Table of Content

10 August 2021, Volume 43 Issue 4

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A Study on the Mechanical Properties and Acoustic Response Characteristics of Marble in the Process of Triaxial Mechanical Loading

MENG Yingfeng, YANG Fengqiang, LI Cheng, YIN Hanxiang, LIU Houbin

2021, 43(4):  1-10.  DOI: 10.11885/j.issn.16745086.2021.04.29.04

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The mechanical environment of deep stratum rocks is extremely complex. The study of rock mechanical properties, failure laws and acoustic response characteristics under high stress can provide a basis for the scientific design of key parameters of deep wells. This paper carried out triaxial mechanical experiments on marble under different confining pressure conditions, analyzed the stress-strain and acoustic characteristics of marble during loading under different confining pressure conditions, and discussed the applicability of strength criteria and rock stress-strain and dynamics, acoustic response mechanism. Research shows that during the mechanical loading process, the wave velocity of the rock sample can be divided into two types: “rapid increase-uniform increase-step down” and “rapid increase-uniform increase-maintain stability-step down”; before the rock sample is damaged, the wave speed of the rock sample and amplitude of the rock sample decrease in different degrees; with the increase of confining pressure, the compressive strength and elastic modulus of marble increase, and the failure mode of the rock sample changes from splitting to shearing. Confining pressure has an inhibitory effect on the occurrence of secondary cracks in marble; the linear Morh-Coulomb criterion and the single parameter Bieniawski criterion are applicable to the description of the strength characteristics of marble under low and high confining pressures, respectively. The research in this paper provides an important theoretical basis for the prediction of rock fracture in deep wells and the establishment of strength criteria.

Mechanical Mechanisms of Wellbore Instability of Deep Anisotropic Shale in Southern Sichuan

LI Zhengtao, ZHANG Zhen, WU Pengcheng, MA Tianshou, FU Jianhong

2021, 43(4):  11-25.  DOI: 10.11885/j.issn.16745086.2021.04.28.05

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Wellbore instability problem usually occurs during horizontal drilling in deep shale reservoirs in Southern Sichuan basin, due to the deep buried depth, complicated lithology, hard brittle texture, well-developed bedding planes and complicated in-situ stresses, which seriously hinders the efficient development of deep shale gas. Thus, taking the deep Longmaxi shale in Southern Sichuan basin as the research objective, the anisotropic elastic and strength parameters of the shale were determined by experiments, the bedding occurrence was analyzed by using imaging logging data, and the in-situ stresses and its orientation were determined based on the indoor experiments and logging data. The wellbore collapse pressure model of anisotropic shale was established, and the collapse pressure with different formation conditions were analyzed and verified by taking the deep shale gas well in Southern Sichuan basin as an example. The results indicated that the cohesion and internal friction angle of shale matrix are 13.00 MPa and 39.50°respectively, while the cohesion and internal friction angle of bedding plane are 11.10 MPa and 28.50°respectively. The dip angle of bedding plane is 5～15ånd the azimuth angle of the bedding plane is NE110°～NE120°. The maximum horizontal stress, minimum horizontal stress and vertical stress are 114.69～117.23 MPa, 93.79～94.57 MPa and 108.42～112.81 MPa respectively. The collapse pressure is the highest when simultaneously considering the elastic anisotropy and strength anisotropy and the effect of strength anisotropy is obviously greater than that of elastic anisotropy. The stability of horizontal wells drilled along the direction of maximum horizontal principal stress is the best, followed by vertical wells and small-angle directional wells, while the stability of horizontal wells drilled along the direction of minimum horizontal principal stress is the worst. The actual drilling condition of well L20X is basically consistent with the predicted results, which confirms the accuracy of the model presented in this paper.

A Study on Stabilization Strategy of Jurassic Coal Strata in Northern Kuche Structural Belt

LI Ning, ZHENG Heguang, LU Jun'an, SHAO Changchun, HE Shiming

2021, 43(4):  26-34.  DOI: 10.11885/j.issn.16745086.2021.04.29.02

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This research aims to solve the problem of the poor wellbore stability in Jurassic coal strata in the northern Kuche structural belt of Tarim Basin. The microstructure and physical and chemical properties of coal samples were analyzed by means of X-ray diffraction and scanning electron microscopy, and the mechanism of coal seam instability was determined. Then the prediction model of coal seam collapse pressure considering the influence of multiple cleat surfaces is established. According to the model, the geomechanical parameters of coal and wellbore stability are evaluated and the horizontal well trajectory optimization is carried out. Meanwhile, based on the microstructure of coal, the optimization of drilling fluid blocking material is carried out to form the stabilization strategy of horizontal wall in coal seam. The results show that the Jurassic coal strata in the northern Kuche structural belt are unstable. The well inclination and drilling fluid density are 45°～80ånd 1.70～1.74 g/cm³, the blocking material system of the horizontal well borehole stabilization strategy is 3% sulfonated asphalt+3% emulsified asphalt+3% calcium carbonate. The research is instructive for the safe and development of petroleun of the Jurassic coal seam in the northern kuche structural belt of Tarim Basin.

Annular Aerated Gas-lift Reverse Circulation Drilling Technology and Key Parameters Design

LI Qian, ZHANG Xiaolin, LI Zhengtao, LI Juan, DAI Feng

2021, 43(4):  35-43.  DOI: 10.11885/j.issn.16745086.2021.04.29.01

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Using gas lift reverse circulation drilling technology to solve the leakage problem of Changning shale gas surface drilling, it is necessary to solve the problems of conventional gas lift reverse circulation drilling technology, such as open wellhead and inaccurate bottom hole pressure control. Therefore, on the basis of conventional gas-lift reverse circulation drilling technology, a new annular aerated gas-lift reverse circulation drilling technology is designed. The new technology is equipped with rotary blowout preventer, drill pipe cock and other tools, which has well control ability. By adjusting key construction parameters such as gas injection rate and drilling fluid displacement, bottom hole pressure is controlled to reduce drilling leakage. Based on the multiphase flow theory, the calculation model of bottom hole pressure for annular aerated gas-lift reverse circulation drilling technology is established. The influence of key construction parameters on bottom hole pressure is analyzed, and the design method of key parameters is established. The key construction parameters are optimized within the scope of safety window to ensure safe drilling. The results show that the bottom hole pressure first decreases and then rises with the increase of gas injection rate, and there is a critical gas injection rate; the bottom hole pressure rises with the increase of well depth; the bottom hole pressure rises with the increase of drilling fluid displacement. The field test was carried out in the surface drilling of a well in Changning. Compared with the lost circulation formation in the same section, the lost circulation was reduced by 83.6% using the conventional drilling technology. The research results provide a new technical measure to solve the problem of lost circulation in the surface drilling of Changning shale gas in Sichuan.

Application of Gas Continuous Circulation Drilling Technology in Gravel Layer in Bozi Block

LI Luchun, LIAN Zhanghua, PU Keyong, YAN Hai

2021, 43(4):  44-50.  DOI: 10.11885/j.issn.16745086.2020.08.03.01

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There are over 5 000 m of gravel layers in the upper strata of deep and ultra deep wells in Bozi Block. It has high hardness and poor drillability, which leads to low ROP and long drilling cycle of conventional drilling at the upper large-size borehole when conventional drilling is used. In recent years, a variety of speed-up tools and technologies have been tested, but the effect is unsatisfactory. Aiming at the difficulties of low ROP and long drilling cycle of conventional drilling for the huge-thick gravel layer in Bozi Block, using underbalanced drilling method, the scheme of gas continuous circulation drilling technology is designed, and technical measures such as well deviation control, grits cleaning and mud conversion have been formulated, and a gas continuous circulation drilling process technology suitable for the geological characteristics of huge-thick gravel layer is formed. This technology has been applied to 6 wells for 9 times in total, which has significantly improved the ROP, shortening the drilling cycle, and greatly saving the drilling costs. The gas continuous circulation drilling technology overcomes the difficulty that the stands cannot be connected when the sand setting is more than 30 m. It effectively prolongs the drilling footage of gas drilling in micro water-producing formation, and controls the maximum well deviation within the safe range of 5ånd does not affect the implementation of casing running and completion modification technology in the later stage. The gas continuous circulation drilling technology is becoming a key technology for speeding up and improving the efficiency drilling of the huge-thick gravel layer in the Bozi Block, which provides strong technical support for the trillion cubic meter capacity construction target in the Bozi-Dabei Block.

Mechanical Specific Energy Analysis and Optimization of Drilling Parameters for Nanyuan Deep Formations in Junggar Basin

LU Zongyu, XU Shengjiang, JIANG Zhenxin, TIAN Long, ZHONG Yinming

2021, 43(4):  51-61.  DOI: 10.11885/j.issn.16745086.2021.04.29.11

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The deep formations in Nanyuan Area of Junggar Basin are characterized by large burial depth, high compressive strength of rock, poor drillability, and strong hold-down effect from drilling mud, which result in frequent drilling accidents and extended drilling cycle (average drilling period 335 d for single well). The average rate of penetration in deep formations are below 2 m/h. It is of urgent necessity and massive challenge to improve ROP in drilling the Nanyuan deep Formations. Aiming at the complicated conditions in Nanyuan deep Formations, a relationship between the load at wellhead and the load on bit is established, and the output characteristics of positive displacement motor is analyzed. Then the Mechanical Specific Energy (MSE) model for the compound drilling method is derived. Based on the drilling data of the ϕ190.5 mm section in the Hutan No.1 Well, focusing on various geological formations, analyze the relationship between MSE and ROP in Nanyuan deep Formations, which is helpful for optimizing drilling method and locating proper ranges of drilling parameters. According to the MSE analysis, ROP in Lianmuqing Formation is bot sensitive to rpm (rotation per minute), where conventional drilling method with improved WOB (weight on bit) is recommended. Compound drilling method with a properly increased WOB is suggested in Shenjinkou and Hutubihe formations. The improvement of ROP is not obvious in Qingshuihe and Kalazha formations when torsional impactor is used. Thus are novel tools suggested to boost ROP and consider the parametric combination of high WOB and moderate RPM. The research results are expected to provide reference for enhanceing penetration rate in Nanyuan deep formation.

A Study on Deep Well Deviation in Shuangyushi Block, Sichuan Basin

MI Guangyong, YUAN Heyi, WANG Qiang, MIAO Yao, DONG Guangjian

2021, 43(4):  62-70.  DOI: 10.11885/j.issn.16745086.2021.05.05.01

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To solve the problem of well deviation in Shuangyushi Block, We establish a model of drill bit deflection force with consideration of interaction between bit and formation, formation dip, inclination, angle, formation anisotropy index, formation deflection force and drill bit deflection force. And we explore the law of well deviation change under different influencing factors and verify the correctness of the model based on actual drill data. The results show that, compared with drilling fluid drilling, the formation deflection force increases under air drilling conditions. When the inclination angle is smaller than the formation dip, the formation force is a deflection force, which causes the drill bit to deviate upwards. Increasing the drilling pressure will cause the inclination angle to increase. As the formation dip angle increases, the natural forming force of the formation increases. As the inclination angle increases, the combined lateral force decreases. The resultant force of the bit has a balanced point. With the increase of the formation anisotropy index, the natural deflection ability of the formation increases, leading to the increase of the well inclination angle. According to the results of theoretical analysis, an integrated quantitative measure is proposed for drilling tool assembly, bit optimization, and drilling parameters, which reduces the inclination angle of ultra-deep formations with easy deflection to 0.50°～1.15ånd increases the ROP by 40%～85%. The results of this paper can provide important reference and guidance for the study and efficient control of ultra-deep well deviation rules.

An Analysis of Drilling Pipe Buckling Characteristics in Curved Hole Under the Effect of Torque

PANG Dongxiao, LU Qi, DENG Hu, PENG Chi, FU Jianhong

2021, 43(4):  71-80.  DOI: 10.11885/j.issn.16745086.2021.06.03.03

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This paper analyzes the buckling characteristics of drilling string in curved wells under the influence of torque. The buckling differential equation and the buckling critical load calculation model of drilling string in a curved wellbore are derived. The finite element models of drilling string in buildup and drop-off sections are established for ϕ127.0 mm drilling pipes in a ϕ215.9 mm wellbore, and the buckling characteristics of drilling strings with and without torque are compared and analyzed. The results show that the sinusoidal buckling configuration of drilling string is not related to torque. It is only decided by the axial load on drilling string. Torque has great influence on spiral buckling configuration. The relative error between the critical buckling loads of drilling strings obtained by numerical simulation under different build-up rates and analytical solution is within 15%, which proves the reliability of numerical simulation. When torque is applied in curved wellbore, the helical angular displacement increases, and the corresponding critical sinusoidal and helical buckling loads decrease. Sinusoidal buckling of drilling string does not change much, and torque has more influence on spiral buckling deformation. In the buildup section, sinusoidal and spiral buckling start at the top of the drilling string, while the bottom of the drilling string maintains a balanced state. In the drop-off section, when the torque is not considered, the initial sinusoidal buckling and spiral buckling first appear at the lower part of the drilling string. When the torque is considered, the initial sinusoidal buckling appears at the upper part of the drilling string, while the initial spiral buckling appears at both the upper and lower parts of the drilling string. Compared with the buildup section, the critical axial loads of drilling string buckling in the drop-off section are very small, which indicates that the possibility of buckling in the buildup section is relatively small, while that in the drop-off section is much easer to occur. The results are expected to provide a reference for the design, buckling control, and strength check of drilling strings in horizontal and extended reach wells.

Molecular Simulation for Inorganic Salts Inhibition Mechanism on Illite Hydration

LIU Meiquan, PU Xiaolin, ZHANG Qian, SU Junlin

2021, 43(4):  81-89.  DOI: 10.11885/j.issn.16745086.2021.04.29.15

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In order to study the microscopic mechanism of illite hydration expansion and the mechanism of inorganic salt inhibitors, two illite crystal models (1M-tv and 1M-cv), which are common in oil and gas reservoirs, were established through molecular simulation technology. Furthermore, molecular dynamics method was used to study the microscopic distribution of illite interlayer particles and the hydration parameters of inorganic salt cations entering the interlayer. The results show that the interlayer spacing of illite increases with the number of adsorbed water molecules. It reaches saturation when the number of molecules is 20; 1M-tv configuration is more prone to hydration expansion than 1M-cv configuration; K⁺ and Ca²⁺ are the smallest hydration numbers and hydration radius of monovalent cations and divalent cations, respectively. In addition, K⁺ can be embedded in the silicon-oxygen six-membered ring of the tetrahedral sheet, making it difficult for ion exchange to occur, which can effectively prevent water molecules from entering the inside of the illite crystal structure. The experimental results well verify the inhibition of KCl and CaCl₂ on illite hydration expansion. The results have important theoretical significance for the study of shale anti sloughing mechanism and drilling fluid hydration inhibitor.

Cleanup of Oil-based Drilling Fluids Using In-situ Emulsification of Thermoresponsive Surfactants

REN Yanjun, LI Miao, JIANG Qihui, SHI Jingkang, YI Duo

2021, 43(4):  90-98.  DOI: 10.11885/j.issn.16745086.2021.04.29.14

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Efficient removal of residual oil-based mud in wellbore is the key for excellent cementing and wellbore intergrity. To clean the oil-based mud at high temperature and high salt efficiently, the cleaning agent was used to form micro- and nanoemulsion in-situ with the oil-based drilling fluid based on the thermoresponsive phase inversion effect and cloud point effect. By simulating the law of temperature change in the well and testing the cloud point and conductivity, the cleaning agent formula was identified as the mixture of 3% fatty alcohol-polyoxyethylene ether and 2% isomeric carbonyl polyoxyethylene ether. The cleaning agent was thermoresponsive. It could seperate out from its aqueous solution at 75℃ and therefore facilitate the oil phase shedding; then the oil phase mixed with the cleaning fluid and was then solubilized in the cleaning fluid by forming the micro-and nano-emulsion. The effects of cleaning agent concentration, temperature, salinity and time on the cleaning results were investigated by testing the cleaning rate, contact angle and cementing strength. The results showed that the cleaning rate of over 99% of oil-based mud could be reached using 5% thermoresponsive cleaning agent at 50～90℃ and 20% CaCl₂ with 10 minutes, the wall surface could return to water wetting state from the oil wetting state, and the cementing strength was enhanced observably.

A Study on Anti-collapse Mechanism and Synthesis of Hyperbranched Polymer HP-NH₂

WEI Yunjin, HUANG Xuegang, ZOU Yuanhong, XU Weining, XIE Gang

2021, 43(4):  99-108.  DOI: 10.11885/j.issn.16745086.2021.04.30.01

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Wellbore stability is the current technical difficulty in drilling deep shale gas with water-based drilling fluids. Using diethylenetriamine and N, N-methylenebisacrylamide as raw materials, the hyperbranched polymer HP-NH₂ was synthesized by Michael addition reaction. The structure and characteristics of the hyperbranched polymer HP-NH₂ were characterized by infrared spectroscopy, liquid chromatography-mass spectrometry, gel chromatography, particle size analysis and thermal weight loss analysis. The study found that the number average molecular weight of the hyperbranched polymer HP-NH₂ was 3 371 g/mol, the particle size was in the range of 280～1 900 nm, and the temperature resistance reached 280℃. Through the artificial mud cake method, linear expansion experiment, recovery rate experiment and mud cake dispersion experiment, the anti-collapse performance of hyperbranched polymer HP-NH₂ as an anti-collapse agent was analyzed. We found that with the increase of HP-NH₂, the linear expansion rate of bentonite gradually decreased (the lowest was 19.11%), the rolling recovery rate of cuttings gradually increased (the highest was 75.18%), and the permeability of the artificial mud cake gradually declined. Hyperbranched polymer HP-NH₂ can be inserted between the crystal layers of montmorillonite to exchange cations that are easily hydrated between the layers, tighten the substrate spacing, and inhibit the hydration and dispersion of montmorillonite; HP-NH₂ can also effectively block mud cake's micro-nano pores. Therefore, the hyperbranched polymer HP-NH₂ can be used as a potential anti-collapse agent in water-based drilling fluid systems.

An Experimental Investigation on Pressure Bearing and Tight Plugging of Fractured Formations in Deep and Ultra-deep Wells

WANG Qiang, YUAN Heyi, LIU Yang, MI Guangyong, LUO Le

2021, 43(4):  109-117.  DOI: 10.11885/j.issn.16745086.2021.05.06.01

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Lost circulation is extremely easy to occur during drilling in deep and ultra-deep fractured formations. The failure of plugging zone formed by lost circulation materials (LCMs) may be aggravated under the complex environment of high temperature, high pressure and high in-situ stresses, which results in failure to reach the plugging rate and the fracture plugging effect expectation. Based on the idea of multi-stage and multi-grain bridge plugging, WNDK-1 rigid LCM and high-temperature resistant rubber particles often used in Shuangyushi block located in western Sichuan are selected to conduct LCMs performance evaluation and fracture plugging simulation experiment under high temperature aging environment. The experimental results show that after aging in 150℃ drilling fluid for 24 h, the particle size distribution of WNDK-1 does not change significantly. The maximum friction coefficient and compressive strength of WNDK-1 rigid material decrease by 1.89% and 1.15%. The particle size distribution D₉₀ value of rubber particles increase by 3.55%, the friction coefficient increases by 1.59%, and the compressive strength remains the same. If rigid materials, elastic materials and fibrous materials are mixed with drilling fluid at appropriate concentrations to plug the fractures, the pressure-bearing capacities of the formed plugging zone are generally higher than 13 MPa. Besides, the plugging zone has the characteristics of low porosity and low permeability. The observation of structure and morphology inside fractures after the plugs failure demonstrates that friction/comprehensive failure and shear instability in the plugging zone formed by multi-stage and multi-grain bridge plugging under high temperature aging environment.

Environment Cracking and Surface Protection of C110 Casing in Sour and Deep Well

DENG Hu, LI Yufei, ZHANG Zhi, LU Qi, HOU Duo

2021, 43(4):  118-128.  DOI: 10.11885/j.issn.16745086.2021.04.28.06

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The casing served in deep and ultra-deep wells with complex geological characteristics, well trajectory and high temperature, high pressure and containing H₂S, bears severe wear, corrosion, fracture and other failure risk, frequent environment sensitive cracking occurs to C110 casing when in corrosive environment and high level tensile stress. The slow strain rate test (SSRT) was used to study the environmental cracking behavior of C110 casing plated with Ni-W alloy in the simulated gas field formation water, and the protective effects of Ni-W alloy coating on C110 casing against environment sensitive cracking. The results show that the C110 casing has high sensitivity of SCC in the simulated formation water, risk of environmental cracking exists even if the corrosion rate is low, and it is characteristic of brittle fracture under the action of slow strain rate tensile test. The tensile strength and yield strength of C110 casing after Ni-W alloy surface were increased by 2.3% and 6.3%; after Ni-W alloy coating were heated, the tensile strength, yield strength, elongation and strain energy of C110 casing in simulated formation water increased significantly, the SCC sensitivity is reduced, and the fracture mode is changed into ductile brittle mixed fracture. The phenomena indicates that the Ni-W alloy coating of C110 casing and heat treatment can effectively reduce the risk of corrosion, cracking and damage of C110 casing, significantly improve its ability to resist environmental cracking and applicability in deep and ultra-deep wells.

A Study on Stress Corrosion Test Method for Welded Joint of Whole Pipe Section of Composite Pipe

ZENG Dezhi, LI Zuolong, LI Fagen, WU Ze, YAN Jing

2021, 43(4):  129-137.  DOI: 10.11885/j.issn.16745086.2021.04.30.09

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A southwest high acid gas field planed to use bimetallic composite pipes as internal corrosion control means. In order to evaluate the corrosion resistance of welded joints of composite pipes, a finite element mechanical model of the whole pipe section was established according to its structural characteristics. The accuracy of the finite element model was verified by loading stress-strain test, and the reasonable form of stress groove and total test pressure of welds were determined. Finally, the corrosion resistance evaluation of welded joint of X52/825 metallurgical composite pipe under severe stress and corrosion conditions was carried out by independently developed corrosion evaluation device and method for the whole pipe section of bimetallic composite pipe. The results show that the welded joint of X52/825 metallurgical composite pipe is not cracked or punctured after 720 hours corrosion test under stimulated working conditions. The results of 100% X-ray flaw detection show that the welded joints of X52/825 metallurgical composite pipes have no cracks, showing its good corrosion resistance. The results of this research provide theory and reference for the performance evaluation and safety use of composite pipes in acid gas fields.

Optimization and Evaluation of Carbamide Composite Flooding Corrosion Inhibitor for Stimulation Well

ZHAO Haiyan, YI Yonggang, YU Huiyong, LIU Zhendong, ZENG Dezhi

2021, 43(4):  138-146.  DOI: 10.11885/j.issn.16745086.2021.04.29.06

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In a western oil field, the temperature of the wellbore of a carbonamide combined flooding well can be as high as 260℃, and the partial pressure of CO₂ is expected to be 0.4 MPa. The wellbore string is facing a higher risk of corrosion. To solve the problem of wellbore corrosion, according to the characteristics of the service conditions of the wellbore of the carbonamide combined flooding huff and puff well, a set of corrosion inhibitor optimization evaluation technical process suitable for the carbonamide combination flooding huff and puff well is proposed. First, the compatibility of the preselected corrosion inhibitor is evaluated, and then the hot roll resistance is performed. Temperature test and electrochemical test, select the corrosion inhibitor with good temperature resistance and corrosion inhibition performance, and then use the high-temperature autoclave to simulate the on-site corrosion conditions to evaluate the corrosion inhibitor protection effect; finally, combining the process characteristics of the injection-simmering-production of the carbonamide combined flooding huff and puff well and the corrosive environment at different stages, the optimal injection timing of the corrosion inhibitor in the cooling stage of the simmering well is determined. The research results show that under the conditions of temperature 180℃ and CO₂ partial pressure 0.4 MPa, the optimized corrosion inhibitors XCN2-21 can reduce the corrosion rate of N80 pipes to 0.044 1 mm/a, meeting the requirements of oilfield corrosion control.

Design of Nitrogen Injection Well String and Wellhead Device in Deep Reservoir of Jidong Oilfield

HUANG Xiaomeng, WANG Lei, XU Jingjing, LU Haiwei, XING Lijie

2021, 43(4):  147-156.  DOI: 10.11885/j.issn.16745086.2021.04.29.09

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The Gao 5 Block of Jidong Oilfield has the characteristics of high reservoir pressure, large inclination of gas injection wells, and deep drilling of the pipe string. According to the current gas injection well structure, surface facilities and gas injection efficiency requirements, a commingled gas injection string structure scheme was designed. Self-programming of erosion model was used to optimize tubing size. The high temperature and high pressure dynamic corrosion simulation experiment was carried out under different working conditions to explore the wellbore corrosion mechanism of nitrogen injection wells and determine the influence of various factors on the corrosion rate. Finally, the remaining strength and service life of the pipe string were evaluated. The results showed that choosing ϕ73 mm and 13Cr tubing as the general gas injection string has a good service life under the influence of erosion and oxygen corrosion caused by nitrogen injection. Safety devices such as surface safety valves, can greatly reduce the safety risks of high-pressure gas injection wells. This design scheme can meet the production needs of deep nitrogen flooding in Jidong Oilfield and has a wide range of application prospects.

An Open-hole Wall Support Technology Based on Multi-material Composite String and Its Application

HUANG Liang, DENG Kuanhai, WANG Sen, FU Hao, LIN Yuanhua

2021, 43(4):  157-166.  DOI: 10.11885/j.issn.16745086.2021.04.30.05

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The use of sidetracking technology for completed ultra-deep open-hole horizontal wells is one of the main means to develop the remaining oil economically and efficiently. The traditional milling steel casing sidetracking technology has some problems, such as large running friction and long milling time and so on. Therefore, an ultra-deep horizontal well open-hole wall support technology based on multi-material composite pipe string is proposed, combined with the well conditions of the ultra-deep open-hole horizontal well that has been drilled in the Northwest Oilfield in China. The paper firstly proposes a multi material composite string combination and design method based on “aluminum alloy + carbon steel”. Secondly, the applicability evaluation of the critical buckling load calculation model based on the actual buckling test data of the string is carried out, and the buckling critical load calculation model suitable for “aluminum alloy + carbon steel” multi material composite string is optimized; thirdly, the trip-ability analysis software of multi material composite string is developed, which comprehensively considers the torque, friction, rigidity, borehole conditions, string strength and material. The feasibility evaluation of composite string of Shunbei X1, X2, X3 and X4 wells in Northwest Oilfield is evaluated by using the trip-ability analysis software. The technology has been successfully applied in Well X1 and Well X2, which further demonstrates the feasibility of field application of the technology. The result provides a theoretical reference for the design of composite string of ultra-deep openhole horizontal wells wall support and trip-ability evaluation.

Design Method of Cement Stone Performance for Large Scale Fracturing in Horizontal Wells

YIN Hu, ZHAO Xiuwen, LI Qian, ZHONG Shouming, LI Weixuan

2021, 43(4):  167-174.  DOI: 10.11885/j.issn.16745086.2021.04.28.09

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Large-scale fracturing of horizontal wells requires the higher performance of cement, which requires not only a certain compressive strength but also a certain deformation ability. The current commonly used standardisno longer meet the requirements of cement sheath sealing for horizontal well large-scale fracturing. Based on the stress analysis model of the cement sheath under fracturing conditions, a design method for cementing cement performance that can meet the requirements of largescale fracturing sealing is proposed with consideration of cement sheath tensile failure and interface failure. We established a cementing stone performance index control chart including elastic modulus, Poisson's ratio, and tensile strength (or yield strength), and quantified the Young's modulus, Poisson's ratio, and tensile strength (or yield strength) of the cement stone. The performance design of cement stone based on the index control chart can effectively reduce the risk of damage to the cement ring. A control board for the performance index of large-scale fracturing horizontal well cementing cement in the Mahu Oilfield was established. According to the requirements of the cement stone performance index control chart, the elastic-toughness cement slurry system was selected, and more than 10 wells were applied in the field. The reservoir fracturing reconstruction with a surface construction pressure of 90 MPa was successfully completed.

A Study of Precisely Managed Pressure During Casing Running in Deep Water Wells

TONG Chuanxin, ZHANG Hairong, XU Bihua, ZHAO Hu, CUI Ce

2021, 43(4):  175-182.  DOI: 10.11885/j.issn.16745086.2021.04.28.07

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Deep-water formations with a narrow pressure window pose a huge challenge to casing running. The surge-swab pressure generated during casing running will cause the wellbore pressure to exceed the upper limit of the formation pressure window and cause lost circulation. When a lost circulation occurs in offshore operation, the operating time will be greatly increased. In order to prevent excessive surge-swab pressure from exceeding the safety pressure window when using conventional methods for casing running, the casing running speed can be reduced. This reduces the leakage risk to a certain extent, it increases drilling costs, but does not apply to some wells. Therefore, for leakage risk during casing running in deep-water narrow safety pressure window formations, a deep-water narrow safety pressure window wellbore pressure control model is established on the basis of dynamic surge-swab pressure; and a deep-water precise-control managed pressure method for casing running is proposed. Using the established dynamic surge-swab pressure model to analyze the influencing factors of wellbore pressure, it is found that the surge-swab pressure of the wellbore increases with the increase of casing depth, maximum running speed, drilling fluid density, yield value and viscosity of drilling fluid. The calculation results show that shows that the precise managed pressure in deep water not only reduces the leakage risk, but also shortens the operation time and reduces the operation cost.

Integrity Evaluation of the First Interface of Cement Sheath Based on Tensile Bonding Strength

YANG Yuanguang, FANG Zhongqi, YUAN Bin, XIE Yingquan, YAN Shuang

2021, 43(4):  183-190.  DOI: 10.11885/j.issn.16745086.2021.04.29.13

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The decrease of working fluid density in oil and gas wells or oil and gas exploitation will inevitably lead to a decrease in the internal pressure of the wellbore, causing the first interface of cement sheath to be stretched, causing the cement sheath and casing to peel off, resulting in micro-annular gaps. In the past, due to the lack of measuring device and method of the tensile bond strength, the shear bond strength or hydraulic bond strength of the first interface of cement sheath was used in the research and design of the sealing integrity of cement sheath, which resulted in deficiencies in the optimization of cement slurry system and the design of the sealing integrity of cement sheath. Therefore, according to the stress process of the cement sheath, a test device for the tensile bond sealing strength of the first interface of cement sheath is developed, and the corresponding measurement method is proposed. Based on the device, the cement slurry system is optimized and the integrity of the first interface is evaluated. By comparing the tensile bond strength and shear bond strength of the first interface of cement sheath, it is found that the former is about 0.37～0.45 of the latter. The use of shear bond strength to optimize the cement slurry system or evaluate the sealing integrity may lead to the first interface of cement sheath seal failure. It is found that the first interfacial tensile bond strength of latex cement slurry system is greater than that of expansion toughened cement slurry system and selfhealing cement slurry system under high pressure curing condition. Taking the tensile bond strength of the first interface of cement sheath as one of the important basis for the experimental design of cementing slurry and the evaluation of the sealing integrity of cement sheath can ensure the safe production and operation of oil and gas wells.

Prediction of CO₂ Corrosion of Cement Paste Based on Solid Calcium Content

YUAN Bin, YUAN Kunfeng, XU Bihua, YAN Shuang

2021, 43(4):  191-198.  DOI: 10.11885/j.issn.16745086.2021.04.29.03

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CO₂ will corrode the cement sheath of oil well, leading to its strength decline, make it lose the function of protecting casing and sealing off oil, gas and water. Thus shorten the life of oil and gas well and cause huge economic loss. If the corrosion depth and corrosion laws of CO₂ downhole can be predicted, the life of the oil well can be predicted, and the corrosion resistance of the cement ring can be improved. However, most of the current CO₂ corrosion depth models are semi empirical models based on experimental data fitting, which are not universally applicable. To solve this problem, a prediction model of CO₂ corrosion depth was established based on the law of mass conservation, diffusion convection equation and calcium precipitation rate. The model takes into account many factors, especially the diffusion of CO₂ and the precipitation of calcium ions, so it has strong applicability. The reliability of the model is verified by CO₂ corrosion test, and the change law of cement matrix change after corrosion is analyzed by using the model. The results show that with the increase of corrosion time, the porosity and permeability of cement sheath increase, and the tortuosity decreases, which leads to the acceleration of material convection diffusion and corrosion rate; the closer to the corrosion end, the greater the porosity and permeability, and the smaller the tortuosity.

Sweet Spot Prediction of Shale Oil Reservoir Based on Logging Data

XIA Hongquan, LAI Jun, LI Gaoren, YANG Yun

2021, 43(4):  199-207.  DOI: 10.11885/j.issn.16745086.2021.04.28.10

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The prediction of sweet spots in shale oil reservoirs is affected by many factors. Clarifying the main controlling factors and assigning reasonable weights is the key to fast and efficient sweet spot evaluation. The traditional prediction method of shale oil sweet spots is not reasonable in terms of weight and classification standard. For this reason, firstly, the grey correlation analysis method is used to analyze the main control factors; then, the multiple correlation coefficient method is used to reasonably assign the optimal main control factors according to the type of reservoir to which they belong; finally, the geological engineering sweet spot coefficient of the reservoir is established to realize accurate prediction of shale reservoir sweet spots. Taking the Chang 7 shale oil reservoir in Longdong Aera of Changqing Oilfield as the research object, by using logging data, 10 representative geological and engineering parameters are selected from the aspects of source rock property quality(SQ), reservoir physical property quality(RQ) and completion quality(CQ) and the geological engineering double sweet spot eva-luation standard of shale oil reservoir in the work area is established: X_SQ>0.581, X_RQ>0.494 and X_CQ>0.715. The sweet spots predicted by this standard are in good agreement with the actual high and low production areas, which confirms the accuracy of the method. The research can provide a theoretical basis for the sweet spot prediction and well location layout of the Chang 7 shale oil reservoir in Longdong area.

A Study on Downhole Complex and Three Dimensional Distribution of Fractures in Changning Shale Formation

LIU Houbin, YU Xingchuan, ZHANG Zhen, WAN Xiumei, SUN Hangrui

2021, 43(4):  208-218.  DOI: 10.11885/j.issn.16745086.2021.04.29.05

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The Changning Block is rich in shale gas reserves in the Longmaxi Formation and is an important support point for China's oil and gas resources to increase production. However, the frequent occurrence of well leaks and blocking accidents in the drilling of shale reservoirs in the Changning Block has hindered the process of beneficial development of shale gas resources in the area. By combing the complex information data of drilled wells in the key well areas of the Changning Block, the ant-tracking method was used to predict the fracture development and 3D spatial spreading pattern of the shale formation based on the 3D seismic data body of the block, and the response relationship between well leakage, jamming complexity and fracture development was comparatively studied to lay the foundation for early warning and safe drilling of horizontal shale drilling wells in the Longmaxi Formation of the Changning Block. The results show that the fracture prediction method based on ant body is better applied in the Changning Block, the response between well leakage complexity and fracture prediction results is high, and fracture development, cross penetration of oblique intersection and laminated joints are the main causes of well leakage and jamming accidents. The fracture development in the shale of the Maokou and Wufeng Formations has threedimensional spatial variability, and the well leakage and jamming complexity is more prominent in the high tectonic areas and near large faults where fractures are developed and the rock is more fragmented. The results of the study explain the causes of well leakage and blocking complexities in the Changning Block and predict the three-dimensional fracture spreading pattern in the Changning Block, which can provide a scientific basis for the prediction of downhole complexities and the formulation of prevention and control measures in the Changning Block at a later stage.