Table of Content

25 June 2024, Volume 31 Issue 3

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Summary

Research Status on Mechanism of Enhanced Oil Recovery by Nanofluids

Hao Long, Hou Jirui, Liang Tuo, Wen Yuchen, Qu Ming

2024, 31(3):  1-10.  DOI: 10.3969/j.issn.1006-6535.2024.03.001

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As a new technology to enhance oil recovery, nanofluid flooding has advantages over traditional surfactants and polymer solutions flooding. However, the current research on the mechanism of this technology is not systematic. Based on the research progress of nanofluids at home and abroad,this study summarizes the main EOR mechanism of nanofluid flooding. Meanwhile,the current difficulties faced by this technology and the future research direction are pointed out.The results show that the EOR mechanism of nanofluids flooding includes: reducing oil-water interfacial tension;forming a wedge-shaped film in three-phase (oil-water-solid) zone, which results in the pressure of structural separation; improving the mobility ratio to expand the swept area; altering the wettability of rock;enhancing foam stability; reducing injection pressure and selecting the porous channels of water plugging.Future research can focus on improving the stability of nanofluids, reducing costs, conducting synergistic studies on the mechanisms of enhanced oil recovery, and developing efficient nano-flooding systems.This study lays a theoretical and experimental foundation for the large-scale application of nanofluids in enhanced oil recovery process.

Geologic Exploration

Practice of Digital Wellbore Inversion of Coal Seam Structure and Evaluation Method of Organic Matter Abundance

Huang Hongxing, Yang Xiuchun, Chen Guohui, Zhu Wentao, Shi Binbin, He Rui, Zhao Haoyang

2024, 31(3):  11-17.  DOI: 10.3969/j.issn.1006-6535.2024.03.002

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Coalbed methane is one of the typical types of self-generated and self-stored gas reservoirs.Methane gas is mainly generated by the thermal evolution of organic carbon in coal seams and adsorbed in large quantities on the surface of pores.The content of organic components is crucial for the gas generation and storage capacity of coal seams.However,the reservoir structure of coalbed methane is complex,with interbedded development of organic coal-rock component structures and inorganic leat.The strong heterogeneity within the layers and the difficulty in visualizing the longitudinal coal seam structure in a single well greatly hinder the evaluation of organic matter abundance and efficient development of coalbed methane in single wells.Full-diameter CT scanning is an important experimental technique for large-scale core analysis in recent years.The grayscale values obtained by this technique are closely related to the composition of coal and rock.The higher the grayscale value,the higher the ash content,density,and mineral content of coal and rock,and vice versa.Taking the Daning-Jixian Block as the research object,this study combines full-diameter CT scanning technology with wellbore interpretation technology to explore the relationship between rock grayscale and logging data such as gamma, density,and sonic transit time.A mathematical model for digital well logging inversion of coal seam structure is established,and combined with the relationship between grayscale and organic components,a method for evaluating organic matter abundance in single wells using full-diameter CT scanning is proposed.The comparison between the inverted cross-section of Well A1 in the study area and the real grayscale cross-section obtained by full-diameter CT scanning shows a high degree of similarity,and the distribution pattern of inverted grayscale values is consistent with that of real CT grayscale values,indicating that the inversion model can effectively reflect the heterogeneity structure changes within the coal seam.The digital wellbore inversion and organic matter abundance evaluation method can provide strong technical support for fracturing section selection and production capacity prediction of coalbed methane wells in the study area.

Characterization of Tight Sandstone Reservoirs and Prediction of Favorable Reservoirs in the Fourth Member of the Upper Triassic Xujiahe Formation in the Tianfu Area of the Sichuan Basin

Qiu Yuchao, Xu Qiang, Li Yibo, Deng Wei, Zheng Chao, Zhao Zhengwang, Jin Zhimin, Tan Xiucheng

2024, 31(3):  18-26.  DOI: 10.3969/j.issn.1006-6535.2024.03.003

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The fourth member of the Upper Triassic Xujiahe Formation in the Tianfu Area of the Sichuan Basin (referred to as the T₃x₄) is characterized by thick sandstone layers interbedded with thin mudstone layers,making it a crucial exploration target for tight sandstone gas reservoirs.However,due to limitations in seismic vertical resolution,thin mudstone layers are hard to be determined by conventional methods,hindering the precise characterization of sandstone in this interval.This limitation has impeded the understanding of the distribution patterns of sand bodies in the basin and identification of favorable target areas.To address these issues, this study integrates drilling data with seismic facies and establishes a mapping relationship between the two.Under the constraint of natural gamma-ray curves,optimal inversion frequency parameters were sought to enhance the accuracy of inter-well sand body prediction.The distribution characteristics of the T₃x₄ sandstone in the Upper Triassic Xujiahe Formation were well characterized with waveform indication simulation method.The results indicate that the T₃x₄ sandstone exhibits characteristics of being thick in the west (55.0 to 100.0 m) and thin in the east (15.0 to 50.0 m).Compared to the third member of the Xujiahe Formation,the transition zone of sand body thickness variation (slope break zone) has migrated towards the interior of the craton.Analyzing data from 18 wells within the seismic working area,it was observed that beneath the slope break zone,the sandstone has a high average porosity (8.0%) and a large reservoir thickness (45.4 m).Considering tectonic evolution, sedimentary facies distribution,sandstone thickness,and porosity variations,it is proposed that the T₃x₄ follows a pattern of structures control slope break zones,and slope break zones control reservoirs.Regions below the slope break zones exhibit favorable reservoir properties with good physical characteristics and significant thickness,making them promising exploration areas.The study results provide valuable insights for the in-depth exploration of the T₃x₄.

Characteristics and Genesis Mechanism of High-quality Clastic Reservoirs in Zhuanghai Area of Jiyang Depression

Wei Min, Yu Shina, Tong Huan, Wu Yanjia, Zhou Hongke, Shi Xiaoxiao, Guo Hougen

2024, 31(3):  27-36.  DOI: 10.3969/j.issn.1006-6535.2024.03.004

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Oil and gas resources are rich in Zhuanghai Area of Jiyang Depression. The proven reserves are high in the Neocene and buried hill strata, while the degree of drilling and understanding of the Paleogen is low, and the characteristics of the Paleogen reservoirs and the genesis mechanism of high-quality reservoirs are poorly understood. For this reason, we have studied the petrological characteristics, type of diagenesis, spatial characteristics of clastic reservoirs and the controlling factors of high-quality reservoirs in the Lower Dongying Member-Shahejie Formation in Zhuanghai Area through the means of analysis such as cores, thin sections, scanning electron microscopy, etc. The results show that the clastic reservoirs of the Lower Dongying Member-Shahejie Formation in Zhuanghai Area have a small distribution range and thin thickness, and the reservoirs are dominated by sandstones deposited rapidly near sources with low rock maturity. The compaction is the main reason for destroying the pore space of the reservoirs. The cementation, mainly carbonate cementation, has caused further damage to the pore space. The dissolution of organic acid is an essential factor for forming secondary pore space and increasing pore space in the reservoir. The overall pore type is dominated by secondary pores and primary pores are only retained to a relatively high degree in the reservoirs of the Lower Dongying Member in the northwest source area, and the reservoir fractures are few. The high-quality reservoirs is mainly controlled by three conditions: sedimentary conditions, diagenetic modification, and fracture communication. The glutenite reservoirs are favorable exploration targets with high rock maturity, far from calcium sources, close to acid sources, and fracture communication. This study can provide the geological basis for the exploration and development of the Paleocene system in Zhuanghai Area.

Characteristics of Sedimentary Facies and Lithofacies Distribution of Deep Shale of Wufeng Formation-Longmaxi Formation

Li Wei, Lei Zhian, Chen Weiming, Meng Senmiao, Chen Li, Pu Bin, Sun Chaoya, Zheng Jie

2024, 31(3):  37-44.  DOI: 10.3969/j.issn.1006-6535.2024.03.005

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The deep shale gas of Wufeng Formation-Longmaxi Formation in the western Chongqing is an essential area for the next exploration and development of shale gas in China. However, the shale petrographic components in this block are complex and spatially variable, and the fine description of the distribution characteristics of sedimentary facies and lithofacies is lacking, which affect the identification and preference of shale gas “sweet spot” sections. To this end, the sedimentary facies and lithofacies of the deep shale of Wufeng Formation-Longmaxi Formation in western Chongqing were studied in detail through core thin section, scanning electron microscopy, Xray diffraction, FMI imaging, nuclear magnetic resonance, organic carbon analysis and other research tools. The results show that the whole study area is in the deposition of offshore shelf phase, which can be divided into three subfacies: shallow water shelf, semi-deep water shelf, and deep water shelf that have different controlling effects on the shale gas; 10 main lithofacies are developed, and the lithofacies are more stably deposited from the northwestern to the southeastern part of the area, and Longyi₁¹ layer develops three main lithofacies: siliciclastic shale, mud-rich siliciclastic shale, and mud-siliciclastic shale. According to the distribution characteristics of the sedimentary facies and lithofacies, it is predicted that the resource potential of this section in Well Z203 Area is huge, and it is a favorable exploration area in the next step. The results of this study deepen the understanding of the vertical distribution of shale reservoirs in the study area and provide data support for the subsequent efficient development of shale gas.

Structural Evolution and Fault-controlled Hydrocarbon Accumulation Mechanism of Aogula Fault Zone in Songliao Basin

Sun Guoqing

2024, 31(3):  45-51.  DOI: 10.3969/j.issn.1006-6535.2024.03.006

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The Aogula Fault Zone in the Songliao Basin exhibits complex kinematic characteristics, making the study of the spatiotemporal matching relationships of various reservoir-forming controlling factors challenging, which brings challenges to the accurate characterization of reservoir morphology. To address this issue, a comprehensive analysis of geological, seismic interpretation, and drilling data was conducted to understand the structural development history and fault causes. Based on the understanding of reservoir formation, the mechanisms of fault-controlled oil and gas accumulation were clarified. The fault-dense zone in the study area can be divided into two levels, and the faults are classified into three types based on their relationship with stratigraphic position. The study reveals that the Aogula Fault Zone is composed of two antithetic "S"-shaped main faults, experiencing three stages: faulting, sagging, and inversion. During the faulting stage, the activity intensity and elongation percentage of the fault zone are the highest, gradually weakening during the sagging stage, accompanied by multiple reverse regulating faults, and strengthening again during the inversion stage. Oil-source faults serve as vertical migration pathways connecting source rocks and traps, controlling the formation of fault-related traps and providing favorable conditions for oil and gas migration and accumulation. The research results can provide theoretical support for further exploration targeting and reserve enhancement.

Study on Response Mechanism and Brittle Evaluation of RockFracture Acoustic Emission Signals Based on Wavelet Packet Decomposition

Wei Jingyi

2024, 31(3):  52-60.  DOI: 10.3969/j.issn.1006-6535.2024.03.007

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Rock brittleness is a key indicator controlling rock fracturing, determining the formation of an effective fracture network during fracturing. Studying the characteristics of rock brittleness and fracturing behavior through acoustic emission signals can directly reveal the relationship between rock fracturing and brittleness. However, the relationship between acoustic emission signal characteristics and rock fracturing patterns has not yet been established, making it difficult to accurately determine the brittleness characteristics of rocks. Therefore, based on the wavelet packet decomposition method, this study divides rock acoustic emission signals by energy frequency bands and builds the relations between different brittle rock fracturing patterns and the energy distribution of acoustic emission signals. Through the analysis of the three-dimensional spatial distribution characteristics of acoustic emission signals, a rock brittleness index evaluation method based on the energy distribution of acoustic emission signals is proposed. The study shows that the energy characteristics of acoustic emission signals are related to rock brittleness, with the main frequency of energy distribution mostly below 750 kHz. The higher the content of brittle minerals in the rock, the larger the proportion of energy signals,the more concentrated the energy distribution of rock fracturing acoustic emission signals, the stronger the mutability of rock fracturing, the more apparent the transition between fracture tension, shearing, and mixed evolutionary expansion modes, and the greater the brittleness index. The results can provide reference and guidance for research on rock fracturing warning and other related aspects.

A Calculation Method of Deep Tight Reservoir Saturation Based on Acoustic Parameters and Pore Structure Classification

Yuan Long, Liu Wenqiang, Luo Shaocheng, Wang Qian, Li Nan, Cao Yuan

2024, 31(3):  61-69.  DOI: 10.3969/j.issn.1006-6535.2024.03.008

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Deep tight reservoirs present intricate geological conditions, posing challenges in accurately calculating gas saturation. To address the issue, the structural belt of northern Kuqa in Tarim Basin is taken as an example. Based on the theories of rock conductivity and acoustics, using petrophysicaltesting methods and data analysis techniques, combined with mercury intrusion porosimetry and nuclear magnetic resonance data, the reservoirs are divided into four categories based on their pore structures.According to the parameters of rock fractures, the relation equation between water saturation and acoustic parameters at different conditions of pore structures is established. An innovative saturation calculation model based on acoustic parameters and pore structure classification has been developed. According to the research results, compared with Archie equation of electrical petrophysical variations in rocks and Gassmann-Wood saturation equation, the simulator better matched with the test data. This method has proven to be feasible. The method provides a new approach to calculate the saturation of deep tight reservoirs using non-electric logging data, and promote efficient exploration and development in deep tight gas reservoirs.

Reservoir Engineering

Physical Property Law of Coalbed Methane Well Fracturing Development Enhanced by Microwave

Li Xiaogang, Qin Yang, Liu Ziwei, Zhu Jingyi, Yang Zhaozhong, Xie Shiyi, Jin Xinxiu, Gao Chenxuan

2024, 31(3):  70-77.  DOI: 10.3969/j.issn.1006-6535.2024.03.009

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To study the effects of microwave heating on the properties of coal, the temperature rise and cracking laws of coal under microwave action were studied through physical and numerical simulation methods.The research shows that the water cut of coal has a direct impact on the microwave heating effect, and the temperature rise rate inside the coal mass is anisotropy, which leads to the uneven heating of different parts, thereby generating thermal stress.Higher microwave power leads to more severe coal fragmentation.Microwaves applied to the fractured coal can further expand the artificial cracks.The additives in the fracturing fluid can improve the dielectric properties of coal.The production-increasing effect of slickwater fracturing fluid is significant, and it is expected to replace active water (KCl) for efficient production-increasing technology transformation. In the early stage of implementation, hydraulic fracturing technology is used for the fracturing fluid fill into the depth of the fracture as much as possible,which can enhance the dielectric properties of the coal seam. In the later stage,using microwave heating technology can enhance the effectiveness of coalbed methane recovery.This study can provide guidance for the refracturing process of coalbed methane development.

A Calculation Method of CO₂ Storage Capacity in Low Permeability Reservoirs Based on "Four-zone" Method and Its Application

Wang Xiangzeng, Chen Xiaofan, Li Jian, Chen Fangping, Fan Qingzhen, Wang Jian

2024, 31(3):  78-84.  DOI: 10.3969/j.issn.1006-6535.2024.03.010

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CO₂ flooding and storage have the potential to significantly mitigate greenhouse gas emissions and contribute to achieving carbon neutrality objectives. Nevertheless, current methods for calculating CO₂ storage capacity primarily concentrate on rough estimates of static CO₂ storage, neglecting variations in CO₂ storage during actual production processes. In view of the above problems, the process of CO₂ flooding and storage is divided into gas phase zone, two-phase or near-miscible phase zone, diffusion zone and oil phase zone by using CO₂ dissolution, CO₂ swept volume and oil displacement mechanism. Based on the four-zone method, the CO₂ storage capacity is calculated, and the dynamic CO₂ storage capacity under varying hydrocarbon pore volume multiples, injection pressure and gas injection rate is obtained. The research results are applied to the low permeability reservoir of W oilfield. The results show that the pore volume multiple, pressure and gas injection rate of injected hydrocarbons are positively correlated with the total storage capacity. When the pressure increases from 12 MPa to 30 MPa, the total amount of CO₂ storage increases by 15.53×10⁴ t; when the gas injection rate increases from 5 t/d to 30 t/d, the total amount of peak CO₂ storage increases from 3.51×10⁴ t to 12.62×10⁴t. The research results can provide new ideas for the development of CO₂ flooding and storage projects in similar reservoirs.

Component Transformation Path in the Process of Heavy Oil Fire Flooding Based on Core Analysis

Sun Xin′ge, Yang Fengxiang, Li Haibo, Zhan Hongyang, Gao Chengguo, Yuan Shibao

2024, 31(3):  85-90.  DOI: 10.3969/j.issn.1006-6535.2024.03.011

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To address the issues of unclear conversion of oil components in in-situ combustion process,the heavy oil reservoir in Hongqian-1 well field of Xinjiang Oilfield was taken as an example:by using infrared spectroscopy and GC-MS method, combined with core analysis results and heavy oil reaction mechanism, the reaction variation of heavy oil components are divided into hydrocarbon oxidation, chemical bond cleavage of heavy oil components and coking formed by aryl radical condensation.The research results are applied to the analysis of the conversion path of heavy oil components.The findings show that the reaction of heavy oil components is the most concentrated in the low temperature stage,which is dominated by condensation and thermal cracking reactions;in the combustion stage,the aryl radical condensation produces coke, and the C-H bond is broken and oxygenated to form oxides such as OH,CHO,and CO;at the high temperature oxidation stage,the coke and heavy components of gum and asphaltene combust.The findings of this study confirm the conclusion that coke combustion induces a rise in temperature and a decrease in viscosity of heavy oil,leading to the transformation of heavy oil components into coke during the fire drive process,thereby enhancing our understanding of the significance of fuel formation and transformation during fire flooding.

Seepage Characteristics and Water Injection Strategy of Fractured Reservoir

Fang Na, Liu Zongbin, Yue Baolin, Wang Shuanglong, Gao Yue

2024, 31(3):  91-97.  DOI: 10.3969/j.issn.1006-6535.2024.03.012

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A research for the optimization of oil recovery rate and water injection method was conducted with numerical simulation and field practice so as to improve the accuracy of numerical simulation of fractured reservoirs and provide effective water injection strategies.In the study,a buried hill reservoir in JZ25-1S oilfield in the Bohai Bay Basin,a large fractured reservoir, is taken as a study object. With the use of digital core technology and multiphase flow simulation, the characteristics of micropores and throats in the upper and lower semi-weathered crust and the characteristics of relative permeability curve are defined. The results indicate that development degree of microfractures plays an important role in controlling the shapes of relative permeability curve and the capillary curve of matrix block; the more developed the microfractures in the core, the higher the oil displacement efficiency, and the more conducive to matrix imbibition; the length and frequency of microfractures follow a power-law distribution, with a power-law index of approximately 1.5; in the initial stage of development, the production are mainly from the fracture system, while in the middle and later stages of development, the production from the matrix system gradually increases; the cumulative production contribution ratio of the fracture system to the matrix system is approximately 2∶1;the recovery rate is expected up by 4.5 percentage points through the optimization of the oil recovery rate in the initial stage and utilization of cyclic water injection during the water-cut rising stage. This study provides important guidance for development strategies and water injection schemes for fractured reservoirs.

Experimental Study on Flow Pattern of Two-phase Slug (Gas and Liquid) in Fractures

Ni Xiaoming, Zhao Yanwei, Guo Shengqiang, He Qinghong, Yan Jin, Song Jinxing

2024, 31(3):  98-105.  DOI: 10.3969/j.issn.1006-6535.2024.03.013

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To address the lack of consensus on the slug flow pattern and control factors in the middle section of reservoir fractures, the flow pattern of slug in fracture is studied, the influence and fluctuation characteristics of key parameters of slug flow are analyzed, such as fracture diameter and gas-liquid phase flow rate. The results indicate that the two-phase slug flow presents periodic characteristics in the fracture by using micro-scale flow simulation experimental device of two phase (gas and liquid) migration and production,. From formation to disappearance, the gas slug can be divided into three stages: formation, expansion and dominance. The fracture diameter is linearly negatively correlated with the slug frequency and linearly positively correlated with the fluctuation range of gas and liquid phase flow. As the liquid flow rate was set at 15.00, 20.00, 25.00 and 30.00 cm³/min, the variance ratio of slug frequency decreased with increasing gas flow rate, being 0.130,0.012,0.009 and 0.007, respectively.Similarly, the variance ratio of liquid holdup decreased with increasing gas flow rate, being 1.503,1.175,0.918 and 0.820, respectively. The findings have guiding significance for the study of slug flow mechanism in the process of coalbed methane development.

The Variation Law of Water Flooding Reservoir in Low Permeability Tight Sandstone Reservoirs

Shi Lihua, Shi Tiaotiao, Liao Zhihao, Xue Ying, Li Lusheng

2024, 31(3):  106-115.  DOI: 10.3969/j.issn.1006-6535.2024.03.014

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To address the issues of unclear understanding of the variation law of clay minerals and micro-porosity structure within low permeability tight sandstone reservoirs before and after water flooding, the Chang 2 and Chang 6 Reservoirs in Yanchang Oilfield of Ordos Basin were taken as research objects. The types and contents of rock and clay minerals, the characteristics of microscopic pore throat structure are studied by using experimental methods such as casting thin sections, X diffraction and high pressure mercury intrusion, and the variation law of reservoirs before and after water flooding was analyzed. The results show that the microscopic heterogeneity of Chang 6 tight reservoir is stronger than that of Chang 2 low permeability reservoir. After water flooding, the content of illite/Montmorillonite mixed layer and illite in Chang 2 Reservoir increased, chlorite content and illite/smectite mixed layer ratio decreased, and the content of illite/smectite mixed layer, chlorite content and illite/smectite mixed layer ratio in Chang 6 Reservoir decreased. When the pore throat radius is large, the injected water improves the pore throat. On the contrary, the pore throats are damaged by the injected water. The larger the core permeability is, the faster the pressure propagation velocity is, the faster the injected water advances, and the more water is visible at the outlet end. This study can provide technical reference for water flooding development of low permeability tight sandstone reservoirs.

Condensate Water Output Mechanism and Production Prediction Method of HTHP Condensate Gas Reservoir in South China Sea

Xiong Yu, Xiong Feng, Feng Pengxin, Lei Tingting, Wang Linghong

2024, 31(3):  116-122.  DOI: 10.3969/j.issn.1006-6535.2024.03.015

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To address the issues of condensate water output rising in high temperature and high pressure (referred to as the HPHT)condensate gas reservoirs in the South China Sea,Dongfang Gas Field is taken as the research object.By using the method of formation water evaporation experiment,water/gas ratio tests were conducted in PVT cells and long cores under formation conditions,and the calculation method for condensate water output in HPHT condensate gas reservoirs was proposed to predict the output rate of condensate water in Dongfang Gas Field.The research indicates that the fluid in HPHT condensate gas reservoir is affected by the small pore size of the porous medium and the shifting of critical properties,the saturated vapor pressure of which is altered, resulting in a significant increase in the production of condensate water.The numerical simulation method considering the interphase mass transfer between water and gas can be accurately predicted the variation of the condensate water output in HPHT condensate gas reservoir. Daily water production of Dongfang Gas Field was simulated with the bottom hole flowing pressurelimit of 1.5 MPa and the prediction time of 5 a.The accuracy of history in water output was over 90%, the condensate water output can be clearly predicted by the method.This study is of guiding significance for formulating a reasonable development plan and improving the development effect in HTHP condensate gas reservoirs.

Optimization Design of CO₂ Huff-n-Puff Parameters in Fuyu Tight Reservoir

Yao Tongyu, Sun Linghui, Cui Chuanzhi

2024, 31(3):  123-128.  DOI: 10.3969/j.issn.1006-6535.2024.03.016

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To address the issues of high production decline rate and low recovery rate in the early stage of tight oil development,considering the rock and fluid properties of Fuyu tight oil reservoir, numerical simulation methods were used to study the effects of injection timing and injection pressure on the effect of CO₂ huff-n-puff.The engineering parameters were optimized, the characteristics of reservoir pressure distribution and interfacial tension distribution are analyzed,the operability of CO₂ huff-n-puff development of tight oil is studied.The results indicate that CO₂ huff-n-puff should be implemented when the oil reservoir pressure coefficient drops to around 0.65.At this time,CO₂ can fully contact with crude oil,which is beneficial for the dissolution,thereby improving the recovery efficiency.As the injection pressure approaches the minimum miscible pressure,CO₂ in a separate phase enters the matrix,making full contact with the tight oil deep within.The dissolved miscible phase improves the development effect.After reaching dissolution equilibrium,the interfacial tension between CO₂ and tight oil increases rapidly over time,the time of well shut-in can be determined at this moment.The optimal engineering parameters for CO₂ huff-n-puff recovery in this study are:the injection volume is 8000 t,the injection rate is 120 t/d, the injection pressure is 27 MPa,and the time of well shut-in is 30 d. According to this scheme, the increase of crude oil production is calculated to be 3 084 t,and the oil replacement ratio is calculated to be 0.39. The in-situ test of horizontal well also confirms that the application of CO₂ huff-n-puff technology in Fuyu tight oil reservoir increased the recovery rate by 1.38 to 3.33 percentage points.The research results are of great significance for further expanding the application of CO₂ huff-n-puff technology in tight oil reservoirs.

Effect of Pressure on Oxidation of Heavy Oil Pseudo-Component

Chen Lijuan, Li Shutong, Chen Long, Wang Tiantian, Pan Jingjun, Zhao Renbao

2024, 31(3):  129-135.  DOI: 10.3969/j.issn.1006-6535.2024.03.017

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To further clarify the performance of reaction kinetic and the variety pattern of oil components in the process of in-situ combustion, the heavy oil of well Hongqian-1 (Xinjiang) was divided into pseudo-components with different boiling ranges by the method of true boiling point (TBP).The effect of system pressure on the oxidation performance of pseudo-components with boiling range higher than 350 ℃ was studied. The combustion pool was tested with linear temperature program to study the variety pattern of system pressure on the gas volume fraction, temperature and thermal intensity while the pseudo-components of heavy oil reacted with injected air.The results show that the system pressure has significant effects on the oxygenation reaction and decarboxylation during the low-temperature oxidation stage, and the effect on the pseudo-components of low boiling range is further pronounced.The CO and CO₂ production of the pseudo-components with a boiling range of 350 ℃ to 420 ℃ at a system pressure of 3.0 MPa is 10 times higher than its production at 1.0 MPa. Although the pseudo-components with high boiling range are less affected by the system pressure, the thermal intensity of the pseudo-components with a boiling range greater than 500 ℃ can be as high as 12.26 kJ/g;the upgrading of heavy oil in the process of in-situ combustion depends on the consumption of heavy components and pyrolysis reactions.The research results provide reference for the analysis of components loss and reasonable analysis and prediction of reservoir temperature in the process of in-situ combustion.

Coke Generation Law and Physicochemical Characteristics in Fire Flooding Process of Naphthenic Heavy Oil

Chen Dengya, You Hongjuan, Chen Ang, Guo Wenxuan, Chen Long, Wang Xusheng, Guo Yong

2024, 31(3):  136-142.  DOI: 10.3969/j.issn.1006-6535.2024.03.018

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The process of coke generation of naphthenic heavy oil during air injection combustion was studied to clarify the law of coke generation and the basic properties of petroleum coke in the fire flooding process of naphthenic heavy oil, and the composition and structure of generated petroleum coke were characterized through a dynamic airflow oxidation crude oil device. The study showed that the coke generation from naphthenic heavyoil starts at about 250-300 ℃, and the coke yield increases gradually with the rise of the reaction temperature and the highest coke yield of 10.8% is obtained when the temperature reaches 450 ℃; the combustion reaction of coke intensifies, and the yield decreases to 4.6% when the temperature is 500-600 ℃. The coke is observed as a dense lamellar structure with 0.05-1.00 μm carbon layers superimposed on each other by electron microscope, and it could be classified into two categories with 400 ℃ as the demarcation point. The H/C atomic ratio of coke generated is 0.78-0.82 at 300-350 ℃, which is oxidized coke containing many alkyl chains. The cracking reaction intensifies at 400 ℃, and the process of heavy oil coke formation transforms from the primary oxygenation reaction to that of cracking, and most cokes generate at 400 ℃. The H/C atomic ratio of the coke generated at 400-600 ℃ is 0.33-0.47, which was cracked coke with higher aromaticity and graphitization degree. This study is of some significance for the ignition and fireline control during fire flooding development of naphthenic heavy oil.

Optimized Proration Plan Based on Stable Production in Luojiazhai High-sulfur Gas Reservoir

Lan Yunxia, Jia Changqing, Liao Huawei, Zhang Junliang, Xue Jiangtang, Zhang Qiang, Li Xiang

2024, 31(3):  143-149.  DOI: 10.3969/j.issn.1006-6535.2024.03.019

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The gathering system of Luojiazhai high-sulfur gas reservoir is affected by sulfur accumulation, the interlock alarm is frequently triggered by the gas well pressure after the secondary throttling. To avoid the interlocking shut-in, the production can only be maintained by means of pressure reduction. By analyzing the relationship between average daily gas production and sulfur accumulation of single well during the phase, it is found that as the ratio of gas production per well increased, the sulfur accumulation also shows an increasing trend. In order to extend the continuous production cycle of single well and reduce the sulfur accumulation and its impact on the gathering system, the traditional production proration method currently used can no longer meet the production requirements. To obtain the optimal stable production period of gas reservoirs, this paper starts from various factors affecting production proration of single well, and establishes a nonlinear mathematical model with the optimal stable production period as the objective function through node analysis. Taking into account constraints such as gas well productivity, critical sulfur-carrying production, maximum production limit of safety valve after throttling, and maximum production corresponding to interlocking pressure after secondary throttling, the single well production proration corresponding to the optimal stable production period is obtained by iterative computation based on differential calculus. According to the optimal production proration plan, production adjustments were made to 6 production wells. After the secondary throttling, the average pressure drop of single wells is about 0.3 MPa, the average stable production days increased from 106 d to 201d, and the sulfur accumulation in gathering system decreased by 121 kg. The method effectively guides the production of gas wells and also provides reference significance for optimizing production allocation in similar gas fields.

Study on Dynamics of Gas Hydrate CO₂ + N₂ Displacement

Guan Fujia, Su Xiangguang, He Wanjun

2024, 31(3):  150-157.  DOI: 10.3969/j.issn.1006-6535.2024.03.020

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To determine the displacement dynamics constant of mixed gas ( CO₂ + N₂ ) injected during the process of natural gas hydrate development, research methods such as indoor experiments and chemical thermodynamic calculations were used. Under the conditions of displacement temperature of 274.15 K and displacement pressure of 3.7 MPa, the driving force, displacement rate and hydrate displacement rate constant during the displacement of natural gas hydrates by 3 groups of mixed gases were studied. And the hydrate displacement dynamics constant was obtained based on inversion of displacement dynamics models. The results show that during the hydrate displacement process, the displacement driving force of the mixed gas ( CO₂ + N₂) and the hydrate displacement rate decrease rapidly with the increase of the displacement time. The generation dynamics constants of CO₂ hydrate in the 3 groups of experiments are relatively close, with an average value of 0.025 2 mol/( h·m²·MPa ), and the degradation dynamics constant of CH₄ hydrate are also relatively close, with an average value of 0.0316 mol/( h·m²·MPa ). Under the same conditions of hydrate system, the dynamics constant of hydrate displacement is only related to the temperature and pressure. The research results have important guiding significance for the dynamic prediction of natural gas hydrate development by using mixed gas (CO₂ + N₂) displacement.

Drilling & Production Engineering

Analysis of the Relationship Between Fracturing Fluid Imbibition and Typical Production Rules of Gas-rich Shale Gas Wells

Wang Ke, Lu Shuangfang, Lou Yi, Li Nan, Li Haitao, Ye Kairui, Zhang Yan, Li Songlei

2024, 31(3):  158-166.  DOI: 10.3969/j.issn.1006-6535.2024.03.021

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The imbibition of fracturing fluid can alter the original gas-water occurrence in reservoirs,thereby affecting the production behavior of shale gas wells.However,through dynamic displacement and nuclear magnetic resonance experiments,combined with previous research results,we analysis of the original gas-water distribution in pores,the effects of fracturing fluid imbibition on gas occurrence in through-pores and blind-pores,and the relationship between gas well production behavior and fracturing fluid imbibition were carried out.The study reveals that the occurrence of original gas and water in pores is related to mineral types,fracture apertures,and reservoir humidity.There is bound water in small pores and pore corners.The adsorption area of methane in large pores decreases with increasing reservoir humidity,and the amount of adsorbed gas is influenced by the proportion of organic matter pores and maturity.A zone affected by imbibition exists near fractures,where fracturing fluid imbibition can promote desorption of adsorbed gas,displace free gas in non-equilibrium imbibition through-pores,and compress free gas in equilibrium imbibition through-pores and blind-pores.The gas displaced from non-equilibrium imbibition through-pores is one of the main sources of free gas in hydraulic fractures.Only when the fracturing fluid retained in the wellbore,fracture and influence area is discharged,was gas be produced from various areas,resulting in the phenomenon that the gas production curve lags behind the water production curve,and the fracture drainage resistance is much smaller than that of the matrix pore,which is the main reason for the "L"type decline of gas production and water production curve.The research findings focus on the impact of fracturing fluid imbibition on the occurrence and production of gas,contributing to the enrichment of shale gas reservoir protection theories.This has guiding significance for the efficient development of gas wells.

Study on Electronegative High-efficiency Polymer Degrading Agent and Field Pilot Test

Wen Yuchen, Hou Jirui, Lou Zeyang, Pan Yinuo, Qu Ming

2024, 31(3):  167-174.  DOI: 10.3969/j.issn.1006-6535.2024.03.022

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An electronagtive high-efficiency polymer degradation agent is developed in this study to address the problem of polymer plugging caused by long-term polymer injection during polymer flooding. In this study, the microscopic morphology, system stability, degradation performance, and viscosity reduction performance of the degradation agent were studied by means of electron microscopy, Zeta potentiometer, and multi-angle light scattering instrument, and the dynamic degradation effect of the degradation agent in porous media was evaluated. The results show that the optimal dosage of the degradation agent is 1% by mass fraction, both the viscosity reduction rate and degradation rate of the polymer can reach over 95% at this dosage. The degradation agent exhibits strong capacity of degradation for polymers and polymer clusters, with a significant effect on removing blockage. And on the other hand, the degradation agent is featured with low corrosiveness and causes little damage to wellbore and pipelines. A field test of plugging removal by polymer injection was conducted in Well W4-4 of the JZ9-3 Oilfield, and the field plugging removal effect of electronegative high-efficiency polymer degradation agent was verified. This polymer degradation agent can provide technical support for the efficient development of polymer flooding.