Table of Content

25 October 2024, Volume 31 Issue 5

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Geologic Exploration

Characteristics and Main Controlling Factors of High-quality Reservoir in the Mound-beach Body of the Fourth Member of Dengying Formation in Gas Zones of the Central Sichuan Basin

Fan Yi, Xia Maolong, Jia Song, Li Yiwen, Chen Hongwei, Wang Shilin, Dang Yijia, Lei Baoze

2024, 31(5):  1-10.  DOI: 10.3969/j.issn.1006-6535.2024.05.001

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In order to clarify the development characteristics and main controlling factors of high-quality reservoirs in the Fourth Member of Dengying Formation in Penglai gas zones,based on the analysis of reservoir lithology and porosity-permeability properties,the main controlling factors of high-quality reservoir development are systematically studied and the reservoir deposition and formation model are established by drilling core,rock slice observation,logging and physical property test.The results show that the dominant reservoir rocks of the Fourth Member of Dengying Formation are algae concretion dolomite,algae stromatolite, and algae laminated dolomite.The main reservoir space is formed by dissolution pores and microfractures.The diameter of dissolution pores decreases toward the bottom of the reservoir, and three types of reservoirs are developed:matrix pores, dissolution pores,and dissolution pore-microfractures.The dissolution pore-microfracture type is concentrated in the upper part of the Fourth Member of Dengying Formation;the dissolution pore type is mainly developed in the middle and upper part;the matrix pore type is concentrated in the lower part.The reservoir as a whole has the characteristics of low porosity and ultra-low permeability.Vertically, the mound base and mound core are developed as high-quality reservoir layers.Supergene freshwater dissolution,buried hydrothermal dissolution and hydrocarbon migration organic acid dissolution in Tongwanian jointly promote the development of dominant reservoirs and control the intensity and mode of dissolution.The fault can provide a channel for infiltration and migration of fluids, connect isolated pores,accelerate the development of karstification, and control the intensity of karstification and the spatial distribution of dissolution pores.Based on the characteristics of reservoir space and vertical distribution,starting from the genetic mechanism,three types of reservoir development models of vertical and horizontal superimposed seepage fracture solution and horizontal hyporheic layer solution of the surface mixed karst body of the Fourth Member of Dengying Formation in Penglai gas zones are established.The results have certain reference significance for the efficient development of the Fourth Member of Dengying Formation in Penglai gas zones.

Identification of Natural Gas Hydrates and Natural Gas Reservoirs Based on SMOTE and XGBoost

Du Ruishan, Huang Yupeng, Fu Xiaofei, Meng Lingdong, Zhang Yi'nan, Jin Mingyang, Cai Hongbo

2024, 31(5):  11-19.  DOI: 10.3969/j.issn.1006-6535.2024.05.002

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Natural gas hydrates identification and characterization are the key tasks throughout the exploration and development phase of marine energy resources.However,due to the complex nonlinear relationship between logging data and reservoirs,as well as the imbalance of logging data, traditional reservoir identification methods often show low accuracy,which severely limited the progress of energy exploration in the study area.To address the above issues,a composite method for reservoir identification is proposed.The improved SMOTE algorithm is used to increase the number of minority class reservoir samples and denoise the data,which effectively solves the issues of data imbalance.The XGBoost algorithm is then used to identify reservoirs.The results show that compared with traditional machine learning method,the RLSMOTE-XGB method has higher effectiveness and accuracy in reservoir identification.This method addresses the limitations of traditional machine learning methods in the case of imbalanced sample classes,increasing the reservoir identification accuracy from 66.7% to 86.4% and significantly improving the algorithm′s performance.This study can effectively improve the identification effect of natural gas hydrates and natural gas reservoirs,which is of great significance for achieving intelligent reservoir identification.

Method for Predicting Formation Pressure in Anomalously High-Pressure Mudstone Section at the Southern Margin of Junggar Basin

Tian Shanchuan, Gan Renzhong, Xiao Lin, Ding Yi, Wei Ruihua, Chen Xiaowen, Xu Yonghua, Liang Lixi

2024, 31(5):  20-30.  DOI: 10.3969/j.issn.1006-6535.2024.05.003

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The mudstone section of Anjihaihe Formation in the southern margin of Junggar Basin is characterized by abnormally high pressure.The genetic mechanism of abnormal pressure is not clear.Moreover,the mudstone section exhibits strong fluid sensitivity,which can lead to deviation in well logging data and is not conducive to formation pressure prediction.To address this issue,considering the impact of fluid actions on the physical parameters of mudstone,a well logging property correction method is established.On this basis,the formation mechanism of abnormal high pressure in mudstone formation is revealed by combining sound velocity-density curve and comprehensive compaction curve.A multi-parameter formation pressure prediction method for the mudstone section has been developed,combining effective stress theory and multiple regression analysis method.The results indicates that the genetic mechanisms of high pressure in the mudstone formation at the southern margin of Junggar Basin include undercompaction,tectonic compression,and tectonic compaction.After logging correction,with the increase of effective stress,the longitudinal wave time difference,porosity,natural gamma and average horizontal stress of mudstone decrease,while the density increases.The effective stress is linearly related to the longitudinal wave time difference,density and porosity,and exponentially related to the natural gamma and average ground stress.Based on these findings,a well logging prediction method for mudstone formation pressure is constructed and its applicability has been verified.The research outcomes contribute to the design of drilling schemes in the study area and are of significant importance for the efficient exploration and development in the southern margin of Junggar Basin.

Microscopic Pore Structure Characteristics and Classification Evaluation of Tight Sandstone Gas Reservoirs in the Zhongba Area of Western Sichuan

Liu Lin, Liu Xiangjun, Sang Qin, Xiong Jian, Li Wei, Liang Lixi

2024, 31(5):  31-40.  DOI: 10.3969/j.issn.1006-6535.2024.05.004

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The tight sandstone reservoirs in Zhongba Area of western Sichuan have experienced complex sedimentation and diagenesis evolution,resulting in poor reservoir physical properties,complex pore structure and strong heterogeneity,which makes it difficult to effectively evaluate the reservoir and to predict the"sweet spots".To address this issue,this study focused on the tight sandstone reservoirs of the Xujiahe Formation in the Zhongba Area,and systematically evaluated the microscopic pore structure characteristics of the reservoirs based on rock physics experiments including porosity and permeability from gas logging,casting thin sections,high-pressure mercury intrusion,and scanning electron microscope.Additionally,reservoir classification and well logging evaluation are conducted to determine the development and distribution characteristics of high-quality reservoirs with flow unit theory and BP neural network technology.The results show that the physical properties of the target reservoirs in the study area are poor,the reservoir space is mainly secondary intergranular pores and dissolution pores,the throat types are mainly point-like and necked,and the pore size is mainly nano-submicron.Reservoirs are classified into four types(Ⅰ-Ⅳ)based on flow unit.The Flow Zone Index (FZI) can effectively characterize the pore structure and flow characteristics of the reservoirs.Type Ⅱ and Ⅲ reservoirs are mainly developed in the second member of Xujiahe Formation in the study area,and type I reservoirs are developed in the ZB9-65-46 well field in the middle of the gas reservoir.The main structural reservoirs in the Zhongba Area are thick,predominantly type Ⅱ,while the reservoirs in the Laopingba and structural saddles are mostly thin layers,mainly type Ⅲ.Vertically,the effective reservoirs are primarily developed in the upper part of the second member of Xujiahe Formation,and its thickness accounts for 73.60% of the total reservoir thickness.The research results provide a scientific basis for reservoir evaluation and efficient development of similar tight sandstone gas reservoirs.

Volcanic Rock Identification Method Based on Machine Learning and Its Application

Zhu Bohan, Shan Xuanlong, Yi Jian, Shi Yunqian, Guo Jiannan, Liu Pengcheng, Wang Shuyang, Li Ang

2024, 31(5):  41-49.  DOI: 10.3969/j.issn.1006-6535.2024.05.005

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In the southern part of Songliao Basin, Chaganhua Area, the lithology of the Huoshiling Formation volcanic rocks is complex and variable. Traditional methods such as two-dimensional intersection and step-by-step classification based on conventional well logging data are difficult to accurately identify the lithology of volcanic rocks. To address the issues, a proposal is developed to use machine learning algorithms for intelligent identification of volcanic rock lithology. By observing sample cores and thin section analysis, the lithology of volcanic rocks in the sampled section is determined. The logging data set of the coring section is divided into training set and test set. The training set is used to match the object function, and the test set is brought into the model to predict results, and use integrate models with ensemble learning to conduct blind well prediction.The fusion model establishes a quantitative mathematical relationship between the characteristics of each well log curve, integrates the characteristics of multiple machine learning, and improves the learning efficiency of the model based on accurate lithology data set labels.The results show that the prediction accuracy of the integrate model for blind wells achieves 95.10%. The model has wide applicability, which can accurately identify and predict the lithology of volcanic rocks. This study can provide support for the intelligent exploration of volcanic rock oil and gas.

Depositional Microfacies and Sedimentary Patternsof Sandy Conglomeratein the Carboniferous Bachu Formation of the Tahe Oilfield

Wang Jia'nan, Zhao Weiwei, Diao Xindong, Huo Zhipeng, Gao Jianbo, Li Wenping, Sun Ningliang, Jiang Dong

2024, 31(5):  50-58.  DOI: 10.3969/j.issn.1006-6535.2024.05.006

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To address the issues of unclear sedimentary patterns of sandy conglomerate and insufficient research on the depositional microfacies in the Carboniferous Bachu Formation of Tahe Oilfield,the sedimentary patterns,depositionalmicrofacies type and distribution characteristics of the study area are studied by using lithology, drilling,and logging data.The results show that the study area mainly develops three types of rocks:conglomerate,sandstone and mudstone,with a high content of calcareous materials.There are 3 sedimentary facies,5 sedimentary subfacies and 13 sedimentary microfacies in the study area.The sedimentary microfacies vary rapidly in lateral,and multiple phase of the identical sedimentary microfacies are superimposed in vertical.The western part of the study area is delta sedimentary system with braided channels,which interbeds with the tidal flat on its side.A kekule by the Akkulak Uplift, the eastern part is closer to the source area.It develops a fan delta sedimentary system,which interbeds with the tidal flat in the south.The diamicton position of braided channels delta front and tidal flat shows good evidence of oil,gasand water,which is a favorable sedimentary facies belt.The main sedimentary pattern of the study area is the diamicton of braided channel delta front,fan delta front and tidal flat.The findings have guiding significance for the subsequent exploration and development of Tahe Oilfield.

A Prediction Method for Heterogeneous Reservoir Parameters Based on Transfer Learning

Gao Guohai, Wang Xin, Jiang Wei, Wang Yongsheng, Zhang Enli, Zhou Yan, Li Liang

2024, 31(5):  59-66.  DOI: 10.3969/j.issn.1006-6535.2024.05.007

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To address the issue of traditional methods neglecting flow mechanisms and parameter correlations,a prediction model of reservoir parametert hat integrates seepage theory with transfer learning is proposed.By using the oversampling algorithm of SMOTE,the issue of imbalanced samples is effectively addressed.The discriminative model of lithology andseepage capacity is established by using random forest,which provides the information of seepage mechanism for reservoir parameter predicting.Combined with parameter correlation,the technology of transfer learningis used to build a reservoir parameter prediction model.The results show that the correlation analysis between reservoir parameters can be conducted by introducing lithology and seepage capacity discrimination technology,which can effectively improve the prediction accuracy of reservoir parameters.The prediction error of the model in porosity and permeability parameters is 3.51% and 15.17%,respectively,and the prediction accuracy is significantly improved.This method can effectively address the issues of parameters prediction in heterogeneous reservoir, and provide reference for the research combining artificial intelligence technology with physical models.

The Relationship between Overpressure Systems and Hydrocarbon Accumulation in the Dongying Sag

Gao Zhiqiang

2024, 31(5):  67-76.  DOI: 10.3969/j.issn.1006-6535.2024.05.008

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The overpressure system plays a crucial role in the process of oil and gas migration and accumulation in Dongying Sag.Due to the lack of dynamic characterization of the overpressure system,this study investigates the pressure gradient characteristics and its control on oil and gas in a sequence of "point-line-surface".The research indicates that the areas with high and low gradient values overlap longitudinally.Planarly,the high gradient value in the lower part of the Fourth Member of Shahejie Formation is distributed in the northern deep sag and gradually diminishes from north to south.The high gradient value ranging from the upper part of the Fourth Member to the lower part of the Third Member of Shahejie Formation is distributed around each hydrocarbon generation sag.There exist five types of pressure gradient structures.The high pressure-low gradient zone is located at the center of the sag,followed outward by the high pressure-high gradient zone,the transition-high gradient zone,the transition-low gradient zone,and the normal pressure-low gradient zone.Source,reservoir and seal formations constitute the material basis,boundaries and structural types of the pressure gradient,with tectonic evolution controlling the vertical attenuation and spatial distribution of the pressure gradient.The oil and gas controlled by faults related to the pressure gradient are distributed in a ring-like pattern,and the related lithologic reservoirs are scattered.This study holds significant theoretical significance for rolling exploration and desert prediction in Dongying Sag.

Reservoir Engineering

A Productivity Prediction Method for Tight Gas Wells Based on Knowledge Graph and Random Forest Algorithm

Li Wenyi, Hou Mingyu, Quan Hang, Yu Jie

2024, 31(5):  77-84.  DOI: 10.3969/j.issn.1006-6535.2024.05.009

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The productivity prediction of gas well is influenced by various factors such as geology and engineering.Traditional methods like mathematical analysis and numerical simulation struggle to quickly and accurately predict the productivity of tight gas wells.To address this issue,an innovative method combining knowledge graph and the random forest algorithm is proposed based on big data and machine learning concepts to develop a productivity prediction method of tight gas wells.Data preprocessing standardizes different types of basic data,and entity recognition and linking technologies integrate entities from various data sources into the knowledge graph.Relationship extraction and modeling techniques are used to establish relationships and attributes among entities,developing a complete knowledge graph for accurate productivity prediction.On this basis,a productivity prediction model for tight gas wells is developed using the random forest machine learning algorithm,and the model predicts the productivity of tight gas wells in the Qiulin Block with an accuracy of 89.7%.This method allows for rapid and accurate productivity predictions in the early stages of development,significantly improving prediction accuracy and providing decision support for productivity deployment and high-yield well cultivation in tight gas development.

Applicability Analysis of Fractal Model with Poricidal Fracture in Coal Bed Based on Low-field Nuclear Magnetic Resonance Technology

Ren Haiying, Wen Shupeng, Hou Jianjun, Kong Lingfei, Zhou Zeni, Guo Zhijun

2024, 31(5):  85-94.  DOI: 10.3969/j.issn.1006-6535.2024.05.010

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To study the heterogeneity of pore and fracture distribution in coal bed and reveal the occurrence state and transmission characteristics of coalbed methane reservoirs, taking the high-order coal samples of the Permian Longtan Formation at 9 Blocks in the multi-coal seam development zone of western Guizhou as an example, the nuclear magnetic resonance (NMR) test of coal samples is performed using saturation-centrifugation test method to define the distribution of movable water and irreducible water, and the characteristics of pore fracture structure. Quantitative characterization of the heterogeneity of pore fracture distribution in coal samples is conducted through single tractal theroy and multi-fractal theory, and the correlation between different fractal dimension values and pore structure parameters is discussed. The results show that the single-fractal model can better characterize the heterogeneity of different fluid states or pore distribution range, while the multi-fractal model is more suitable for characterizing the heterogeneity of pore fracture distribution. In the single-fractal parameters, the fractal dimension value (D₂) of the seepage pore and the total fractal dimension value (D_T) show a weak negative correlation; in the multi-fractal parameters, with the increase of the pore volume of the adsorption pore, the spectral width (D_-10-D₁₀) increases linearly and has a slight variance. The samples can be divided into two categories, that is, adsorption pore development and seepage pore development. The type of adsorption pore generally develops fine pores,with a "bimodal pore size" distribution, while the type of seepage pore generally develops large pores with a "uni-modal or bimodal pore size" distribution. As the volume of the seepage pore decreases, the heterogeneity of the seepage pores and the overall distribution of pores increases. The adsorption porosity is the main factor affecting the distribution of pore and fracture, and it plays a significant controlling role in the multi-fractal characteristics of pore and fracture structure in coal reservoirs. There are certain differences in physical significance between the single-fractal model and the multi-fractal model, but both provide important theoretical support for the study of the pore fracture structure of coal reservoirs.

Prediction Method of Wellbore Pressure Distribution for Gas Wells with High Water/Gas Ratio

Wu Ruidong, Li Yuansheng, Ma Lian, Lu Ying, Shi Meixue, Liao Ruiquan, Cheng Yang

2024, 31(5):  95-101.  DOI: 10.3969/j.issn.1006-6535.2024.05.011

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Gas reservoirs in the East China Sea gas fields,which operated by water flooding,are commonly developed.Some gas wells produced along with water,the water/gas ratio of which are high.The traditional two-phase flow wellbore pressure calculation methods were employed to predict wellbore pressure distribution.However,it was found that the current models suffer from low prediction accuracy,which affects the diagnosis of liquid loading conditions in gas wells and the selection of drainage and production measures.To address this issue,gas-liquid two-phase flow patterns under different well diameters,inclinations,and water/gas ratios were analgzel and,a new flow pattern discrimination method suitable for high water/gas ratios was derived based on the discrimination model by corrected Kaya flow pattern.Additionally,the empirical coefficients related to the Mukherjee-Brill liquid holdup calculation formula were revised.Introducing dimensionless parameters characterizing the water/gas ratio and angular correction factors.On this basis,a new pressure drop model applicable to high water/gas ratio gas reservoirs was established.The research shows that the new liquid holdup calculation model achieves an average calculation error of less than 15% for liquid holdup under different pipe diameters and inclinations.The new wellbore pressure drop calculation method improves the prediction accuracy of wellbore pressure in water-injected gas wells in the East China Sea by over 50%,with an error not exceeding 8% compared to measured values.The new method has significant application value for predicting the pressure distribution of two-phase flow in high water/gas ratio gas wells and is of great guiding significance for optimizing operational systems and selecting drainage and gas production processes in the middle and later stages of water-drive gas reservoir development.

The Influence of Low Permeability Interlayers Distribution on the Seepage Law of Positive Rhythm Heterogeneous Reservoir

Zhang Xiangyu, Yu Tiantian, Li Aifen, Zhang Zhongping, Zheng Wangang, Chu Wei, Ma Aiqing, Feng Haishun

2024, 31(5):  102-109.  DOI: 10.3969/j.issn.1006-6535.2024.05.012

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Aiming at the problems occuring after water flooding,such as intensified reservoir heterogeneity,rapid increase of water content,complex distribution of remaining oil,and difficult to improve oil recovery,the study on the influence of low permeability interlayers distribution on the seepage law of positive rhythm heterogeneous reservoirs was conducted.The reservoir numerical simulation software CMG was adopted to simulate the water flooding process of the positive rhythm reservoir with interlayer.At the same time,the sand filling physical model of the positive rhythm heterogeneous reservoir was established based on the similarity criterion,and the water flooding experiment was carried out.Also,the accuracy of the numerical model is verified by the results of physical simulation experiments.On this basis,the influence of interlayer position on the seepage law of positive rhythm reservoir is analyzed.The research shows that the existence of interlayer increases the interlayer heterogeneity,plays the role of seepage barrier,and expands the swept volume.After water flooding,the remaining oil in the positive rhythm reservoir would display a distribution pattern featuring "more top and less bottom" under gravitation,which indicates that the area near the interlayer is the main region of remaining oil enrichment.This study is of great significance for further tapping the potential of remaining oil and sustainable development of oilfield.

Foaming Performance Evaluation of Hydroxysulfobetaine Surfactant under High Salt Conditions

Li Longjie, Ge Jijiang, Pan Yan, Chen Pengfei, Chu Pengju, Zhang Tianci

2024, 31(5):  110-118.  DOI: 10.3969/j.issn.1006-6535.2024.05.013

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In order to address the issue of gas channeling during the development of high-temperature and high-salinity oil reservoirs, which seriously affects the development effect, the long carbon chain hydroxysulfobetaine and the short carbon chain hydroxysulfobetaine were compounded according to the mass ratio of 2∶1 to construct a temperature-resistant and salt-resistant foaming agent MHSB12, and the gas mobility was controlled by the foam generated by this foaming agent. The performance of MHSB12 in two kinds of simulated formation water with salt content of 11.19×10⁴ and 22.38×10⁴ mg/L was comprehensively compared by conducting experiments such as determination of critical micelle mass fraction, interfacial rheology and static adsorption capacity, evaluation of bulk phase foam performance foaming capacity in porous media. The results show that under high salt conditions, MHSB12 has lower critical micelle mass fraction, higher surface expansion modulus, stronger foaming performance and foam stability. When the mass fraction of MHSB12 is 1%, the foaming agent solution meets the surfactant adsorption standard for enhanced oil recovery, which is suitable for deep clastic rocks in Tarim Oilfield and other similar reservoirs. Under the same mass fraction of MHSB12, the adsorption capacity of foaming agent solution with higher salt content is slightly lower. Compared with low salt conditions, the mobility control ability of MHSB12 in the core under high salt conditions is significantly weaker. Hence the evaluation of flow experiment should be paid attention to when screening foaming agents for field use. The study has practical guiding significance for screening foaming and evaluating foaming agents forenhancing oil/gas recovery.

Study on the Diffusion Law of Dimethyl Ether in Various Crude Oil Components Based on Molecular Dynamics Simulation

Wang Zhoujie, Li Songyan, Feng Shibo

2024, 31(5):  119-126.  DOI: 10.3969/j.issn.1006-6535.2024.05.014

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Dimethyl ether,an important solvent,shows promising potential in the technology of co-injection of solvent and steam to enhance heavy oil recovery.In order to address the issue of unclear diffusion behavior and distribution characteristics of dimethyl ether in heavy oil,a system model is established for C₆-C₃₀ oil phase and dimethyl ether with varying carbon num-bers.Molecular dynamics simulation is used to calculate the coordination number,diffusion co-efficient,density distribution,and interaction energy of dimethyl ether in the oil phase.The study aims to investigate the diffusion law of dimethyl ether in crude oil components.The results indicate that as the carbon number increases,the density distribution of alkanes and dimethyl ether molecules becomes uneven,resulting in a decrease in the number of alkanes surrounding dimethyl ether molecules.At 120 ℃ and 5 MPa,the diffusion coefficients of dimethyl ether in dimethyl ether-C₆ and dimethyl ether-C₃₀ systems are 2.19×10^-8 m²/s and 1.10×10^-8 m²/s,respectively.The diffusion coefficients of alkanes and dimethyl ether gradually decreased with an increase in carbon number,which can be attributed to the increase in molecular size and complexity of shape of alkanes.Under the influence of Van der Waals force,the interaction energy between alkanes and dimethyl ether initially increases and then decreases.As the carbon number decreases,the mass transfer rate at the oil-dimethyl ether contact increases,resulting in greater dissolution of dimethyl ether in the oil.This leads to an expansion in the volume of crude oil and a decrease in viscosity,resulting in a significant improvement in oilfield development.This study provides theoretical support for optimizing the application of dimethyl ether in the oilfield,and has important implications for future research in this area.

Drilling & Production Engineering

Study on the Casing Erosion Mechanism in Staged Fracturing of Horizontal Wells

Zou Linhao, Song Yang, Su Yinao, Li Wei, Zhao Huan, Gai Jingming, Li Zhuolun, Jiao Shengjie

2024, 31(5):  127-135.  DOI: 10.3969/j.issn.1006-6535.2024.05.015

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The staged fracturing of horizontal wells is an effective method for the development of shale oil.During the fracturing process,long-term operation under high pump pressure and high sand volume conditions leads to erosion of the casing holes,resulting in thinning of the casing wall thickness and uneven reaming at the hole.To address this issue, a numerical simulation study of liquid-solid two-phase flow for casing hole erosion is conducted using the CFD-DDPM method.In this study,the erosion mechanism of casing hole in the process of close-cut volume fracturing is analyzed,and the erosion law of perforation hole and its adjacent area under the influence of different perforation angles,sand content,inlet velocity and particle size is studied.The results show that with the increase of casing perforation angle,the maximum erosion rate at the hole opening exhibits a "tooth" shape variation;with the increase of sand content and inlet velocity,the maximum erosion rate increases gradually,and the erosion area on the inner wall of the pipeline also significantly increases;with the increase of particle size,the maximum erosion rate decreases first and then increases.The numerical simulation of liquid-solid two-phase flow for casing hole erosion based on CFD-DDPM method reveals the mechanism of hole erosion during staged fracturing,which has significant guiding for balanced fracture initiation and extending tool lifespan.

Three-Dimensional Simulation of Competitive Multi-Fracture Propagation in Shale Reservoirs with Consideration of Natural Fractures

Ling Xingjie, Chen Qi, Huang Zhiqiang

2024, 31(5):  136-145.  DOI: 10.3969/j.issn.1006-6535.2024.05.016

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Traditional simulations of shale fracture propagation are mostly limited to two-dimensional models,and do not consider the mechanism of multi-fracture synchronous competitive propagation.Construction schemes for fracturing are mostly based on statistical analysis and field experience,which cannot effectively screen out the influence parameters of strong sensitivity in complex fracture network fracturing effects.To address this issue,a numerical model of fractured geomechanics in three-dimensional space is developed using the actual reservoir of W204H well area in the Weiyuan Block as a case study.The model takes into account the synchronous competitive propagation of fractures in multi-cluster fracturing and analyzes the impact of in-situ stress on construction parameters.Furthermore,the model quantitatively studies the influence of in-situ stress difference,cementing strength of natural fractures,perforation parameters,and fracturing fluid discharge on the volume of complex fracture networks.The results indicate that a higher in-situ stress difference leads to a lower activation probability of natural fractures and a less complex fracture network.Additionally,a lower cementation strength of natural fractures results in easier activation and capture of hydraulic fractures.When the cementation strength is less than that of the rock matrix,hydraulic fractures will penetrate natural fractures and propagate along the fracture height.Furthermore,a higher perforation combination density results in a smaller perforation pressure drop and a weaker ability to alleviate inter-cluster stress interference.The hydraulic fracture is more likely to propagate along natural fractures with an increase in the injection rate of fracturing fluid.To ensure the uniform propagation of multiple clusters when controlling dense perforation clusters,it is recommended to use large discharge and high viscosity fracturing fluid.This study provides a theoretical basis for the optimal design of volume fracturing in fractured shale reservoirs.

Research on Perforation Impact Vibration Model and Vibration Suppression of Shock Absorber in Ultra-Deep Well

Liu Jun, Jian Yilin, Chen Yili, Zhou Xinzhong, Liang Shuang, Yuan Mingjian

2024, 31(5):  146-154.  DOI: 10.3969/j.issn.1006-6535.2024.05.017

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During the perforation operations in ultra-deep wells,the huge shock wave generated by the detonation of perforating charges can cause severe vibration of the pipe string,potentially leading to string damage.To address this issue,Hamilton's principle is used to develop a longitudinal-transversal-torsional coupling nonlinear dynamic model of the pipe string system,considering the influence of factors such as string-casing wall contact and high temperature and high pressure environment in ultra-deep wells.By integrating the detonation pressure field with the dynamic model,accurate predictions of string vibration under detonation state can be obtained.The research findings are used to study how explosive perforation affects the longitudinal-transversal-torsional coupling vibrations of the pipe string in ultra-deep wells under high temperature and high pressure conditions.The results show that appropriate arrangement of dampers below the packer (20~25 m) can effectively reduce the load on pipe string,suppress the vibration of the string,and prevent adverse collisions between the pipe string and casing and thus protect the dampers from damaging.When a single damper is insufficient for perforation conditions,a dual-damper arrangement can effectively suppress pipe string vibration,reduce the load to pipe string,and extend the service life of the pipe string tools.These findings are of significant importance for reducing the risks of perforation strings.

Development and Application of Equilibrium Dual Unseating Water Injection Packer

Xiao Guohua, Liu Yufei, Zhang Xin, Wang Yuanzheng, Zhang Jianzhong, Lu Ming, Wang Yao

2024, 31(5):  155-161.  DOI: 10.3969/j.issn.1006-6535.2024.05.018

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To address the issues of high unsealing load and short sealing validity period in the multi-stage injection string unsealing of highly deviated deep well in Jidong Oilfield,an equilibrium dual unseating water injection packer is developed through 3D simulation and optimization design of key structures based on the analysis of the packer structure.The packer is equipped with an independent dual unseating mode to minimize unseating load.It utilizes the spring and the pressure difference between the interior and exterior of the water injection string for automatic closing of the backwash valve.A combination of rubber extrusion and metal wire seating methods is employed to enhance the seating reliability of the backwash valve.High-strength fluorine rubber material has been selected to improve the temperature and pressure resistance of the rubber cylinder,while a equilibrium piston is designed to protect the unseating pin,eliminating the risk of premature unseating due to high pressure difference.The technology has been successfully deployed in 11 wells,achieving a flawless construction success rate of 100% and surpassing a seating validity period of 30 months.The unseating load of the lifting has been reduced by 17.2%.With an extended seating cycle and decreased unseating load,this packer effectively meets the long-term seating requirements for high-pressure separate injection in highly deviated deep wells,presenting promising economic benefits and application prospects.

Analysis of Factors Affecting the Fracture Conductivity in Deep Shale Gas Reservoirs of Southern Sichuan

Yang Yadong, Zou Longqing, Wang Yixuan, Zhu Jingyi, Li Xiaogang, Xiong Junya

2024, 31(5):  162-167.  DOI: 10.3969/j.issn.1006-6535.2024.05.019

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Deep shale reservoirs are characterized by high closure pressure,high formation temperature,and high elastic modulus,making it challenging to achieve high conductivity fractures through fracturing.To effectively support and maintain pressure fractures in deep shale gas reservoirs,taking the deep shale of the Weiyuan Block in Sichuan as an example,based on an self-developed testing platform of API fracture conductivity,this study investigated the impact of high closure pressure (82.8 MPa) and high formation temperature (160 ℃) on the conductivity of supported fractures.The differences in conductivity under different types and combinations of proppants are compared,and the adaptability of self-supported fractures under deep shale conditions is analyzed.The results indicates that the high closure pressure is the main controlling factor for the decrease of fracture conductivity.Under the high closure pressure of 82.8 MPa,the positive effect of increasing sand concentration on conductivity is no longer evident;the high-temperature environment increases the degree of proppant breakage and embedment,with higher temperatures resulting in lower conductivity;the fracture conductivity of 40/70 mesh ceramsite proppant is significantly greater than that of 70/140 mesh quartz sand and micro-nano proppants;the conductivity of quartz sand and ceramsite (mass ratio of 1∶1) placed in sections is more economical and has higher conductivity than the whole mixed placement;the convex points on the wall of self-supported fractures are easily crushed under high closure pressure,showing strong stress sensitivity in conductivity,which is not conducive to maintaining conductivity in the later stages of production.The results of this study provide theoretical support for the design of fracturing parameters and optimization of conductivity in deep shale gas reservoirs.

Research on Wellbore Collapse Instability Areas in Stratified Shale Oil Reservoirs

Zhang Haijun, Wang Lihui, Zhang Ligang, Jing Haiquan, Yang Yanyun, Qu Yonglin, Liu Zhaoyi, Liu Yueqiu

2024, 31(5):  168-174.  DOI: 10.3969/j.issn.1006-6535.2024.05.020

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During the drilling process of stratified shale oil reservoirs,if the wellbore stability is not well enough,it is prone to frequent incidents such as wellbore collapse and stuck pipe.A numerical model for the stress around the wellbore and a collapse zone prediction method in shale oil reservoir is developed to reveal the influence of bedding plane on wellbore collapse and enlargement of shale oil reservoir in accordance with MG-C strength criterion and multi-weak plane criterion.Also,the collapse and enlargement patterns for single and multiple weak planes under different stress mechanisms are calculated.The results show that the presence of multiple weak planes causes simultaneous damage to the matrix and weak planes,leading to more complex wellbore collapse patterns.While collapse expansion induced by weak planes is significantly higher than that of the matrix.Compared to the strike-slip stress mechanism,the collapse of wellbore under normal stress mechanisms shows a more significant change with stress ratio,and the collapse risk is higher.Under normal stress mechanisms,when the stress ratio exceeds 1.5,the wellbore expansion rate exceeds 100% and wellbore increases sharply.The research findings provide theoretical guidance for preventing wellbore collapse during drilling in shale oil reservoirs under different regional stress mechanisms.