Table of Content

25 October 2023, Volume 30 Issue 5

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Summary

Geologic Characteristics of Passive Continental Margin Basin on Both Sides of the South Atlantic Ocean and Its Impact on Exploration

Zhang Yi, Zheng Qiugen, Hu Qin, Chen Wenlin, He Shan

2023, 30(5):  1-10.  DOI: 10.3969/j.issn.1006-6535.2023.05.001

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Nearly 20 passive continental margin basins developed on both sides of the South Atlantic Ocean, and a number of major oil and gas discoveries obtained in deep-water areas, however, a lack of understanding of the geological characteristics of the passive continental margin basins resulted in the low level of exploration in deep-water areas, and the amount of oil and gas resources to be discovered was enormous. For this reason, on the basis of comprehensive analysis of a large amount of data and cases, and fully combining the study results and understanding in recent years, the key geological characteristics of the passive continental margin basins in the middle and southern part of both sides of the South Atlantic Ocean were systematically summarized, and the impact of the geological features on the exploration of oil and gas was deeply analyzed. The study shows that igneous rocks of 3 orogenic types and 3 active phases are widely developed throughout the rifting period in the basin complex on both sides of the South Atlantic Ocean; in the southern section, the thick, seaward-tilted reflective wedges are widely developed in the volcanic passive continental margin basin; in the middle section, the salt rock planar distribution and thickness change rule of the salt rock development type basin complex has both similarity and difference between the two banks, and the salt cementation of the subsalt sandstone reservoir may occur; dirty salts are developed in the Gabon Basin, Santos Basin, Campos Basin, and Lower Congo Basin; the Kwanza Basin in the middle section of West Africa has become the only saltstone-hard gypsum-carbonate interbedded sedimentary basin on both sides of the South Atlantic Ocean due to the cyclonic evaporation pattern, and the thinner inter-salt carbonate layer is both a hydrocarbon source rock and a reservoir. The above geologic characteristics have a great impact on the prediction of the subsalt seismic imaging, subsalt reservoirs, hydrocarbon source rocs and inter-salt carbonate rocks, and increase the multiplicity of solutions for seismic exploration. This study provide a basis for the precise positioning of the future technical attack direction of the passive continental margin basin.

Geologic Exploration

Comprehensive Evaluation Technology of Metamorphic Submarine Reservoir Effectiveness Mud-Logging in Bohai Sea Area

Tan Zhongjian, Deng Jinhui, Zhang Guoqiang, Li Hui, Li Hongru, Zhang Zhihu, Liu Zhiwei

2023, 30(5):  11-17.  DOI: 10.3969/j.issn.1006-6535.2023.05.002

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The lithology of metamorphic buried hill is complex and diverse, and the reservoir is highly non-homogenous. The conventional mud-logging method can only qualitatively judge the development location of the submarine reservoir based on oil and gas shows, and it is difficult to accurately identify the physical properties of the submarine reservoir. To address this problem, the light-colored mineral mud-logging characterization technology of metamorphic buried hills, the bedrock mechanics mud-logging characterization technology of metamorphic buried hills, and the drillability evaluation technology of metamorphic buried hills were innovatively established through starting from comprehensive geological experiments and microdrilling simulation experiments and applying the XRF elemental logging, the XRD whole-rock diffraction mud-logging and engineering mud-logging data to form comprehensive evaluation techniques and quantitative evaluation standards for mud-logging on the effectiveness of metamorphic buried hill reservoirs based on various perspectives such as rock mineralogy, rock mechanics and rock fragmentation. The research results have been applied to more than 50 wells in the metamorphic buried hills of the Bozhong Depression in the Bohai Sea area, and the average compliance rate of the reservoir effectiveness mud-logging interpretation is as high as 81.7% when compared with the conclusion of the logging interpretation. This technology plays an important role in the discovery and evaluation of reservoirs in the metamorphic buried hills of the Bozhong Depression in the Bohai Sea area, and provides a reliable basis for major exploration decisions such as midway testing and completion drilling, and has a good application prospect.

Architecture Model of Sandy Braided River Island in the Lower He8 Member of Sulige and Residual Gas Distribution Rule

Zhou Heng, Ma Shizhong, He Yu, Lu Zhiyuan

2023, 30(5):  18-27.  DOI: 10.3969/j.issn.1006-6535.2023.05.003

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The sandy braided channel bar is an important reservoir type for oil and gas development with complex internal inhomogeneity. To address the problems of poorly understood sedimentary characteristics, architecture model and residual gas distribution rule of the braided channel bar in the Lower He8 Member of the Sulige Gasfield, a study was conducted with the analytic hierarchy process as a guide, in combination with the analysis of architectural elements and the data from ancient outcrop, modern sedimentation, coring and dense well pattern logging. The results show that the braided river sand bodies have the characteristics of mutual superposition in the longitudinal direction and large contiguous distribution in the horizontal direction, and the variation of the accommodable space controls the contact relationship of the sand bodies, and four types of sand body vertical superposition patterns are proposed during the rise and fall of the datum; as the sand body is internally hierarchical, the different seepage barriers (interbeds) are formed during the deposition of the braided river, which are divided into five types, such as river bottom stagnant mud and gravel deposits, channel bar silt layer, side-deposited fine-grained deposits, abandoned river fill deposits and overbank fine-grained deposits, and the silt layer is the main factor controlling the oil and gas seepage inside the channel bar; according to the difference of water flow energy characteristics and deposition characteristics in different locations of the channel bar, the channel bar is divided into the architectural elements such as head, center, wing, tail and silt layer, etc. The tail is obviously more inhomogeneous than the head, and the upper part of the channel bar with more developed silt layer is more inhomogeneous than the lower part; the tail and upper parts of the channel bar are the residual gas enrichment areas. The results of the study are useful for the exploration and development of reservoirs in the study area and similar braided river depositional environment.

Application of Dynamic Velocity Iterative Technique to Steering While Drilling in Complex Tectonic Zones

Feng Bing, Wang Sheng, Li Gangquan, LanBaofeng, Wu Song, Li Shaopeng

2023, 30(5):  28-34.  DOI: 10.3969/j.issn.1006-6535.2023.05.004

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Since the shale gas development area at the edge of the Sichuan Basin is subject to superimposed transformation of multi-stage tectonics and tectonic complexity, it results in large changes in formation velocity, makes difficult to accurately build a velocity model with poor well-seismic matching, and affects the drilling catching rate of horizontal wells. To this end, we combined conventional 3D seismic well calibration, established a standard formation, analyzed the anisotropy of the formation, clarified the longitudinal and transverse variation characteristics of the formation velocity, corrected the burial depth coefficient and the occurrence, and took the longitudinal velocity and transverse velocity superposition calculation to establish the velocity field;dynamically update the velocity field by applying the formation information of the wells to ensure the accuracy of the drill-following models,improving the accuracy of seismic prediction target depth and the judgement of the occurrence variation pattern. In the practical application of this method, the error between target prediction and actual drilling is reduced to 7.7 m, which is much higher than the seismic resolution, which proves the rationality and adaptability of this technical method, and can be widely used in geosteering of horizontal wells.

Characteristics of Well Logging Response and Evaluation of Favorable Zones in Tight Sandstone Reservoirs

Zhao Hui, Qi Huaiyan, Wang Kai, Cai Tao, Meng Lihua, Zhou Bing

2023, 30(5):  35-41.  DOI: 10.3969/j.issn.1006-6535.2023.05.005

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To address the difficult problem of complex and variable well logging response characteristics of sandstone reservoirs in Well K39 Area of Jiyuan Oilfield and the difficulty of identifying effective reservoirs, the well logging principles and methods were applied to analyze well logging interpretation models of effective reservoir parameters, clarify well logging phase patterns in the study area, innovate and carry out cluster analysis of well logging response characteristics parameters such as porosity, oil content saturation and permeability, and establish four types of flow unit identification criteria for dense sandstone reservoirs. The study results show that in Well K39 area, the established well logging model is used to reinterpret 8 oil layers, totaling 91 m, 13 oil-water layers, totaling 108 m, and 7 water with oil layers, totaling 77 m, releasing a total of 23.8×10⁴t of reserves, which is effective in practical application, and evaluating the favorable Class I and II flowing unit reserves of Jiyuan Oilfield totaling 202.5×10⁴t. The research results enrich the study of effective reservoir identification and favorable zone preference in Ordos Basin, and have reference value for development adjustment and well deployment of tight reservoir in Ordos Basin.

Reservoir Space and Physical Characteristics of Shale Oil with Different Lithologies

Zhou Zhijun, Zhang Guoqing, Cui Chunxue, Wang Jingyi, Wang Juan

2023, 30(5):  42-49.  DOI: 10.3969/j.issn.1006-6535.2023.05.006

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In view of the lack of deep understanding of the reservoir characteristics of the Shahejie Formation in the Jiyang Depression, it affects the exploration and development of shale oil. Taking key coring wells of the lower sub-section of the Member 3 and the upper sub-section of the Member 4 of the Paleogene Shahejie Formation of in the Jiyang Depression as the study object, X-Ray Diffraction was used to determine the mineral composition of shale reservoirs, and the shale was classified into mudstone Shale, limestone and dolomite, the mudstone in the lower sub-section of the Member 3 of the Shahejie Formation is relatively developed, and dolomite in the upper sub-section of the Member 4 of the Shahejie Formation is relatively developed; the reservoir space and pore size of shale with different lithologies were quantitatively described with the scanning electron microscopy, and the predominant lithology of shale reservoirs was analyzed. The results of the study show that the medium to large pores were developed in dolomite, and the oil are mainly occurred in the dolomite intergranular micropores and dissolved pores; the medium to small pores were developed in mudstone, and the oil were mainly filled in the intergranular micropores and intergranular micro-fractures; the small pores were developed in limestone, and the oil were mainly occurred in the intergranular micropores and locally in the dissolved pores; the dominant lithologies of shale reservoirs were, in order of preference: dolomite, mudstone and limestone; the oil content and oil test data of different lithologies showed that the upper sub-section of the Member 4 of Shahejie Formation in the Jiyang Depression is a high-production oil section, and it can be determined as the “sweet spot”reservoir of the Shahejie Formation in the Jiyang Depression. The study results have guidance significance to the exploration and development of shale oil in Jiyang Depression.

Reservoir Characteristics and Main Controlling Factors of Tight Sandstone in Member 2 of the Xujiahe Formation in Northwest Sichuan

Lei Yue, Huang Qian, Wang Xuli, Yang Tao, Tian Yunying, Li Honglin, Tang Xiao, Liu Bai

2023, 30(5):  50-57.  DOI: 10.3969/j.issn.1006-6535.2023.05.007

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In order to clarify the distribution characteristics of tight reservoirs in the Upper Triassic Xujiahe Formation in the northern part of the Western Sichuan Depression, we obtained reservoir sedimentary microfacies types and mineral composition characteristics by core description, logging interpretation and X-ray diffraction mineral analysis, combined with the diagenesis obtained from the rock thin section and field emission scanning electron microscopy observation, pore permeability and mercury injection test and the analysis results of the reservoir pore structure, and carried out a study on the factors influencing the tightness of reservoirs in the Member 2 of the XujiaheFormation.A comprehensive classification evaluation standard was established for the reservoirs Member 2 of the Xujiahe Formation in Northwest Sichuan. The result shows that the main reason for the development of tight reservoirs in the Member 2 of the Xujiahe Formation is that the reservoir porosity is controlled by lithology, and the differences in mineral components make the sandstone in the Member 2 of the Xujiahe Formation characterized by strong cementation, moderate compaction and weak fracture. The study results provide a basis for the screening of favorable sites for the development of tight reservoirs in this area and the favorable target areas for further exploration of tight gas reservoirs.

Microscopic Pore Structure of Shale Reservoir in Member 1 of Qingshankou Formation, Daan Area of southern Songliao Basin

Zhang Yue, Tang Jiguang, Hu Meiling, Qin Dechao, Feng Tao

2023, 30(5):  58-66.  DOI: 10.3969/j.issn.1006-6535.2023.05.008

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The microscopic pore structure characteristics of reservoirs are important factors in evaluating the reservoir performance, resource abundance and "sweet spot" of shale reservoirs. To address the problem of unclear microscopic pore structure characteristics of the shale in Member 1 of Qingshankou Formation, Daan Area of southern Songliao Basin, argon ion polishing and scanning electron microscope, high-pressure mercury injection, constant-speed mercury injection,CT scanning and nuclear magnetic resonance were used to jointly characterize the reservoir space type, pore throat radius distribution, connectivity and fluid mobility of the Member 1 of Qingshankou Formation. The results of the study show that the reservoir in Member 1 of Qingshankou Formation, Daan Area belongs to clay shale facies and mixed shale facies, with an average porosity of 5.95% and an average permeability of 0.041 6 mD, and belongs to extra-low pore ultra-low permeability, ultra-low pore ultra-low permeability tight and ultra-tight reservoir space. Eight types of reservoir space types are divided.Micro-fractures and throats connect the reservoir space, improving the reservoir physical properties, and the percentage of mobile fluid in the reservoir ranges from 27% to 47%. Combined with the oil-bearing characteristics of the reservoir, it is predicted that the area of favorable reservoir of Class I in the study area is about 300 km², and that of Class Ⅱ is about 650 km², which has good prospect of exploration and development. The study results can provide a more accurate scheme for microscopic pore structure evaluation of shale reservoirs in the study area.

Comprehensive Evaluation Method of Low-resistivity Reservoirs in Gravelly Sandstone with Complex Pore Structure in Beibuwan Basin

Wu Jian, Zhang Hengrong, Hu Xiangyang, Liu Tuliang, Yang Dong

2023, 30(5):  67-76.  DOI: 10.3969/j.issn.1006-6535.2023.05.009

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Within the Beibuwan Basin in the western part of the South China Sea, there are a large number of low-resistivity reservoirs in gravelly sandstones, with complex pore structure and fluid distribution, and the water saturation of the formation calculated by conventional methods is on the high side, which makes it difficult to distinguish between irreducible water, movable water and movable oil, and makes it difficult to discriminate between the fluid properties of the formation and to evaluate the oil saturation of the reservoir. In view of this, the cause of low resistivity in gravelly sandstone reservoirs was analyzed from internal and external factors based on core assay data such as mercuric pressure and thin section, and combined with logging and mud-logging characteristics. On the basis of the reservoir classification, the petroelectric parameters of the low-resistivity and high-resistivity reservoirs were finely etched to obtain the evaluation parameters applicable to different microporous structures. At the same time, the variable T₂ cutoff value processing was performed based on the "morphology method" and in conjunction with nuclear magnetic resonance (NMR) logging. Compared with the traditional fixed T₂ cutoff value, the obtained results can effectively evaluate the content of irreducible and movable fluids, which significantly improves the accuracy of formation fluid analysis and the compliance rate of logging interpretation of low-resistivity reservoirs, and then forms a set of comprehensive fluid identification and reservoir evaluation method for low-resistivity reservoirs with complex pore structure in gravelly sandstone. By applying this evaluation method, a large number of low-resistivity reservoirs were found in the old wells, which confirms that this method is highly practical and operable. This study is of great significance to the fluid identification and evaluation of the low-permeability and low-resistivity reservoirs with complex pore structures.

Reservoir Engineering

Study on the Production and Injection Performance of Clustered Boreholes in Horizontal Multilateral Wells in Tight Reservoirs

Yi Xianzhong, Qiao Shaofeng, Liu Hangming, He Dongxu, Yao Xiutian, Chen Hui

2023, 30(5):  77-83.  DOI: 10.3969/j.issn.1006-6535.2023.05.010

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The multi-stage and multi-cluster fracturing of horizontal wells is mostly used for the development of the tight oil, the fracture shape and extension are difficult to be precisely controlled, and the fracturing fluid contamination seriously jeopardizes the surrounding water bodies. To address the above problems and to utilize tight oil reservoirs more safely and efficiently, the clustered borehole type horizontal multilateral well pattern was proposed for oil production, and an oil drainage model based on the oil-water phase seepage distribution mechanism was established by simulation software. Through the capacity prediction, the fishbone-shaped horizontal multilateral well with optimal branching angle and location arrangement was selected, and this well pattern was applied to the clustered borehole type horizontal multilateral well and its production and injection performance was studied. The study shows that the clustered borehole well pattern has wider oil drainage area and faster water breakthrough. The clustered borehole well pattern has excellent production and injection performance, shorter construction period, higher safety and efficiency. After 10 a of depletion production under the same conditions, the clustered borehole well pattern increases production by about 11.12% compared with the fishbone-shaped horizontal multilateral well; the average daily oil production of the LD-1 platform Chang 71 well group increases by 13.21% after 4 a of simulated production using the clustered borehole well pattern; neighboring wells were extracted under simulated injection and extraction conditions for 20 a, the clustered borehole well pattern is used as the injection well, and the cumulative oil production of a single well increases by about 23.7% compared with that of the fishbone-shaped horizontal multilateral well. The study can provide technical references for the structural optimization of the clustered borehole well pattern and the efficient exploitation of tight reservoirs.

Dynamic Characteristics and Numerical Simulation Research of Waterflood-induced Fractures

Cheng Linsong, Yang Chenxu, Cao Renyi, Jia Pin, Pu Baobiao, Shi Junjie

2023, 30(5):  84-90.  DOI: 10.3969/j.issn.1006-6535.2023.05.011

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Traditional reservoir numerical simulation software just only consider the static fractures but cannot simulate the impact of fracture expansion on well network development during long-term water injection, and the simulation results are poorly fitted to the actual production dynamic data. To address this problem, a characterization method for complex dynamic fracture networks is developed based on the embedded discrete fracture model (EDFM) with dynamic improvement of pre-processing algorithms. The study shows that an efficient waterflood-induced fracture simulation method is established by coupling cross-fracture discrimination criteria, which can efficiently simulate the dynamic changes of fractures during reservoir injection and development and reflect the influence of natural fractures on the main fracture propagation path; the results are consistent compared with Abaqus software, while the computational efficiency is higher. The application example shows that the method has good adaptability for numerical simulation of waterflood-induced fractures, can accurately achieve the fitting of the water cut rising rules of the field water injection well cluster, which and has good reference significance for production dynamic analysis of ultra-low permeability reservoirs and later well pattern adjustment.

Quantitative Characterization of the Influence of Volcanic Rocks on the Physical Properties of Sandstone Reservoirs and Its Application

Ma Kuiqian, Li Chao, Yuan Jianwei, Yang Ming, Zhang Bo

2023, 30(5):  91-97.  DOI: 10.3969/j.issn.1006-6535.2023.05.012

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To address the problem that it is difficult to quantitatively characterize the influence range of volcanic rocks on the physical properties of sandstone reservoirs, the three typical contact relationships between volcanic rocks and sandstone reservoirs were summarized using dynamic and static data from the Bohai X Oilfield. A multi-layered commingled production composite reservoir model was established using the theoretical method of well testing, the "homogeneous + composite reservoir" model was used as an example to analyze the influence of the boundary radius of homogeneous layer, the mobility ratio of inner and outer zone of composite layer, the storage capacity ratio of inner and outer zone of composite layer, and the radius of inner zone of composite layer on the pressure dynamic curve, and the model was applied to the well testing interpretation of Well A1 in Bohai X Oilfield. The study results show that the pressure dynamic curve can be divided into five stages according to the pressure derivative curve morphology, which are wellbore storage stage, early transition flow stage, transition flow stage from the inner zone to the outer zone of the composite layer, mixed flow stage between the outer zone of the composite layer and the homogeneous layer, and multi-layer boundary response stage. The high fitting accuracy of Well A1 test well further verified the reliability of the model. The physical properties of the sandstone reservoir deteriorated within about 300.0 m from the volcanic rocks. Combined with the range of volcanic rocks portrayed by the static data, the network deployment of the 2 adjustment wells in the Bohai X Oilfield was guided, and the specific productivity index after production was 4.0 to 6.0 times of that of Well A1, with remarkable effect. The research results have some implications for well testing interpretation, well network deployment and efficient development of sandstone oilfields in volcanic rock development area.

Numerical Simulation Method of Large-Scale Discrete Fracture Dimensionality Reduction Based on Control Unit

Zhang Yaxiong, Lyu Xinrui

2023, 30(5):  98-104.  DOI: 10.3969/j.issn.1006-6535.2023.05.013

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To address the problem that the strong inhomogeneity of large fracture reservoirs makes it difficult to depict the special hydraulic conductivity with conventional continuous media models, a discrete fracture network model was used for numerical simulation, in which large fractures are subjected to a dimensionality reduction to be explicitly characterized in the grid system, unstructured grids were used for the reservoir geometrical sectioning, the fracture was taken as a virtual boundary and discretized into a dimensionality reduction control unit, and the finite-difference numerical computation format of the flow mathematical model was constructed by calculating the conduction coefficients of bedrock-bedrock, bedrock-fracture, and fracture-fracture respectively, and taking into account the dimensional differences of the neighboring control units. The method was applied to numerical simulation of fractured reservoir in Well Group T-6 in a certain western oil field. The results show that the production history fit compliance rate of the well group is more than 80%, the type of residual oil is mainly the shielded residual oil in fracture channel and the unused residual oil between wells, and the proportion of residual oil between wells and below the production interval accounts for more than 60%, which is more in line with the actual understanding. The study results have good applicability to the numerical simulation and residual oil distribution study of reservoirs containing large-scale fractures, and the dimensionality reduction treatment of large fractures can significantly improve the computational efficiency.

Experiment on the Mechanism of Enhanced Recovery by Branched Pre-Crosslinked Gel Particles

Meng Qingchun, He Gang, Guo Fajun, Li Huabin, Wang Li

2023, 30(5):  105-112.  DOI: 10.3969/j.issn.1006-6535.2023.05.014

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In order to solve the problem of decreasing viscosity and oil displacement effect of the oil displacement system due to polymer aging of the chemical displacement in a certain block in Dongying, Hejian, Huabei Oilfield, the microscopic oil displacement mechanism of branched pre-crosslinked gel particles (B-PPGs) as well as the oil displacement effect of parallel cores were clarified based on a variety of experimental means such as viscosity and viscoelasticity tests, particle size distribution measurements, nuclear magnetic resonance (NMR) tests, and parallel core displacement. The results show that the branched structure of B-PPG molecules has better water solubility, which can improve the apparent viscosity of the solution; the B-PPG reticulated molecular structure enters into the formation in a crushed or deformed manner, which improves the mobility ratio and increases the sweep efficiency of the non-homogeneous reservoirs; when the permeability gradient of the main reservoir is 7.4, the recovery enhancement of the B-PPG system with a mass concentration of 2000 mg/L is 20% higher than that of the water displacement. The study shows that: The B-PPG system has good viscoelasticity, strong deformation ability, and ideal salt resistance, which can form an effective seal in the highly permeable layer and substantially increase the recovery rate. This study provides a reference for improving the profile of high-temperature and high-salt reservoirs, sealing the preferential channels of high permeability, and substantially increasing the reservoir sweep efficiency and recovery rate.

Nuclear Magnetic Resonance Experiment for Enhanced Recovery of Adsorbed Methane from Shale through Carbon Dioxide Injection

Zhang Tianjin, Wang Yanfeng, Li Jun, Yuan Qing

2023, 30(5):  113-120.  DOI: 10.3969/j.issn.1006-6535.2023.05.015

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It is very important to clarify the adsorption-desorption law of shale-adsorbed CH₄ under the action of CO₂ to improve the recovery of shale gas. On the basis of clarifying the pore structure of the target shale, the nuclear magnetic resonance (NMR) testing technique was introduced, the CH₄ injection and CO₂ injection adsorption-desorption experiments were carried out separately to quantitatively characterize the absolute adsorption amount of CH₄, and the influence law of CO₂ on the adsorption-desorption of CH₄ in shale was investigated. The results show that CH₄ in shale occurs in three states: adsorbed state on the shale surface, unbound state in pores and free state between particles; the absolute adsorption amount of CH₄ calculated by NMR is greater than the excess adsorption amount determined by thermogravimetry; CO₂ can desorb the adsorbed CH₄ with molar fraction of 21.8%~33.2%; the adsorbed CH₄ will remain in the pores as unbound state after desorption, but cannot escape from the pore space as free state; while CO₂ is injected to improve the recovery of adsorbed gas, the secondary hydraulic fracturing or CO₂ dry fracturing technology shall be adopted to improve the conversion efficiency of unbound state CH₄ to free state. The research results can provide reference for improving the recovery of shale-adsorbed CH₄.

Formulation Optimization of Oxygen-Reduced Air Flooding Foam System for High Temperature and High Salinity Reservoir in Gasi Block

Cheng Tao, Zhang Yi, Song Yingzhi, Dang Yangbin, Jia Zhiwei, Zhu Xiuyu, Pu Lantian, Shao Liming

2023, 30(5):  121-126.  DOI: 10.3969/j.issn.1006-6535.2023.05.016

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Gasi E₃¹ reservoir in Qinghai Oilfield is a typical high temperature and high salinity reservoir, and after decades of development, it faces development challenges such as rapid rise in water content, rapid decrease in production, and difficulty in stabilizing production. The conversion of the development method to oxygen-reduced air foam flooding has become an important technical direction for further recovery enhancement in this reservoir, but the conventional foaming system has the problem of poor temperature and salinity resistance, which is not applicable in Gasi E₃¹ reservoir. Based on the FC foaming agent (the main components are cetyl hydroxypropyl sulfobetaine and sodium dodecyl sulfate), a new foam stabilizer is synthesized by polystyrene modification of nano-SiO₂ particles, and the two are compounded to form FC-W temperature-resistant and salinity-resistant foam system. The static evaluation experiment shows that the comprehensive value of the system at 160 ℃ is 90.9% of that at 25 ℃, and the comprehensive value of the system at 15×10⁴mg/L mineralization is 94.8% of that at distilled water, and the temperature resistance and salinity resistance are significantly higher than those of the FC foam system. The dynamic simulation shows that the air flooding sweep volume can be expanded by 9.4% when the air flooding is implemented after the foam plug. If the water flooding is continued after the foam plug, the water flooding enhanced recovery rate can continue to increase by 20 percent points, and the ability of the system to expand the sweep volume and improve the displacement efficiency is outstanding. The research results show that the new temperature-resistant and salinity-resistant foam system has good adaptability to the oxygen-reduced air foam flooding applied in this reservoir.

New Method for Evaluating the Volume Fracturing Fracture Network Sweep Volume in Shale Oil Horizontal Wells and Its Application

Zhang Kuangsheng, Xue Xiaojia, Tao Liang, Xiong Zuowei, Chen Wenbin, Wu An'an, Yan Guang

2023, 30(5):  127-134.  DOI: 10.3969/j.issn.1006-6535.2023.05.017

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In response to the difficulty of quantitative characterization of the fracture network sweep volume of sandwich-type shale oil volume fracturing in Qingcheng Oilfield, Ordos Basin, a fracture network stimulation volume prediction model is established by applying physical model experiments and mine field data and coupling key geological engineering parameters, and a multi-scale pore percolation model considering fluid pressure is established to quantitatively characterize the fracture network percolation volume based on rock microstructure and wettability, and a new concept of fracture network sweep efficiency is proposed to quantitatively evaluate the fracture network sweep volume. The study shows that the shale oil volume fractures generally show a strip-like fracture network with the main fractures dominating and branch fractures supplementing, and the shape is similar to a "cactus"; the main controlling factors affecting the sweep volume of the fracture network are fracturing fluid volume, fracture density, brittleness index, pumping rate, horizontal stress difference, reservoir thickness, and sand addition; the stimulation volume of fracture network is the main contributor to the fracture network sweep volume of the horizontal well volume fracturing. The percolation volume of fracturing fluid in the fracture network is mainly used to replenish the energy of the formation, the fracture network sweep efficiency of the typical platform is 33.2% to 63.6%, and the fracture network sweep volume needs to be further improved. The research results can provide a reference for the evaluation of the volume fracturing effect of shale oil horizontal wells.

Dynamic and Static Integrated Classification Model for Low Permeability Tight Gas Wells Based on XGBoost Algorithm

Shang Yongtao, Zhai Shuo, Lin Xinyu, Li Xiangliang, Li Hui, Feng Qing

2023, 30(5):  135-143.  DOI: 10.3969/j.issn.1006-6535.2023.05.018

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The Zimi Gas Field is a typical low-permeability tight gas field with great differences in reservoir physical properties and production characteristics among different gas well reservoirs, so the development strategy is in urgent need of improvement. To address this problem, a classification method for tight gas wells based on XGBoost algorithm is proposed. The input features of the model are determined by calculating Pearson correlation coefficients of the feature parameters and judging the correlation degree of 4 dynamic and 5 static feature parameters with gas well productivity. Then, based on the XGBoost algorithm, the model training is completed and gas wells are classified in Zimi Gas Field through parameter optimization. The study shows that the main factors affecting gas well classification are production allocation, original formation pressure, effective thickness, porosity and open flow capacity; the gas well productivity of Category 1 and 2 in Zimi Gas Field is mainly affected by reservoir thickness and matrix permeability, while the main factors affecting the gas well productivity of Category 3 are effective thickness and gas saturation; the accuracy of the model classification results is 92.3% compared with the expert classification results. This study improves the effectiveness of gas well classification, reduces human subjectivity, and the classification results are in line with the actual mine field, which has a certain guidance for gas well classification management and development strategy formulation.

Drilling & Production Engineering

Experimental on Mechanical Properties Characteristics of Oil Shale in Jiyang Depression

Wang Aimin, Wang Faming, He Wenqing, Wang Shuai, Liu Houbin

2023, 30(5):  144-150.  DOI: 10.3969/j.issn.1006-6535.2023.05.019

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The exploration and development of shale oil in Jiyang Depression had achieved remarkable results, but the wall collapse of highly deviated horizontal wells was extremely prominent, which has repeatedly caused downhole jamming and even drilling accidents. In response to the problems above, the mechanical properties of oil shale and collapse damage law were studied through a series of indoor experiments on rock mechanics, and the mechanism and main control factors of oil shale horizontal wellbore destabilization were analyzed. The study shows that the oil shale rock fractures are extremely developed, mainly of the subparallel low-angle bedding fractures, accompanied by high-angle cross fractures, and the rock fragmentation is high; the mechanical parameters of oil shale have obvious anisotropy characteristics.When the maximum principal stress of the wellbore is perpendicular to the bedding fractures, the overall mechanical strength of the wellbore is the highest, and as the angle between the maximum principal stress and the bedding fractures decreases, the mechanical strength of the wellbore tends to decrease, and when the angle is about 60 °, the mechanical strength is the lowest, and the wellbore stability is the worst.The overall mechanical strength of the oil shale in Jiyang Depression is low, and the maximum compressive strength under downhole stress is 165 MPa, and the lowest is only 52 MPa; there is a difference in the damage mode of the oil shale rock, when the angle between the maximum principal stress and the bedding fracture surface in the wellbore is small, the main damage is the shear slip damage along the bedding fracture, otherwise, the main damage is the oil shale bedrock shear damage. The study results of the destabilization mechanism of oil shale wellbore in Jiyang Depression can provide support for the prediction of wellbore stability and the optimal design of engineering parameters for the highly deviated horizontal wells in oil shale.

Causes of Corrosion and Anticorrosion Strategies of Pipe String in Coalbed Methane Wells at the Southern Margin of Junggar Basin

Wang Junbo, Tian Jijun, Li Fei, Zhang Xuanming, Ji Dongliang, Wang Xianmei, Li Xin

2023, 30(5):  151-157.  DOI: 10.3969/j.issn.1006-6535.2023.05.020

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The corrosion of pipe string in coalbed methane (CBM) wells of low-rank coal genesis in HD block at the southern margin of Junggar Basin is common and serious, which restricts the normal production of CBM wells. Unlike the corrosion of pipe string in CBM wells of middle rand-and high-rank coal genesis, the causative factors of pipe string corrosion in HD block are complex and diverse, which are not only related to the characteristics of produced fluids, but also closely related to the properties of coal rocks in the block, the regional geologic conditions, and the borehole structure of drilled and completed wells. Based on the corrosion characteristics of the pipe string in HD block, combined with the analysis data of produced fluid/gas and corrosive components, the mechanism of pipe string corrosion and the strategy of pipe string corrosion prevention were analyzed and researched. The study shows that the corrosion type of pipe string in HD block wells is mainly the electrochemical corrosion, supplemented by stress corrosion, charged pulverized coal corrosion, potential CO₂ corrosion, etc., and the same kind of corrosion has a potential pattern of concentration by zone or by subzone. According to the corrosion characteristics of gas well pipe strings in HD block, a strategy for preventing and controlling the corrosion of CBM well pipe strings is proposed, and preliminary results have been achieved after application. The results of the study can provide theoretical and technical guidance for the analysis of the corrosion mechanism and the formulation of corrosion prevention strategies for pipe strings in CBM wells of low-rank coal genesis.

Drilling & Production Engineering

Effect of Directional Double-Tooth Structure on Rock-Breaking Efficiency and Lateral Force of PDC Drill Bit

Gai Jingming, Li Wei, Liu Gangjun, Zheng Ruiqiang, Li Bing, Chen Zihe

2023, 30(5):  158-165.  DOI: 10.3969/j.issn.1006-6535.2023.05.021

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To address the problem of premature failure of PDC drill bit due to low rock-breaking efficiency and lateral vibrations and swirls of drill string caused by lateral force surge when drilling in soft and hard interbedded layer,a PDC drill bit with directional double-tooth structure was designed,and the effects of the directional double-tooth structure with different tooth arrangement parameters on rock-breaking efficiency and local lateral force balance were investigated by the finite element simulation of double-tooth cutting. The results show that the directional double-tooth structure can effectively improve the rock-breaking efficiency,the double-tooth spacing has less effect on the mechanical specific power,compared with the single-tooth cutting,the mechanical specific power is reduced by about 5.60%-6.50%; the double-tooth clamping angle can significantly affect the mechanical specific power,when the double-tooth clamping angle is 160 °,compared with the single-tooth cutting,the mechanical specific power is reduced by about 96.36%-113.82%; under the same side turning angle,the lateral force of the double-tooth clamping angle tooth arrangement is reduced by about 33.40%-74.20% compared with that of the single-tooth cutting,which can effectively improve the local lateral force balance of the drill bit in the transitional formation and directional drilling,and thus improve the overall force balance effect of the drill bit. This study can provide a reference and basis for the optimized design of the tooth arrangement of the PDC drill bit.

External Load Analysis of Hydraulic Fracturing Casing in Zhaotong Shale Gas Horizontal Well of Sichuan Basin

Meng Hu, Shen Yinghao, Zhu Wanyu, Li Xiaojun, Lei Derong, Ge Hongkui

2023, 30(5):  166-174.  DOI: 10.3969/j.issn.1006-6535.2023.05.022

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In view of casing damage during hydraulic fracturing in Zhaotong shale gas demonstration area, a numerical model of casing-cement sheath-formation combination was established based on casing deformation characteristics and theoretical analysis to study the mechanism of increasing casing external load by temperature effect, fracture network-induced stress, microfracture, formation slip and cementing quality. The study shows that the hydraulic fracturing leads to non-uniform change of the stress field in the Earths crust in the fracture reconstruction area, and greatly increases the stress field in the Earths crust and the difference between the maximum horizontal and vertical crustal stress in the heel and middle areas of horizontal wells; the induced stresses of fracture network, temperature effect, microfracture, formation slip and cementing quality defects will increase the external load of casing, among which the induced stresses generated by hydraulic fracturing has the most significant effect on increasing the external load of the casing, which may be the main reason for the high probability of damage to the casing in the heel and middle areas of horizontal wells. This may be the main reason for the high probability of casing damage at the heel end and central area of horizontal wells. Due to the high construction pressure and high stress difference, the weak surfaces such as stratigraphy and natural fractures in Zhaotong area are prone to shear slip in the process of hydraulic fracturing, and the casing deformation and force are the largest when the angle of the weak surface and the wellbore is 45°. The study results provide guidance for casing strength design and casing deformation prevention in horizontal wells of shale gas fields.