Table of Content

25 February 2021, Volume 28 Issue 1

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Summary

Research Status and Prospect of Natural Gas Hydrate Petroleum System

Tan Furong, Geng Qingming, Liu Shiming, Du Fangpeng, Liu Zhiwu

2021, 28(1):  1-9.  DOI: 10.3969/j.issn.1006-6535.2021.01.001

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The theory of the natural gas hydrate petroleum system is applied to discusses the progress of research on the complex dynamic physicochemical process of natural gas formation and decomposition,the research progress,problems and development trend of the natural gas hydrate petroleum system are analyzed,and the following conclusions are obtained:most natural gas hydrate gas sources are closely related to the biodegradation; the main factors affecting the temperature-pressure critical curve of the natural gas hydrate are the components of natural gas and pore water,while the ground temperature gradient and permafrost thickness do not change the gas hydrate temperature-pressure critical curve; the static elements and dynamic processes at critical moments of the gas hydrate petroleum system and the coupling relationship between them play an important role in the gas hydrate petroleum system.The study of the gas hydrate petroleum system not only provides a theoretical basis for the process of gas hydrate accumulation,but also provides technical support for the exploration and development of gas hydrate.

Geologic Exploration

Quality Difference Mechanism of Tight Sandstone Reservoir Controlled by Lithofacies in Sulige Gasfield

Li Jinbu, Liu Zihao, Xu Zhenhua, Li Ya, Wang Yan

2021, 28(1):  10-17.  DOI: 10.3969/j.issn.1006-6535.2021.01.002

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In view of the extremely strong heterogeneity, complex pore structure, significant productivity difference between wells and unclear reservoir quality difference mechanism of tight sandstone reservoir, the quality difference mechanism of tight sandstone reservoir controlled by lithofacies was discussed according to comprehensive core and logging data by taking the tight sandstone reservoir in the Member 8 of the lower Shihezi Formation in southern Sulige Gasfield. It is found in the study that the results show that the reservoir quality was controlled by lithofacies by affecting the diagenetic evolution sequence, and the reservoir quality of the fine conglomerate - pebbled coarse sandstone facies was the best, followed by that of the coarse sandstone facies, with a permeability greater than 1.00 mD, and mainly developed with intergranular pores - necked throats;according to the content of lithofacies and plastic debris, the diagenetic evolution sequence of reservoirs was classified by four categories: coarse lithofacies (collective term of fine conglomerate - pebbled coarse sandstone facies and coarse sandstone facies), medium sandstone facies rich in rigid particles, medium sandstone facies rich in plastic particles, and fine sandstone facies;after experienced diagenetic sequence of weak compaction, strong cementation and strong dissolution, the coarse lithofacies had the best physical properties, and were mostly distributed as isolated lenses in sand in striped distributary channel. This distribution feature is the main reason for the obvious difference in productivity between wells. The study results provide a theoretical basis and guidance for the prediction of high-quality reservoirs and the exploration and development of tight sandstone gas reservoirs.

Accurate Description by Well-To-Seismic Integration of Glutenite Fan Sedimentation in the Northern Dongying Sag, Jiyang Sub-basin

Liu Jianwei

2021, 28(1):  18-25.  DOI: 10.3969/j.issn.1006-6535.2021.01.003

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Glutenite fan reservoir is one of the important reservoir types for enhancing reserve and productivity in Shengli Oilfield. In view of the problems of strong heterogeneity and low accuracy of reservoir description, on the basis of core observation, logging response analysis and seismic forward modeling, the glutenite fan in Well Y22 in the northern slope of the Dongying Sag was finely divided with well-to-seismic integration into subfacies and microfacies by such methods as fine stage division, synthetic record calibration, and numerical simulation. It is found in the study that the different sedimentary structure of sedimentary subfacies of glutenite fan brought about different reflection characteristics, and the sedimentary subfacies could be easily distinguished by waveform clustering attribute; the middle-fan subfacies of the glutenite fan in the Yanjia Area was mainly developed with two sedimentary microfacies, namely braided channel and water channel, the lithology of the braided channel was mainly composed of thick pebbled sandstone deposit, and the edge of the water channel was composed of interactive deposit of mudstone and pebbled sandstone; the regional lithology could be easily distinguished with AC, R_LLD and CNL logging curves, and accurate description by well-to-seismic integration of sedimentary subfacies and microfacies of the glutenite fan based on logging curves and variogram simulation, under the constraints of sedimentary facies and tectonic stage models. This technology can accurately describe the sedimentary facies zone of the glutenite fan, and provide technical support for the deployment of glutenite fan exploration and later productivity construction.

Analysis on Accumulation Conditions and Target Optimization of Paleogene - Neogene Gas Reservoirs in Western Qaidam Basin

Tian Jianhua, Dong Qingyuan, Liu Jun

2021, 28(1):  26-33.  DOI: 10.3969/j.issn.1006-6535.2021.01.004

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There are abundant Paleogene-Neogene oil and gas resources in the western Qaidam Basin, but the natural gas exploration level is relatively low. In order to promote future exploration of natural gas in the study area, the study area was selected and evaluated by ideas of source-cap rock controlling hydrocarbon accumulation and structure-deposit-trap controlling reservoir, secondly, experimental analysis, basin simulation and geological evaluation technologies are used to study the characteristics of gas source rock, reservoir stratum, trap, caprock and preservation conditions, and finally, favorable exploration targets were proposed. The findings show that there are 5 sets of gas source rocks (R_o ≥ 0.8%) taken from the western Qaidam Basin, the primary gas source rocks are E₃² and N₁, with significant late hydrocarbon generation characteristics, mainly distributed in the northwest Qaidam Basin and locally in the southwestern Qaidam Basin near the Mangya depression, with an area of about 4 743.40 km² and a total hydrocarbon amount generated of about 1.17×10¹² m³; the reservoirs in the western Qaidam Basin are classified by two types: clasolite and carbonatite, with tight lithology and petrophysical changes; there are 2 sets of regional caprocks in the western Qaidam Basin; the deep caprock is properly preserved while the shallow caprock is poorly preserved; the deep layers of the effective gas source rock area are dominated by structural traps such as late E₃¹ and E₃² (fault) anticlines and fault noses, and consistent in time and space in gas generation period, accumulation period and preservation period. Based on above findings, three favorable natural gas exploration zones, namely Shizigou-Ganchaigou, Xianshuiquan-Youquanzi-Cucumbermao and Nanyishan were selected, and 12 favorable traps and 3 more favorable traps were identified. There is much for reference of the results of the study to the further exploration of natural gas in western Qaidam Basin.

Application of Sedimentary Forward Modeling of the Permian Wutonggou Formation in the East Slope of Jimusar Sag, Junggar Basin

Wang Tao, Li Ting, Guo Wenjian, Zhang Yu, Yang Tongyuan, Yang Yibo, Zhang Jianxin

2021, 28(1):  34-41.  DOI: 10.3969/j.issn.1006-6535.2021.01.005

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In view of the problem of unclear favorable sedimentary facies belts of the Permian Wutonggou Formation in the east slope of Jimusar Sag, Junggar Basin, the sedimentary evolution process of the Permian Wutonggou Formation in the study area was represented with sedimentary forward modeling method based on drilling and seismic data. The results show that braided river deltation-lacustrine sedimentary system was developed in the Wutonggou Formation in the study area under the condition of first water inflow and then water outflow, the provenance mainly came from the Guxi Uplift, and the delta front facies and sand presented a superimposing relationship of first retrogradation and then progradation from bottom to top; lithological traps were formed near the center of the Sag, and fault-lithological traps were formed near the edges of the Sag; lithological traps at the top of the Wutonggou Member 1 and the bottom of the Wutonggou Member 2 were conducive to the formation of light oil reservoirs. This study provides a reference for later exploration deployment and development adjustment in the study area.

Fan Controlled Large-area Accumulation Conditions and Mode of Upper Wuerhe Formation in MH1 Well Zone of Mahu Sag

Lu Honggang, Luo Huanhong, Luo Feifei, Mao Dengzhou

2021, 28(1):  42-50.  DOI: 10.3969/j.issn.1006-6535.2021.01.006

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the Permian System Upper Wuerhe Formation of Mahu Sag is a large continuous conglomerate oil and gas reservoir group found after the Triassic System Baikouquan Formation. Based on the geological, geophysical and analytical data and regional structural background and sedimentary environment, the paper comprehensively analyzes the oil and gas accumulation conditions and control factors of Upper Wuerhe Formation in MH1 Well Zone from 4 aspects, i.e., hydrocarbon source supply, reservoir cap association, migration pathway and preservation condition. Research shows that the glutenite oil reservoir of Upper Wuerhe Formation in MH1 Well Zone has 4 advantages for continuous accumulation: ① The hydrocarbon source rocks of the Fengcheng Formation of Permian System are characterized by high organic matter abundance, high hydrocarbon generation potential and double peak oil generation, which provide high-quality and efficient hydrocarbon source conditions for the Upper Wuerhe Formation Oil Reservoir in MH1 Well Zone; ② The mud-on-sand deposition mode of the Upper Wuerhe Formation retrogradation-type fan body forms a large and effective reservoir cap association vertically; ③ The ternary composite system of faults, surface of unconformity and permeable sand bodies forms a good three-dimensional conducting network; ④ Subfacies dense zone on fan delta plain, lacustrine facies, inter-fan mudstone and faults are mutually distributed, forming a compound multifaceted shielding, which provides well matched preservation conditions for the accumulation in large areas in the front edge of fan delta. We hereby established the fan controlled large-area accumulation model in MH1 Well Zone and carried out integrated deployment of exploration and development with sizable reserves obtained. This research is of guiding significance for the oil and gas exploration of next stage.

Paleo-environmental Characteristics of the Development of High-quality Hydrocarbon Source Rocks in Hetaoyuan Formation of Biyang Sag and Geological Significance

Shi Zhengyong, Jin Yunyun, Luo Jiaqun

2021, 28(1):  51-58.  DOI: 10.3969/j.issn.1006-6535.2021.01.007

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To solve the problem of unclear hydrocarbon generation conditions and development environment of high-quality hydrocarbon source rocks in Biyang Sag, a variety of geological and geochemical analysis methods were used to describe the development scale of the main hydrocarbon source rocks and define the development environment of high-quality hydrocarbon source rocks in the research area, which provides important basis for further oil and gas exploration. According to the hydrocarbon generation conditions and resource contribution analysis, we proposed the Section H₃⁴-H₃² of Hetaoyuan Formation is a high-quality hydrocarbon source rock development section, and the research on paleo-environmental shows that the water gradually become salty from bottom to top in the hydrocarbon source rock sedimentary environment of Hetaoyuan Formation, and the abundance of organic matters in main salting period of Section H₃ upper-H₂ is also relatively high. In conclusion, the comprehensive analysis shows that the annular concave belt is a favorable area for further exploration because of its good hydrocarbon source rock conditions and large amount of remaining resources,This study further clarified the development environment of high-quality source rocks, and provided the basis for further exploration of oil and gas basins.

Development Characteristics of Shale Fractures in Longmaxi Formation of Changning Anticline and the Stage Analysis

Wan Yuanfei, Qin Qirong, Fan Yu, Wu Jianfa, Zhao Shengxian, Ji Chunhai, Fan Cunhui

2021, 28(1):  59-66.  DOI: 10.3969/j.issn.1006-6535.2021.01.008

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Fracture characteristics and formation stages are of great significance for the optimization of shale gas favorable area. For example, we researched the fracture development characteristics and stages of Longmaxi Formation of Changning Anticline in southern Sichuan based on the observation and analysis of outcrop fractures in the field by means of tectonic analysis and tectonic inversion. Results show that the Changning Anticline is dominated by plane shear fractures and sectional shear fractures with high development degree, large dip angle and high filling degree. In this area, fractures in Longmaxi Formation can be divided into 3 types that are formed in 3 stages of tectonic movement. The first type is NNW and NE plane shear fractures and near-EW sectional shear fractures, formed in middle and late stages of Yanshan Movement and fully filled by calcite; the second type is NW and near EW plane shear fractures and NE sectional shear fractures, formed in the final stage of Yanshan Movement-early stage of Himalayan Movement with fracture fillers including calcite and a small amount of clay and pyrite, the third type is near SN and NEE plane shear fractures and NW sectional shear fractures, formed in middle stage of Himalayan Movement-now with a low fracture filling degree and fracture fillers of small amount of calcite. The research results provide some theoretical references for further exploration and development.

Analysis of Heavy Oil Accumulation Mechanism and Exploration Potential in the Eastern Slope of Ruman Sag in Melut Basin, South Sudan

Xue Luo, Shi Zhongsheng, Ma Lun, Zhao Yanjun, Chen Bintao, Shi Jianglong, WangLei

2021, 28(1):  67-73.  DOI: 10.3969/j.issn.1006-6535.2021.01.009

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Aiming at the problem of unclear understanding of the reservoir-forming mechanism and exploration potential of heavy oil reservoirs in the Ruman sag of the Melut Basin in South Sudan, the reservoir-forming mechanism of heavy oil reservoirs in the eastern slope of the Ruman sag was studied by using research methods such as source rock basin modeling and reservoir analysis, and the exploration potential of heavy oil in this area was analyzed. The results show that the primary reservoirs of the Yabus Formation, Gayger Formation and bedrock reservoirs in the eastern slope of the Ruman sag were destroyed due to the tectonic movement during the sedimentary period of the Lau Formation, and have become secondary residual heavy oil reservoirs, thus they have very limited exploration potential. The oil reservoir in the Jimidi Formation is a secondary heavy oil reservoir with late accumulation and favorable exploration area of about 21 km². The oil reservoir in the Galhak Formation belongs to a primary heavy oil reservoir with late accumulation and favorable exploration area of about 105 km². They both have larger exploration potential. This study can provide some reference and guidance for the next oil and gas exploration in the study area.

Prediction of Total Organic Carbon Content of Source Rocks in Paleogene Salinized Lake Basin in Western Qaidam Basin

Tai Wanxue, Liu Chenglin, Tian Jixian, Feng Dehao, Zeng Xu, Li Pei, Kong Hua

2021, 28(1):  74-80.  DOI: 10.3969/j.issn.1006-6535.2021.01.010

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In view of the influence of salinized environment on organic carbon prediction of source rocks in western Qaidam Basin, the optimized ΔlgR method, multiple regression method and BR-BP neural network method were used to simulate organic carbon content according to salinity changes, and the differences of organic carbon prediction results of the three models were discussed. The results show that: The prediction effect of multiple regression model is general.The accuracy of the optimized ΔlgR model is higher than that of the multiple regression model, but its universality is general.The BR-BP neural network model has different performance in high salinity and medium-low salinity areas, but the prediction effect is better. Therefore, we propose applying the neural network model to predict in medium-low salinity areas, and conducting comprehensive calculation by reasonably adjusting the parameters of the neural network model and combining with the ΔlgR model in high salinity areas. The research results can improve the accuracy of source rock identification and guide accurate source rock evaluation in the basin.

Research on the Magmatic Intrusion and Oil and Gas Reservoir Forming Characteristics in Bohai Bay Basin

Xu Tingsheng

2021, 28(1):  81-85.  DOI: 10.3969/j.issn.1006-6535.2021.01.011

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With the continuous progress of volcanic oil and gas reservoir exploration, the volcanic development area has been transformed from a restricted area to a target area for oil and gas exploration, but there are relatively few studies on the petroleum geology of magmatic intrusion oil reservoirs. Based on the characteristics of magma and magmatic intrusion, this paper analyzes the reservoir forming characteristics in Bohai Bay Basin, such as hydrocarbon source rock evolution, surrounding rock storage performance, oil and gas migration, and oil and gas preservation, and further defines the key exploration targets for magmatic intrusion oil and gas reservoirs in Bohai Bay Basin. Results indicate that magmatic intrusion is beneficial to oil and gas reservoir forming for hydrocarbon source rock evolution, reservoir development and trap formation in later stages. This research can promote the exploration and development of magmatic intruded oil and gas reservoirs and the comprehensive geology research.

Semi-Supervised Interlayer Identification Method Based on Self-Encoder

Chen Yan, Jiao Shixiang, Cheng Chao, Huang Cheng, Jiang Yuqiang

2021, 28(1):  86-91.  DOI: 10.3969/j.issn.1006-6535.2021.01.012

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In order to solve the problems such as few interlayer data calibrated by core and unbalanced distribution of interlayers and sandstone samples, the deep self-encoder and semi-supervised learning method were used to calculate the abnormal scores and give the classification confidence to the abnormal scores. According to the classification confidence, the classification results of the interlayers were obtained, and the model was updated. The research results show that the deep self-encoder model using the update algorithm is effective in the interlayer identification, and the comprehensive classification accuracy reaches 85.00%. In addition, compared with other classification algorithms, the optimal model of AE7&UP has the highest F1_score (84.15%), indicating that the identification effect of the model is good and balanced. The research results are of great significance to reconstruct the cognition system of underground fluids.

Reservoir Engineering

Key Technologies and Development Trend of SAGD for Dual-horizontal Wells in Shallow Super-heavy Oil Reservoir in Fengcheng Oilfield

Yang Zhi, Meng Xiangbing, Wu Yongbin, Zhao Huilong, Luo Chihui, Gan Shanshan

2021, 28(1):  92-97.  DOI: 10.3969/j.issn.1006-6535.2021.01.013

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In view of the shallow burial, high viscosity, and strong heterogeneity of crude oil in shallow super-heavy oil reservoir in Fengcheng Oilfield, there are many challenges in the SAGD development of dual-horizontal wells. This paper aims to study SAGD development technology and supporting technologies of shallow super-heavy oil reservoir, optimize the deployment design of double horizontal wells, design SAGD cycle preheating and reservoir project, and study supporting key technologies such as drilling and completion technologies, downhole string structure, and downhole temperature and pressure measurement. According to development practice and technical and economic indicators, the technical limits of SAGD development for super-heavy oil dual-horizontal wells were formulated, and new technologies to improve SAGD development effects were comprehensively discussed, including composite well pattern development, reservoir expansion and stimulation, and solvent-assisted SAGD. The supporting technologies for shallow SAGD series development provide an important reference for the development of similar gas reservoirs.

Experiment on Long Core Pressure Loss and Permeability Damage Evaluation in Tight Sandstone Gas Reservoir

Su Yuliang, Zhuang Xinyu, Li Lei, Li Xinyu, Li Dongsheng, Zheng Jianyang

2021, 28(1):  98-102.  DOI: 10.3969/j.issn.1006-6535.2021.01.014

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Tight sandstone gas reservoirs are characterized by low porosity, low permeability, complex pore structure, and poor pore connectivity, and are susceptible to severe damage by the water blocking effect. For this reason, according to the water blocking mechanism and reservoir conditions of tight sandstone gas reservoirs, long cores corresponding to the formation were selected. Pressure loss evaluation and permeability damage evaluation experiments under different reservoir conditions and different water blocking degrees were carried out. Two concepts were proposed, namely pressure loss rate and permeability damage rate, to characterize the damage degree of water blocking effect to the pressure propagation process and reservoir permeability. The results show that: in the process of gas field development, the water blocking effect badly damages the reservoirs with poor physical properties and low gas permeability, and preventive measures should be implemented in advance; reservoirs with better physical properties and higher gas permeability can be determined whether optimization measures are needed based on the value of its pressure loss rate and permeability damage rate. For different reservoirs, the reasonable implementation of the water unblocking process is of great significance to the production and economic benefits of gas wells.

Study on Reservoir Sensitivity Evaluation and Key Control Factors of Tight Oil Reservoirs

Wang Xiaowen

2021, 28(1):  103-110.  DOI: 10.3969/j.issn.1006-6535.2021.01.015

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Tight sandstone reservoir in Wuqi Oilfield is disadvantaged by poor physical properties, complex pore structure, serious heterogeneity, and poor water injection effect. In response to these shortcomings, the pore structure characteristics and clay mineral composition of Chang₆ Oil-bearing Formation in Wuqi oilfield were analyzed with high-pressure mercury porosimetry, scanning electron microscope and X-ray diffraction and other tests. Nine pore structure parameters were selected as characteristic parameters, and a comprehensive evaluation standard based on reservoir pore structure was established. The reservoirs were divided into three categories: Type I (excellent), Type II (moderate) and Type III (poor). The rock samples of each type of reservoir were evaluated for sensitivity and analyzed for influencing factor. The results of the study show that the sensitivity of the three types of reservoirs was significantly different. Specifically, Type I presented low velocity sensitivity, low water sensitivity, moderately low salt sensitivity, moderately high acid sensitivity, and moderately low alkali sensitivity. Type II presented moderately low to low velocity sensitivity and water sensitivity, moderate to low salt sensitivity, low acid sensitivity, and moderate to low alkali sensitivity. Type III presented moderate to low velocity sensitivity, low water sensitivity, low salt sensitivity, moderately low to low acid sensitivity and low alkali sensitivity. The pore structure and clay mineral composition were the main reasons for the difference in reservoir sensitivity. The study results can provide a reference for optimizing water injection and reservoir stimulation for similar tight oil reservoirs.

Study on the Compatibility of Polymer Surfactant with Reservoirs and the Capability of Liquid Flow Diversion

Wang Wei, Cui Dandan, Yan Xi, Hu Nan, Zhuang Yongtao, Yuan Guangyu, Yan Yungui, Wang Huan

2021, 28(1):  111-117.  DOI: 10.3969/j.issn.1006-6535.2021.01.016

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In order to study the reasons for the great differences in viscosity, salt resistance and oil displacement efficiency between polymer surfactant and polymer, FT-IR spectrogram and scanning electron microscope (SEM) were used to characterize the molecular structure and aggregation morphology of polymer surfactant and polymer. The injectivity, conductivity, fluid diversion and oil displacement efficiency of polymer surfactant and polymer were analyzed by core physical simulation experiment and nuclear magnetic resonance technology. The results show that the viscosity of the polymer surfactant with the same mass concentration (500 to 2,000 mg/L) was 3 to 4 times that of the polymer due to the presence of a reticular aggregate structure; the improvement by the polymer surfactant to the reservoir seepage profile was up to 79.3%, while the improvement by the polymer was only 63.9%; in the medium-high permeable reservoirs, the final recovery present of the polymer displacement was 6.36 Percentages higher than that of the polymer displacement. This study plays an important theoretical guiding role in the application of polymer surfactant as a new displacement agent for greatly improving recovery efficiency in medium-high permeable reservoirs with high water cut and high recovery percent.

Study on Seepage Characteristics of Fluid Between Matrix and Fracture in CO₂ Huff-puff Process in Fractured Tight Reservoirs

Gong Lianhao, Liu Jizi, Wu Xing, Bu Guangping, Huang Fan, Yang Long

2021, 28(1):  118-124.  DOI: 10.3969/j.issn.1006-6535.2021.01.017

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In order to further explore the seepage mechanism of fluid between matrix and fracture in CO₂ huff-puff process in fractured tight reservoirs, a self-designed high-pressure non-magnetic core holder was used to perform pulsed CO₂ huff-puff core experiment. Based on the principle of nuclear magnetic resonance T₂ spectrum testing, the core was scanned online in the process of CO₂ huff and puff to study the micro production characteristics of crude oil in the pores of matrix and the seepage characteristics of fluid between matrix and fracture during CO₂ huff and puff. he results of the study show that after CO₂ entered the fracture, the crude oil was displaced along the fracture, but the crude oil in the matrix could not be produced. At the early stage of well shut-in, the crude oil saturated with CO₂ in the matrix macropores (20.5 ms ＜ T₂ ≤ 716.0 ms) entered the fractures under the action of expansion. At the later stage of the shut-in, the differential diffusion of crude oil concentration caused by CO₂ extraction and the capillary force of pores in different sizes gradually became the the dominant force, and prolongation of shut-in time could effectively increase the production degree of crude oil in the matrix. The recovery efficiency of crude oil was main contributed by the large pores, while the production degree of small pores was poor. With the increase in the pulsed gas injection cycle, the cyclic matrix recovery percent was continuously reduced. The CO₂ huff-puff cycle should be reduced and the shut-in time should be prolonged during the implementation at the mine. In addition, the lower limit of depletion pressure should be set close tot he saturation pressure to improve the overall recovery efficiency during huff and puff. The study results provide reference and basis for enhancing the recovery efficiency of fractured tight oil reservoirs.

Study on Staged Optimization Method of Staged Fracturing for Horizontal Wells in Low-permeability Gas Reservoir

Yang Hao, Li Xinfa, Chen Xin, Chen Xiaoyi, Liu Peng, Feng Qing, Geng Shaoyang

2021, 28(1):  125-129.  DOI: 10.3969/j.issn.1006-6535.2021.01.018

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The staged fracturing in horizontal wells has become an important method for efficient development of low-permeability gas reservoirs. In view of the poor fracturing effect caused by high reservoir heterogeneity and complex distribution of natural fractures, the four-dimensional imaging technology for monitoring fractures was used to establish a weak stress identification method based on logging curve, the weak-stress development evaluation parameter K_tr was introduced and combined with the reservoir development characteristics of horizontal well to optimize the staged parameters, so as to fully stimulate the section with weak stress. The study results show that in the well section higher than the cutoff value of K_tr, the number of fractures detected by four-dimensional imaging was more than the design value, and the productivity after fracturing was higher than the average level. Therefore, the K_tr value can be used to optimize the staged fracturing interval of horizontal wells in combination with the reservoir development characteristics of the well section and to improve the effect of fracturing and stimulation. It plays an important role in improving the fracturing section division method of low-permeability gas reservoirs and improving the efficiency of stimulation and recovery.

Experimental Study on Development Characteristics of Supercritical CO₂ Huff and Puff in Tight Oil Reservoirs

Zhang Yueqi, Gou Lipeng, Qiao Wenbo, Yu Yao, Wang Qingzhen, Chen Ming, Yu Ying, Zhang Maolin

2021, 28(1):  130-135.  DOI: 10.3969/j.issn.1006-6535.2021.01.019

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In view of the rapid decline in oil recovery rate in tight oil reservoirs of the Ordos Basin and the inability to maintain reservoir pressure, the development method with CO₂ huff and puff was discussed, and the experimental study on supercritical CO₂ huff and puff with long core was carried out. The mechanism, development characteristics and influencing factors of supercritical CO₂ huff and puff in enhancing tight oil recovery were explored. The influence of key operating parameters, such as gas injection rate, gas injection phase, pressure holding with N₂, and depletion pressure, on tight oil recovery was discussed. The study shows that: supercritical CO₂ huff and puff can effectively improve tight oil recovery, with effect better than conventional CO₂ huff and puff. Gas injection rate has little effect on oil recovery. Injection N₂ to hold pressure can significantly improve tight oil recovery and oil exchange rage. The first three cycles of huff and puff are of best effect, accounting for 91.5% of the total oil recovery. The research result confirmed the feasibility and huge application potential of supercritical CO₂ huff and puff in improving tight oil recovery, and can provide theoretical basis and technical support for the efficient development of tight oil reservoirs in the Ordos Basin.

Quantitative Characterization and Development Countermeasures of Multi-layer Heterogeneous Interlayer Channeling Flow

Zhang Qinglong

2021, 28(1):  136-141.  DOI: 10.3969/j.issn.1006-6535.2021.01.020

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Aiming at the problem of interlayer channeling interference in water flooding heterogeneous reservoirs, a physical model of heterogeneous long core with multiple layers was designed. The along-way saturation distribution of core group with various permeabilities was calculated using Archie's method. The producing characteristics and interlayer channeling law of multiple layers with water flooding were simulated. The channeling degree was determined by the saturation change at the core end, and interlayer channeling differences under different heterogeneity conditions were compared. The results show that: under the layer-by-layer development mode, the interlayer channeling degree is greatly reduced, with increasement of recovery rate by 4.12 percentage points compared with commingled producing of multiple layers. In commingled producing of multiple layers, the interlayer pressure gradient caused by interlayer permeability differences would lead to channeling or backflow from low-permeability layers to high-permeability layers. The stronger the interlayer heterogeneity, the more serious the channeling, and the lower the recovery rate. The "layer-by-layer commingled producing " method can effectively improve the recovery of multi-layer heterogeneous reservoirs. The research results provide a reference for the formulation of the development plan.

Research and Application of Denoising and Processing Technology for Oscillation Pressure Data when Stopping Pump during Tight Gas Reservoir Fracturing

Li Yueli, Xu Tonghao, Gong Yufeng

2021, 28(1):  142-147.  DOI: 10.3969/j.issn.1006-6535.2021.01.021

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In the fracturing process of tight gas reservoirs, due to the high fracturing displacement, water hammer is prone to occur when stopping pump during fracturing , which causes obvious low-frequency oscillations in the surface pressure data, leading to problems such as poor quality of double logarithmic curve for injection pressure drop, difficulties in applying analysis methods in the field, and etc. Due to the pressure dropping data shows a typical characteristic of low-pass filtering signal when stopping pump, the pump-stopping data can be denoised and processed with three kinds of methods, namely Savitzky-Golay filtering, fast Fourier transform and wavelet transform. Apply this method to some well in Daniudi. The on-site analysis results showed that the data quality improved significantly after denoising and processing with wavelet transform, and the fracture parameters could be obtained accurately with processed data. The research method has a very important guiding significance for improving the speed and accuracy of the effect evaluation after fracturing.

Drilling & Production Engineering

Study on the Law of the Effect of Rock Pore Properties on the Electric Pulse Rock Breaking

Bai Lili, Li Zheng, Wang Ying, Yan Tie, Sun Wenfeng, Li Zhaoxuan, Liu Shicheng

2021, 28(1):  148-153.  DOI: 10.3969/j.issn.1006-6535.2021.01.022

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In order to study on the law of the effect of rock pore properties on the electric pulse rock breaking,by taking the electric field intensity of rock under the action of electric pulse as the evaluation index,a mathematical model of the electric field intensity of rock pores was derived,a two-dimensional simulation method of the electric field intensity of rocks under the action of electric pulses was established,and a numerical simulation study and an experimental study of the law of the effect on electric pulse rock breaking were carried out.The results show that:the existence of pores will cause the electric field distortion of rocks,the electric field intensity at the junction of pores and rock matrix is the largest,and the electric pulse breakdown will occur first at the edge of the pores; the more the number of pores is,and the smaller the spacing is,the greater the electric field intensity is; the pore diameter,pore morphology and orientation have an important impact on the electric field intensity of rocks,and the larger the effective action surface between the pores and the electric field lines is,the greater the electric field intensity is,and the more likely the rock will be broken down.The high-porosity sandstone is more likely to be electrically broken down than the low-porosity marble,and the breakdown probability of sandstone samples also increases with the increase of porosity.The results of the study clarify the important effect of pores on electric pulse rock breaking,which is of great significance for understanding the mechanism of the electric pulse rock breaking.

Study on the Optimization of the Fracturing Perforation Scheme in the Comprehensive Development of Coal-bearing Reservoirs

Zhang Hongjie, Liu Xinjia, Zhang Xiao, Zhang Suian, Shao Bingbing

2021, 28(1):  154-160.  DOI: 10.3969/j.issn.1006-6535.2021.01.023

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For the three typical lithological association occurrence patterns of coal-bearing reservoirs in the Linxing-Shenfu Block on the eastern edge of Ordos Basin,the numerical simulation method is applied to analyze the influence of perforation location and perforation degree on the inter-bed and intra-bed fracture propagation and extension,and the fracture bed-crossing propagation capability and fracture extension length are used as evaluation indexes to study the multi-reservoir fracturing perforation scheme for such three modes as the sandstone-coalseam contact zone,the sandstone-coalseam-mudstone interbed,and the coalseam-sandstone-coalseam multiple coalseams,so as to improve the comprehensive utilization efficiency of oil and gas resources in coal reservoirs.The results show that:the interbed interface has a significant negative effect on energy propagation; the perforation ratio of 50%-80% is appropriate for the coalseam-sandstone-coalseam multiple coalseams and the sandstone-coalseam-mudstone interbed type reservoirs; for the sandstone-coalseam contact type reservoirs,the indirect fracturing process is applicable,and the perforation ratio is 70%-75% for the sandstone and 25%-50% for the coalseam.This study can provide a reference for the optimization of the coal-bearing multi-reservoir fracturing perforation.

Study on the Wellbore Stability in Fractured Carbonate Formations in North Truva Area

Zhang Yanna, Han Zhengbo, Kong Lulin, Chen Qiang, Liu Houbin

2021, 28(1):  161-169.  DOI: 10.3969/j.issn.1006-6535.2021.01.024

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The fracture development of carbonate formation in the Carboniferous Middle Bashkir Order (KT-II) in North Truva area has caused serious wellbore collapse during drilling.In order to improve the drilling efficiency,the fracture occurrence and inter-fracture filler of KT-II carbonate formation were studied,the carbonate matrix and inter-fracture filler components were tested,and the influence of fracture development on the pore permeability characteristics and mechanical strength of the carbonate formation was analyzed.At the same time,a theoretical model of wellbore instability in fractured carbonate formations was established by coupling the ground stress field,seepage field and fracture mechanical weak plane effect,and the prediction of wellbore stability in KT-II carbonate formations in North Truva area was carried out.The results show that:the development of fractures in the KT-II carbonate formation in the North Truva area and the prominent inter-fracture mechanical weak plane effect are the dominant factors inducing the wellbore collapse; the development of fractures strengthens the seepage migration of drilling fluid along the fractures and weakens the supporting effect of the drilling fluid on the wellbore; the fracture occurrence and wellbore trajectory have obvious effects on the stability of the wellbore in the carbonate formations,drilling in the direction of 180° to the dip angle of the fracture plane can bring the optimal wellbore stability,and optimizing the wellbore trajectory can effectively improve the wellbore stability in fractured formations.The results of the study reveal the mechanism and main influencing factors of the wellbore instability in fractured carbonate formations,and provide a reference for the design of wellbore trajectory and the selection of engineering parameters in fractured carbonate formations.

Development of the Equipment Supporting Mobile Electric Igniter Trip in Fire Flooding

Yang Xianzhi

2021, 28(1):  170-174.  DOI: 10.3969/j.issn.1006-6535.2021.01.025

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Fire flooding is one of the most effective development methods for heavy oil fields, and oil layer ignition is a prerequisite for successful fire flooding. The mobile electric ignition operation of traditional coiled tube vehicles has problems such as long well occupation period and high operating cost during fire flooding. In response to these problems, skid-mounted equipment was developed to support electric igniter trip. The equipment is mainly composed of a detachable roller skid and an open injection head skid, which can move the work skid group to other well groups for the operation of electric igniter trip without cut off the coiled tube. The field application results show that the equipment can meet the simultaneous ignition requirements of multiple wells, with high utilization rate and strong applicability, and the operation cost for single well ignition is reduced by more than 60%. This technology can provide a technical support for the large-scale implementation of fire flooding.