layers, and increased difficulty in adding proppant. True triaxial fracturing experiments are carried out on equivalent salt-bearing rock samples prepared according to similarity criteria, and fracture propagation in salt-bearing reservoirs is numerically simulated using the finite element and cohesive element methods. The influence of salt content, viscosity, pump rate, and other factors on fracture propagation is explored. The results show that the fracture opening pressure of salt-bearing reservoirs is greater than that of conventional sandstone reservoirs, and their plasticity is stronger. The existence of pure salt interlayers hinders the longitudinal propagation of fractures. The lower the fluid viscosity , the stronger the dissolution of the salt-bearing reservoir and the lower the operating pressure. The higher the pump rate, the higher the fracturing friction and the operating pressure. In field, high viscosity fluid (120 mPa·s) with clean water and high pump rate （5 m3/min per single cluster） have been adopted to achieve effective penetration into pure salt barriers and form wider fractures. These results have important guiding significance for the stimulation of salt-bearing reservoirs in Mahu area.
Research and Application Progress of Nanofluid for Enhanced Oil Recovery
In view of the limitations of traditional polymer and surfactant solutions in enhancing oil recovery，such as low viscosity retention and large adsorption loss，this paper examines the potential use of nanofluids for enhanced oil recovery and discusses recent research in this field. First，the synthesis of nanomaterials is described，and the methods used for evaluation of the stability of nanofluids in enhanced oil recovery in the field are summarized. Second，six mechanisms by which nanofluids enhance oil recovery are reviewed. These are：reducing interfacial tension，changing wettability，reducing crude viscosity，improving foam stability，structural disjoining pressure，and reducing pressure and increasing injection. Third，the use of nanofluids for enhanced oil recovery in the field is investigated. The “bottleneck” issues that limit the large-scale application of nanofluids in oilfields are also described. For example，there are currently no nanofluid flooding systems available for efficient development of unconventional reservoirs. Furthermore，there has been insufficient theoretical and technical discussion and research. More integrated research is required on inter-related matters such as the development of nanofluids containing two-dimensional nanosheets，determination of the mechanisms for enhanced oil recovery，and field pilot testing of the process. Finally，a direction for the practical introduction and application of nanofluids is proposed.
Dynamic Prediction of Cementing Pressure Through the Whole Process under Eccentric Casing Conditions in Shale Gas Horizontal Wells
Complex problems such as leakage and overflow often occur during the cementing process of shale gas horizontal wells in a particular oilfield in the Permian Shanxi Formation，severely impacting cementing quality. The existing method for calculating casing pressure under concentric wellbore conditions cannot be used to accurately calculate wellbore pressure. A wellbore pressure calculation model is urgently required that considers actual downhole conditions throughout the cementing process to accurately predict and evaluate the distribution of wellbore pressure in real-time，ensuring cementing safety and improving cementing quality. This study considers the effects on wellbore static pressure and circulation friction of various fluid injections under eccentric casing conditions during the cementing process. A correction coefficient for eccentric annulus friction pressure drop is developed，and a calculation model for annulus flow during cementing process with eccentric casings is established. The impact of eccentric casing conditions on annular pressure is analyzed using eccentric casing data from well A. The results show that the annular friction pressure drop under eccentric casing conditions is reduced by 22.7%－33.42% compared to concentric annuli. The predicted values of the pressure calculation model under eccentric casing conditions align well with the actual pump pressure，and the calculation error is within 1.37%. The principle of pressure stability and no leakage during the whole cementing process has been insured with the proposed calculation model. The correctness of the established cementing pressure prediction model and calculation method is verified，providing important guidance for cementing operations in low-pressure and leak-prone formations，avoiding downhole complexities，and ensuring the safety of cementing operations.
Performance Evaluation of the Cement Sheath in Jimsar Shale Oil Wells
Hydraulic fracturing technology is an important method for increasing the production capacity of shale oil wells. The mechanical behavior of well cement sheath during fracturing has an important impact on the safety and efficiency of production. Based on the cement slurry system in Jimsar region，a cement specimen maintenance mold was designed as well as a cement bonding strength test program，maintaining the standard cylindrical specimen of cement and the set cement bond specimen. The uni/triaxial compression experiments were then carried out as well as the cyclic loading experiments and bonding strength experiments under simulated well bottom conditions to investigate the rules governing damage and destruction of cement bodies and cementing interface. The results show that under uniaxial compression，the set cement has significant elastic-brittle characteristics and that macroscopic cracks are formed in the cement body when it is damaged.The average Young's modulus of cement is 3.054 GPa and the average Poisson's ratio is 0.127. Under triaxial conditions，the strength and plasticity of the cement sheath are enhanced. During cyclic loading，there is a bilinear accumulation of cementite plastic deformation，with the amount of plastic deformation being positively correlated with the peak load. The plastic strain after 20 cycles increased from 0.24% to 2.46% when the peak load increased from 22.7 MPa to 53.2 MPa. The cement bonding strength is lower than 1 MPa，which is much lower than the body strength，and the bonding interface is a natural mechanical weak zone，representing a risk point for wellbore seal failure.
Numerical Simulation of the Pressure Law of Energy Charging and Permeability Enhancement in Low Permeability Reservoirs
A number of problems have been experienced with water injection in Yanchang Oilfield，such as high-pressure injection failure，poor flooding effect，and rapid increase of water cut during injection. An innovative method for energy charging and permeability enhancement is proposed to address these issues. Numerical reservoir simulation is used to study the evolution of fractures，matrix pressure，and bottom hole pressure propagation during injection and well shut in. The influence of injection volume on fracturing effect is also analyzed. It is found that，during the injection stage，rapid injection of fluid increases the pressure in the fracture，with the injected fluid passing through the main fracture to the tips of the fractures. The pressure increase mainly occurs around the fracture itself，with little change in the matrix pressure. In shut in stage，as shut in time increases，the matrix pressure in the fracture and near-fracture zone initially decreases rapidly and then tends to become stable after a certain time. If the distance from the fracture is more than 30 m，the matrix pressure first increases and then decreases with the increase of shut in time. The bottom hole pressure also declines rapidly in the early stage of shut in with the rate of decline slowing in the later stage. The greater the injection volume，the higher the formation energy. Analysis of the effect of energy charging and permeability enhancement in field applications for low permeability reservoirs shows remarkable increases in pressure and oil production. This provides a valuable reference for the development of energy charging and permeability enhancement in similar reservoirs.
The Principles and Field Application of Double Throttling Technology in Mahu Oilfield
A high gas-bearing well in Mahu Oilfield suffers from hydrate freezing blockage，which results in considerable fluctuation in gas and liquid production. The surface gathering and transportation capacity is also limited. A double throttling technology is proposed to address this problem. Throttling nozzles are installed downhole and at the wellhead to control hydrates and regulate production. PIPESIM software was used to calculate the temperatures and pressure distribution in tubing and nozzles under double throttling condition. Thus，the principles of hydrate freezing prevention，wellhead production regulation，and discharge for blockage prevention were studied. Double throttling was carried out 51 times in 20 wells and its field application effects was analyzed. 5 test wells were designed to adopt the double throttling technology and thus the average cleaning interval of hydrate in production wells with serious freezing blockage problems was more than doubled. The field application results show that downhole throttle ratio is the core parameter of the double throttling process design. Reducing downhole throttling ratio improves the capacity of hydrate freezing and blockage prevention. Increasing downhole throttling ratio improves the production regulation ability of wellhead nozzle. For high gas production wells，a low downhole throttling ratio design can achieve good hydrate freezing blockage prevention results. For high liquid production wells，a high throttling ratio allows both effective wellhead production regulation and hydrate freezing prevention. This provides a solid theoretical basis for more efficient development of Mahu Oilfield.
Numerical Simulation for Peeling Failure of Cement Plug-Formation Interface in Plugged Section of Old Wells
Due to the small casing diameter and the insufficient gas tightness of casing coupling threads，the old well cannot be converted into an injection-production well for the underground gas storage. Therefore，the old wellbore must be plugged and reconstructed to ensure effective plugging. An old well of the Ye County gas storage in Henan Province was taken as an example，and the well section to be milled and plugged was determined according to the formation lithology and well diameter. A three-dimensional finite element model of the cement plug-formation system based on Cohesive elements was built for the milling-plugging old well section，the peeling failure laws of the cement plug-formation interface under the constant-pressure condition were clarified，and the limit peeling length of the interface was computed. The results show that for old wells without perforated sections，it is necessary to set cement plug barriers in the cap rock and salt sections，respectively，to prevent natural gas leakage in the vertical and lateral directions. Under the gas storage condition with a constant pressure of 20 MPa，the bottom of the cement plug presents slight deformation after being compressed，resulting in the shear deformation of the cement plug-formation bonding interface，and peeling occurs，as such deformation accumulates to a certain extent. With the increasing simulation time，the length of the peeling interface gradually increases and approaches the equilibrium，and the ultimate peeling failure length is 18.4 m. Moreover，the peeling length decreases with the growing elastic modulus of the cement plug. It is recommended to use the high-elastic-modulus cement slurry system for well plugging. The findings of this research help to decide whether the milled-plugged section length of the old well in the field is sufficient，and provide theoretical guidance for ensuring the long-term safe operation of the gas storage.
Numerical Simulation of Combustible Gas Explosions in Nitrogen Injection Wells
There are always some risks of a combustible gas explosion in a wellbore during gas injection for oil displacement. To clarify the consequences of such accidents and establish the evolution law of combustible gas explosions in thermal recovery nitrogen injection wellbores，numerical simulation was carried out for a light hydrocarbon component explosion in a wellbore space with a high length-diameter ratio under high temperature and high pressure. The results show that explosion overpressure is closely related to initial temperature and pressure. Increase in the initial temperature will lead to a decrease in the peak explosion overpressure，and increase in initial pressure will lead to an increase in the explosion peak overpressure. Under the same initial pressure and temperature，the highest explosion pressure occurs at the wellhead when ignition occurs at the well bottom. At both ends of the wellbore，overpressure increases rapidly due to the rebound of the blast shock wave，and the maximum overpressure generated by such an explosion can be more than 300 MPa，sufficient to cause serious damage to tubings，wellbores，and other facilities. These results provide reference for safety measures to prevent explosions and improve control of gas injection wells.
Concentrated Solar Steam Generation Technology Enables Low Carbon Shallow Super-Heavy Oil Production
PetroChina Xinjiang Oilfield Company is carrying out a pilot test of concentrated solar power direct steam generation，in conjunction with existing gas-fired steam boilers，to produce low-carbon，high-quality steam for super-heavy oil production using SAGD technology. This has exemplary significance for the integration of oil and gas field exploration and development using renewable energy technology. Under the double carbon initiative，Xinjiang Oilfield is facing the challenge of reducing natural gas consumption and CO2 emission while maintaining super-heavy oil production. Through combining solar energy concentrating and heating technology，green electricity + electrode molten salt heating technology，and high-temperature heat storage technology，renewable energy supply configuration schemes for different scenarios are proposed for the use of steam injection in shallow heavy oil exploitation，which will provide a specific road map and solution for the scale application of concentrating solar power technology in the heavy-oil operation areas of Xinjiang Oilfield. The schemes will reduce self-consumption of natural gas and provide low-cost steam，and has guiding significance for low carbon，sustainable exploration and development of heavy-oil fields.
Energy Conservation Technology Based on Magnetic Levitation Heat Pump in Oilfields
To make effective use of waste heat resources，different oilfield enterprises use heat pump and other technologies to replace oil and gas-fired boiler，and use waste heat of produced fluid for heat supply of oilfield production and living system. Heat pump has gradually found its application in oilfields. As the only moving part in heat pump system，compressor is the heart of heat pump. Magnetic levitation centrifugal compressor uses advanced magnetic levitation and DC frequency conversion technologies，which can greatly improve unit capacity. To investigate the impact of compressor on COP of water source heat pump and provide reference for model selection of water source heat pump，a case of actual use of waste heat of effluent is used for model selection and comparison of water source heat pump. The results indicate that while the COP of conventional screw compressor heat pump is only 2.4 in terms of waste heat utilization，the COP of magnetic levitation heat pump can reach 3.54，which is higher than the COP standard for waste heat utilization of water source heat pump and 32% higher in energy conservation rate than the original screw compressor heat pump. Counted by unit price of electricity bill of 1.0 yuan/（kW·h），it can save operating power charge of 117.33×104 yuan on a yearly basis. With higher COP，load regulation performance，reliability and stability than other heat pump technologies，the magnetic levitation heat pump can reduce on-site maintenance at the same time，which is more suitable for field application in oilfields.
