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05 December 2024, Volume 20 Issue 4

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OIL AND GAS EXPLORATION

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Settling Plugging Technology for Severe Lost Circulation at the Salt Bottom of Tarim Piedmont Zone

LIU Fengbao, YIN Da, WEI Tianxing, WU Hongyu, WANG Ziwu

2024, 20(4):  1-7.  DOI: 10.12388/j.issn.1673-2677.2024.04.001

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High-pressure gypsum salt beds and low-pressure clastic rock target beds are commonly developed in the piedmont zone of Tarim Basin，and the pressure difference between these two stratigraphic systems generally exceeds 20 MPa. Because of the inaccuracy of geological stratigraphic determination during drilling，it is highly likely to drill through the gypsum salt bed into the target bed and then trigger severe lost circulation. To solve the problem of severe lost circulation at the salt bottom of Tarim piedmont zone，this paper evaluates the performance parameters of plugging materials such as pressure-bearing capacity and settling velocity，identifies the optimal conditions （velocity and mass） of oil- and water- based drilling fluid （OBM and WBM） settling plugging pills respectively，and finally develops the high density large pressure difference settling plugging technology specialized for OBM and WBM，based on the known mechanism of the severe lost circulation problem （drilling induced communication between high and low pressure formations） and the principle of the "bottle plug" plugging practice. Specifically，optimal plugging pills are expected to form in the case of OBM with a oil-water ratio of 50：50 and ES of 200-300 V or WBM with 0.2% XC + 0.2% demulsifier + plugging agents. The developed settling plugging technology has been successfully tested in three wells in Tarim piedmont zone，and Well DB1302 was found with a pressure bearing capacity of 30 MPa，demonstrating excellent application performance.

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Development Status of Wellbore Integrity Inspection Technology for Deep Strata and Deep Wells

SONG Xuefeng , LI Zhibin, , LIU Jinming , WANG Jiangshuai , DING Yida , QIAN Xuesen

2024, 20(4):  8-18.  DOI: 10.12388/j.issn.1673-2677.2024.04.002

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As the development of oil and gas resources is continuously advancing toward deep water areas，deep and ultra-deep strata，complex geology and extreme wellbore conditions pose great challenges to the safe and stable production of oil and gas wells. Conventional wireline logging technology and casing packer leak detection technology can meet the technical requirements of wellbore integrity inspection，but they both require to shutdown wells and the costs of joint inspection are high. In recent years，the application of fiber optic sensing in petroleum industry has become increasingly widespread，opening up new ideas for real-time monitoring of wellbore integrity. By systematically summarizing the current development status of wellbore integrity inspection，the following prospects are made：（1） Permanently or semi-permanently placing optical cables in oil wells enables monitoring the temperature field，pressure field，stress field，sound waves，vibration，and fluids inside the wellbore in real time to provide direct data for evaluating the integrity status of oil and gas wells.（2） Fiber optic sensing technologies such as DTS and DAS are characterized by continuous，real-time and distributed measurement，for which it is preferred to place them during the well construction to deliver full lifecycle monitoring of oil and gas wells. However，the overall cost of fiber optic laying is relatively high，and the fiber optic layout method needs to consider the impact of pipe string insertion and cementing perforation operations. The fiber optic winding method and light source scattering type also have an impact on optical signal transmission and signal-to-noise ratio. Corresponding measures need to be taken to optimize the signal-to-noise ratio，improve the measurement accuracy and stability of the sensor.（3） Processing of fiber optic signals mostly relies on empirical methods determining the signal range in accordance with laboratory simulation experiments. However，considerable differences exist between the laboratory environment and the downhole environment. Therefore，it is still necessary to conduct wellbore experiments and establish measurement interpretation models based on the actual wellbore environment.

OIL AND GAS DEVELOPMENT

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Analysis of BHA Build-Up Performance for Jimsar Shale Oil Horizontal Wells

ZHAO Yanlong , FAN Xu , CHU Haoyuan , LI Xuanxuan , YUAN Yalong , ZHENG Tianhui

2024, 20(4):  19-24.  DOI: 10.12388/j.issn.1673-2677.2024.04.003

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Well trajectory control is a key task in drilling horizontal wells. The purpose of this study is to determine the influences of BHA （bottomhole assembly） parameters on build-up capacity of horizontal wells in Jimsar shale oil. The build-up efficiencies of conventional and rotary steering tools of nine horizontal wells in this block are compared and analyzed. The mechanical properties of different BHAs and the influence laws of different factors on the build-up capacity of drill assembly are investigated via the beam column approach，and suggestions are given for PDM （positive displacement motor） drill parameters. Research shows that from high to low，the build-up capacity in the case of the drill assembly parameters in this block ranks as listed：single bend PDM drill assembly with single stabilizer （sliding drilling），rotary steering tools，single bend PDM drill assembly with double stabilizer （sliding drilling），and finally single bend PDM drill assembly with double stabilizer （compound drilling）. For PDM drill，the following measures can be taken to increase the build-up capacity：increasing bend angle，spacing of stabilizers and first stabilizer size；decreasing the second stabilizer size. The findings of this research provide guidance for the selection of build-up tools for Jimsar shale oil horizontal wells.

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Evaluation Method for Storage Capacity Expansion and Design Production Realization of Underground Gas Storage in Gas Cap Reservoirs：A Case Study of J Gas Storage#br#

XIAN Mengyuan , XI Zengqiang , WANG Xiuwei , JIA Shanpo , ZHANG Bo

2024, 20(4):  27-35.  DOI: 10.12388/j.issn.1673-2677.2024.04.004

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The evaluation method of storage capacity expansion and design production realization is key to achieving efficient underground gas storage （UGS） operation and predicting the future UGS state. In order to understand the operational rules of UGS storage capacity expansion and design production realization，this paper investigates the J gas storage，a UGS of gas cap reservoir as an example. Based on tracking the basic UGS production data and analyzing the development performance of the gas reservoir，this paper analyzes the multi-cycle injection and production operation data of the UGS and evaluates the pressure，storage capacity，gas production capacity of wells and injection-production operation performance of the UGS，using the dynamic evaluation technology of UGS in gas fields. The main factors affecting the storage capacity expansion and design production realization of the J gas storage are clarified. It is suggested that the J gas storage is transitioning from the capacity expansion stage to the stable stage，but the subsequent storage capacity expansion is challenging，because the drainage well fails to drain normally with gas channeling in the reservoir. It is necessary to perform fine reservoir description of the UGS to clarify the reservoir sand body distribution pattern and identify gas channels. Meanwhile，numerical simulation shall be done to develop a reasonable injection-production scheme to avoid impacts of gas channeling in drainage wells and resume storage capacity expansion. A method for evaluating the UGS capacity expansion and production realization has been developed，which provides technical support and references for the construction of similar UGS facilities and realization of UGS storage and production capacity targets.

Key words：gas storage in gas cap reservoir；storage capacity；working gas volume；injection-production

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Intelligent Optimization of Integral Fracturing in Unconventional Reservoirs

ZHANG Li , WANG Bo , LYU Zhenhu , LYU Bei , LI Lizhe , ZHOU Hang

2024, 20(4):  36-43.  DOI: 10.12388/j.issn.1673-2677.2024.04.005

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Integral fracturing is one of the key technologies for the cost-effective development of unconventional reservoirs，which delivers one-time well placement，one-time fracture placement and synchronized initiation of production through batch drilling and batch fracturing. Optimization of well and fracture spacing is of great significance to improve the performance of integral reservoir stimulation. In this work，a typical block of the Mahu conglomerate reservoir is taken as an example to establish a three-dimensional geological model through the geology-engineering-integrated Petrel platform using well logs，mud logs and fracturing treatment parameters. Based on CMG，a numerical reservoir simulator，and logarithmic mesh refinement method，a hydraulic fracturing model of a four-well platform is constructed for production forecasting. Using the particle swarm optimization （PSO） and differential evolution （DE） algorithms，the well spacing and the fracture spacing of the four-well platform are optimized with the well group productivity as the objective function，which realizes the seven-dimensional synchronized parameter optimization. The optimized well group productivity is about 16.3% higher than that of the actual case. The optimized four-well platform presents a longer stable production duration and slower production decline. The optimized hydraulic fracturing treatment is found with a larger affected zone，further promoting the improvement of oil productivity. The findings of this work provide the fundamental model and methodology for optimizing the integral fracturing scheme of unconventional reservoirs.

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Numerical Simulation for Wax Forming Trend Considering Heat Loss in Small Size Vertical Pipes

YANG Zaiguang , LIU Qiwei , WEI Kai, , LIU Yulong, , HAN Xuelong,

2024, 20(4):  44-49.  DOI: 10.12388/j.issn.1673-2677.2024.04.006

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In the production process of flowing wells，the temperature gradients of oil streams are high in the axial and radial directions of wellbores，and the radial wax concentration is affected by Brownian motion and molecular dynamics，leading to highly variable gradients. Due to impurities such as resins and asphaltenes，wellbore precipitates are generated under the control of multiple factors such as oil stream temperature and ambient temperature，with consequences ranging from production reduction to wellbore blockage and thus production interruption. Given the high likelihood of crude oil wax forming with a low freezing point and high thermal sensitivity in an oilfield in western China，a finite element model of oil stream wax forming in small size vertical pipes was established based on thermodynamic model，and the latent heat often ignored in ideal solution models was incorporated. The wax forming trend in a wellbore was simulated using the numerical simulation software. The effects of ambient temperature，oil stream inlet temperature and oil flow Reynolds number on wellbore wax forming were studied. The results show that the wax deposition of crude oil with a low freezing point and high thermal sensitivity increases significantly in a low temperature environment with a low Reynolds number of oil flow. In order to avoid wax forming and blockage caused by heat loss in wellbore，oil stream temperature shall be controlled within 5-10 ℃ higher than the static wax deposition point. The findings of this research provide theoretical support for the prediction of wax forming trend and the determination of wax forming prevention measures during oilfield development.

NEW ENERGY

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CO2 Corrosion Resistance Evaluation of Packer Rubber in CCUS Wells

WANG Sui , SONG Shaohua, , CHEN Sen , ZHANG Liyi , CHEN Zhijun , LI Yanpeng,

2024, 20(4):  50-59.  DOI: 10.12388/j.issn.1673-2677.2024.04.007

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Packers are important tools for CCUS flooding operations，and the reliability of packer rubber directly affects the packer performance. In order to explore the corrosion resistance of packer rubber in a critical CO₂ environment，the corrosion resistance and mechanical properties of nitrile rubber （NBR），hydrogenated nitrile rubber （HNBR） and fluororubber （FKM） in a simulated supercritical CO₂ environment were studied. The high temperature high pressure corrosion tests show that the largest mass increment after tests is observed in the NBR rubber （6.81%） and the least one in the HNBR rubber （2.48%）. The NBR rubber is also found with the largest outside diameter （OD） increment （4.21%），and the HNBR with the smallest OD increment. The largest and lowest drops of tensile strength occur in the FKM and HNBR materials，respectively （66.85% and 37.78%）. The results show that with the increasing service time and ambient temperature，the rate of corrosive media like CO₂ entering rubber is accelerated，which aggravates the damage of the rubber crosslinking network and degrades the mechanical properties. Because weak bonds like unsaturated double bonds in HNBR are fewer than those in NBR，and the bond energy of HNBR is higher than that of the N-O bond of FKM，the stability of HNBR in supercritical CO₂ is better than those of the other two. Therefore，HNBR is recommended to be used as sealing elements of packers in CO₂ flooding injection wells.

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Experimental Study on Etched Fracture Conductivity in Carbonate Rocks for Supercritical CO2 Prepad Acid Fracturing#br#

ZHONG Pengjun , LIU Chao , WEI Chengxiang , ZHOU Weiqin , TIAN Shouceng , WU Jinqiao

2024, 20(4):  60-69.  DOI: 10.12388/j.issn.1673-2677.2024.04.008

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Previously，the carbonate reservoir of Majiagou Formation in Yan'an Gas Field adopted traditional acid fracturing treatments，which ended up with a limited acid-affected range and low post-frac well production. This necessitates the optimization of the acid fracturing practice. The reservoir outcrop samples of the Fifth Member of Majiagou Formation were collected for experiments，and the etched-fracture conductivity meter and 3D laser scanning technology were used for acid etching experiments and fracture conductivity measurement in the cases of supercritical CO₂ （SC-CO₂） prepad acid fracturing and conventional acid fracturing. The effects of  prepad SC-CO₂ and SC-CO₂ soaking time on etched fracture morphology and conductivity were investigated. The results demonstrate that acid fracturing with prepad SC-CO₂，compared with the conventional method，opens natural fractures，enhances the penetration and diffusion of acid solutions into micro-fractures，and thereby improves acidizing effectiveness. After 12 h and 24 h of SC-CO₂ Soaking，the dominant acid etching pattern resembles grooves，with no significant changes in overall etching severity or types.The 48 h soaking ething is associated with the predominant bridge-pier-like etching marks，while the uniform etching is found dominant after 72 h soaking. The difference of fracture conductivity between conventional acid fracturing and SC-CO₂ prepad acid fracturing is minor under the closure pressure below 30 MPa，and yet，a substantial disparity emerges beyond this threshold. The fracture conductivity of SC-CO₂ prepad acid fracturing consistently exceeds that of conventional acid fracturing across the closure pressure range of the experiments. In cases of low closure pressure，the soaking duration is positively correlated with the etched fracture conductivity，whereas excessively prolonged soaking periods under high closure pressure can suppress fracture conductivity. Therefore，48 h well shut-in is recommended. The findings of this research provide theoretical support for the scheme development of compound acid fracturing in carbonate reservoirs.

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EOR Scheme Optimization of CO2 Miscible Flooding in Bohai BZ Reservoir

WU Zheng, LING Haochuan, WANG Jijun, ZHAO Zhuo, LI Shan

2024, 20(4):  70-76.  DOI: 10.12388/j.issn.1673-2677.2024.04.009

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For the first time，CO₂ flooding is attemptively applied to the BZ low permeability reservoir of Bohai Sea area. To capture the rule of CO₂ miscible flooding in the target reservoir，the minimum miscibility pressure （MMP） and displacement efficiency of the target block were determined using two methods，namely the slim tube and long core flooding tests. The results show that the MMP measured via the slim tube test is 32.65 MPa，and miscibility can be realized in the field. The long core flooding incorporates the actual characteristics of the reservoir，and the measured displacement efficiency is more consistent with the field development reality. The conceptual model was built by numerical simulation to analyze the effects of the injector-producer spacing and injection intensity on the sweep efficiency of gas injection. It is shown that during the early development with the high reservoir pressure，injected gas mainly migrates along the high permeability reservoir；it sweeps the medium-low permeability reservoir，as the reservoir pressure declines and the gravity segregation of CO₂，induced by the oil-gas density difference，intensifies. For reservoirs with relatively inferior porosity and permeability，production wells shall be stimulated （e.g. fracturing） to improve the injector-producer pressure difference and ensure sufficient production. In accordance with the development indexes，an injector-producer spacing within 350-400 m and injection intensity of 1×10⁴-2×10⁴ m³/d are preferred. For the optimized scheme，the recovery rate is predicted to be 2.7%/a，associated with an estimated recovery factor of 22.2% and cumulative CO₂ storage of 107×10⁴ t. The findings of this research provide technical support for the gas flooding scheme and well location/spacing determination and are of great practical significance for offshore gas flooding.

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Temperature，Pressure and Phase Transition Characteristics of Supercritical CO2 Pipeline During Commissioning Replacement Process in Xinjiang Oilfield#br#

LI Xinze, , WANG Dezhong , ZHANG Haifan , LIU Shuangquqn , ZHU Tao, , XING Xiaokai,

2024, 20(4):  77-86.  DOI: 10.12388/j.issn.1673-2677.2024.04.010

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Due to the phase characteristics of CO₂，the commissioning and replacement process of supercritical CO₂ pipelines is significantly different from that of crude oil and natural gas pipelines. In this paper，OLGA software was used to simulate the commissioning and replacement process of the supercritical CO₂ pipeline demonstration project in Xinjiang Oilfield，and the dynamics of pressure，temperature，density，phase transition and other parameters were captured. The effects of the initial back pressure，CO₂ injection massive flow of various phases in first station filling and ambient temperature on the replacement time，temperature，pressure，phase transition process and CO₂ consumption were investigated. Findings indicate that in the liquid CO₂ injection stage，gaseous CO₂ is constantly squeezed，and the thermal movement of gaseous CO₂ molecules is intensified due to pressurization，leading to concurrent increases in pressure and temperature. Consequently，the fluids at the pipeline's end transitions directly from the gaseous to the supercritical phase. Enhancing the mass flow rate of CO₂ injection in each phase can expedite the replacement process. To ensure the pipeline's maximum temperature keeping below 50 ℃，the injection flow rate of CO₂ in the liquid phase should be capped at 0.5 m/s. The increase in ambient temperature significantly elevates the liquefaction pressure，prompting the fluids in most pipe sections to transition directly from the gaseous to supercritical phase，which is unfavorable for phase control. Therefore，commissioning during summer is not advisable. To facilitate the assessment of the production plan，three key indexes were proposed：safety index （pipeline temperature），efficiency index （time consumption of commissioning） and economic index （CO₂ consumption），with the safety index taking precedence. The research results offer valuable insights for the development of production plans for supercritical CO₂ pipelines.

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Applications and Exploration of Intelligence Technology for CO2 Transportation Pipelines in the Background of Carbon Peak and Carbon Neutrality#br#

MA Yun , ZHAO Li

2024, 20(4):  87-96.  DOI: 10.12388/j.issn.1673-2677.2024.04.011

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CCUS （Carbon Capture，Utilization and Storage） is widely considered as an indispensable key technology for achieving the goals of global temperature control and China's vision of carbon neutrality，since it can effectively and significantly reduce industrial CO₂ emissions. The transportation of CO₂ from the source to the utilization site is a key link for achieving the technological goals of the CCUS industry. This paper mainly focuses on the pipeline transportation method and firstly outlines the development status of intelligent pipeline technology in both China and other countries. Then，the phase classification and related problems of CO₂ transportation pipelines are addressed，and a problem-oriented investigation of the mechanism of deep fusion between artificial neural network models and pipeline corrosion rate prediction is presented. Moreover，the pipeline leak positioning principles and applications of distributed optical fiber，characteristic spectrum，and robot inspection technologies are clarified，and finally the development orientation of CO₂ transportation pipeline intelligence is discussed. Comprehensive perception，centralized sharing，prediction and early-warning，and collaborative operation of CO₂ pipeline data，driven by the business demand of CO₂ transportation pipelines and delivered by pipeline transportation technology plus ICT technology，are believed to be the future trend of intelligent development of CO₂ transportation pipelines，although no wide agreement has been reached yet. The findings of this research provide references for the sustained development and gradual application promotion of intelligent pipelines in the future.