Table of Content

25 February 2022, Volume 29 Issue 1

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Geologic Exploration

Prediction of Fracture Distribution in Shale Reservoirs in the Dingshan Area and Evaluation of Shale Gas Preservation Conditions

Xie Jiatong, Fu Xiaoping, Qin Qirong, Fan Cunhui, Ni Kai, Huang Manning

2022, 29(1):  1-9.  DOI: 10.3969/j.issn.1006-6535.2022.01.001

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In response to the poor understanding of the fracture development and distribution and the influence of shale gas preservation conditions in Longmaxi Formation, Dingshan Area, the paleotectonic stress field in the study area was simulated through triaxial rock mechanics experiments, rock fracture coefficient was calculated by the Mohr-Coulomb criterion of rock failure, the fracture distribution pattern in the study area was predicted, and the favorable target area of shale gas exploration was optimized. The results showed that the areas with low differential stress and the areas developed with secondary or tertiary fractures were the dominant development areas for fracture development in the shale gas reservoirs, with good preservation effect. Based on the prediction results of fracture distribution, a preservation evaluation system and evaluation standards for Dingshan Area were established with the 12 selected evaluation parameters, and the study area was classified into two types of favorable areas by the weighting method and superposition method, and all of them have the characteristics of gentle tectonic positions, deep burial depth, low differential stress, far distance from large fractures, high pressure coefficient, small scale of fracture development, small dip angle, and early development period. The results of the study provide important reference for subsequent shale gas exploration and preservation condition evaluation.

Study on Identification and Main Controlling Factors of Low-resistivity Oil Reservoirs in Xijiang Oilfield, Eastern South China Sea

Liang Wei, Yan Zhenghe, Yang Yong, Huang Yujin, Xiong Qi, Dong Yifu

2022, 29(1):  10-14.  DOI: 10.3969/j.issn.1006-6535.2022.01.002

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In view of the complex formation of low-resistivity oil reservoirs in Xijiang Oilfield, eastern South China Sea and the difficulty in identification of low-resistivity oil reservoirs, a method for evaluating and identifying low-resistivity oil reservoirs suitable for Xijiang Oilfield was proposed on the basis of comprehensive analysis of mineral field data such as well logging, cores, scanning electron microscopy, clay minerals, and rock wettability, typical Archie′s formula was modified, the reserves of potential low-resistance oil reservoirs were predicted, and the main controlling factors of low-resistance oil reservoirs in Xijiang Oilfield were analyzed. The results showed that the main controlling factors for the identification of low-resistant oil reservoirs in Xijiang Oilfield were wettability indicator, cation exchange capacity and irreducible water saturation. On this basis, the modified Archie′s formula was used to accurately predict that the lower limit of the resistivity curve of oil reservoirs in Xijiang Oilfield, eastern South China Sea to decrease from 10 Ω·m to 2 Ω·m by using the modified Archie′s formula, and the recoverable reserves were estimated to increase by 20×10⁴ t. The study has an important implication to the development of similar oil fields in eastern South China Sea.

Study on Geochemical Characteristics and Genesis of Upper Paleozoic Natural Gas in the Perimeter of Shawan Sag, Junggar Basin

Li Erting, Jin Jun, Liao Jiande, Zhou Bo, Ma Wanyun, Wang Haijing

2022, 29(1):  15-22.  DOI: 10.3969/j.issn.1006-6535.2022.01.003

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To address the problem of poor understanding of the genesis and source of the Upper Paleozoic natural gas in the perimeter of Shawan Sag, Junggar Basin, the simulation and experiment of source rock pyrolysis gas were conducted to work out the genesis and sources of the Upper Paleozoic natural gas in the perimeter of Shawan Sag based on the carbon isotope distribution of pyrolysis gas, as well as the natural gas components, carbon isotopes, light hydrocarbon composition and geochemical characteristics of associated crude oil. The results showed that there were six types of gas genesis in the study area. Type Ⅰ natural gas was derived from the source rocks of Jiamuhe Formation with a δ¹³C₂ value greater than -26.0‰; Type Ⅱ natural gas was biogas with negative δ¹³C₁ value and extremely high aridity coefficient; Type Ⅲ natural gas was derived from the source rocks of Jiamuhe Formation and the source rocks of Lower Wuerhe Formation, with an aridity coefficient of greater than 0.95, and δ¹³C₂ value of about -26.0‰; Type Ⅳ natural gas was derived from the source rocks of Lower Wuerhe Formation, with δ¹³C₂ value ranging from -27.5‰ to -26.9‰; Type Ⅴ natural gas mainly originated from the source rocks of Lower Wuerhe Formation and Fengcheng Formation, among which, the natural gas with a δ¹³C₂ value of not less than -29.0‰ was from Lower Wuerhe Formation and the natural gas with a δ¹³C₂ value of less than -29.0‰ was mainly from Fengcheng Formation; Type Ⅵ natural gas originated from the source rocks of Fengcheng Formation with a δ¹³C₂ value of less than -31.0‰. There are three types of effective gas source rocks in the study area, including humic source rocks of Permian Jiamuhe Formation, humic source rocks of Lower Wuerhe Formation and sapropelic source rocks of Fengcheng Formation. The study results give a answer to the gas source problem of the Upper Paleozoic in the perimeter of Shawan Sag, and provide a basis for the study on natural gas exploration and reservoir accumulation in the study area.

Characteristics and Controlling Factors of Weakly Cemented Glutenite Reservoir in Permian Upper Urho Formation, South Slope of Mahu Sag

Wang Ran, Zheng Menglin, Yang Sen, Zhao Xinmei, Jiang Yiyang, Wan Min

2022, 29(1):  23-30.  DOI: 10.3969/j.issn.1006-6535.2022.01.004

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In order to identify the influence of weakly cemented glutenite reservoirs in the Permian Upper Urho Formation, south slope of Mahu Sag, Junggar Basin, the reservoirs in the study area were studied for reservoir characteristics and controlling factors on the basis of experimental data such as core observation, slice identification, and analysis of chemical examinations. The results of the study showed that the reservoir in the Upper Urho Formation was a weakly cemented glutenite reservoir with low porosity and permeability, mudstone filling at bottom, low structural maturity and composition maturity. The type of pores was dominated by residual inter-granular pores, and there are three types of storage spaces: 1) macro-porous and meso-porous medium to thin throats, 2) micro-porous thin throats, and 3) micropores and fractures; the average porosity of the first type of reservoir was greater than 9.0% and the average permeability was greater than 10.000 mD, showing the pore structure features of macro-porous and meso-porous medium to thin throats. The reservoir quality is directly controlled by the sedimentary environment, and the microfacies in subaqueous distributary channel at the front edge of the delta fan were high-quality reservoir sedimentary facies. The palaeogeomorphic characteristics indirectly affect the reservoir physical properties, with strong hydrodynamics, low shale content and good physical properties in the paleotrench area. The diagenesis in the study area was dominated by compaction, and the improvement of the reservoir space was limited. The gravel support inhibited the compaction, so that a large number of primary inter-granular pores could be preserved. The results enrich the glutenite reservoir characteristics of Permian Upper Urho Formation, south slope of Mahu Sag, and have a certain guiding significance for the further study of glutenite reservoir.

Geochemical Characteristics and Research Significance of Source Rocks of Yixian Formation, Northern Naiman

Liu Haiyan, Pei Jiaxue, Yang Xue, He Shaoyong, Liu Xiaoli, Cui Yujing, Hao Liang

2022, 29(1):  31-37.  DOI: 10.3969/j.issn.1006-6535.2022.01.005

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In view of the problems of unclear stratigraphic attribution and unclear geochemical characteristics of the set of gypsum salt rocks and thick mudstones in northern Naiman Sag, the stratigraphic characteristics of gypsum salt rock and thick mudstone and the characteristics of mudstone source rock were studied by means of paleontological sporopollen fossil identification, stratigraphic logging and seismic characteristics comparison, saturated hydrocarbon chromatography-mass spectrometry and so on. The research results show that the gypsum salt rock and mudstone were the sedimentary products of Yixian Formation in volcanic eruption period, with mudstone thickness of 50 to 400 m, distribution area of about 130 km², 3.40% average organic carbon content, Type I and Ⅱ₁ organic matters, and the average measured Ro value of 0.5% at about 1 880 m, indicating that they were in low-mature to mature evolution stage. The hydrocarbon generation and expulsion period of the source rocks in Yixian Formation was earlier than that of the source rocks in Jiufotang Formation, which is more conducive to hydrocarbon migration and accumulation. This study proves that Yixian Formation has well developed for hydrocarbon generation, which is of great significance for deep oil and gas exploration in the northern Naiman Sag.

Intelligent Identification and Prediction of Lithology of Volcanic Reservoirs Based on Machine Learning

Liu Kai, Zou Zhengyin, Wang Zhizhang, Jiang Qingping, Chang Tianquan, Wang Weifang, Yang Xiao

2022, 29(1):  38-45.  DOI: 10.3969/j.issn.1006-6535.2022.01.006

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To address the problem that the lithology of the volcanic reservoirs of Jiamuhe Formation in Well Block Jinlong 2, Junggar Basin is variable and difficult to be accurately identified by conventional methods, four algorithms in machine learning, including decision tree, random forest, gradient boosting tree and Bayes, were adopted to intelligently identify the lithology of the study area, and then determine eight characteristics parameters such as M and N, which are extremely sensitive to the lithology of volcanic rocks based on the analysis of the geological characteristics of volcanic reservoirs in the study area and the logging response characteristics of different lithologies. The results of the study proved that the random forest method was preferred with the best model, an accuracy rate of more than 90% and the highest model generalization. It was an effective method to identify volcanic rock lithology based on conventional logging curves. According to this study, the volcanic rock lithology can be identified and predicted with a high precision, laying a foundation for subsequent exploration and development of volcanic rock reservoirs.

Method for Identifying Stratigraphic Pinch-out Line Based on Tuner Attribute

Yue Xiwei

2022, 29(1):  46-51.  DOI: 10.3969/j.issn.1006-6535.2022.01.007

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To address the problem of unclear pinch-out lines in the Upper Wuerhe and Fengcheng Formations at Zhongguai Uplift, Junggar Basin, the distribution characteristics of pinch-out lines of target formation were characterized by the tuner attribute method in combination with seismic profile and drilling data. The results of the study proved that within the effective frequency bandwidth, the pinch-out lines identified by the tuned phase attributes of upper Wuerhe Formation at 55 Hz and Fengcheng Formation at 45 Hz were closer to the real positions, but it is difficult to improve the accuracy of the stratigraphic pinch-out line beyond the effective frequency bandwidth. The effect of this method was obviously better than that of instantaneous phase cosine, instantaneous phase and single-frequency phase, and other attributes. This method has reference significance for the characterization of pin-out lines of overburden and eroded formations.

Study on Developmental Characteristics of Coastal Fault in Beidagang Buried Hill and Its Coupling with the Upper Paleozoic Secondary Hydrocarbon Formation

Li Jianhong, Wang Yanbin, Miao Huan, Zhang Yujian, Gong Xun, Xiong Xing

2022, 29(1):  52-58.  DOI: 10.3969/j.issn.1006-6535.2022.01.008

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The formation and evolution of faults are greatly significant for hydrocarbon accumulation. In order to ascertain the control of the coastal faults in the Beidagang buried hills on the hydrocarbon generation and accumulation of upper Paleozoic source rocks, based on a large amount of seismic data and bottom structure drawings of various periods, the development characteristics of the coastal fault was characterized in detail. Combined with the hydrocarbon generation and evolution of the source rocks, the matching relationship was analyzed for hydrocarbon accumulation. The results of the study showed that the eastern section of the coastal fault was the most strong in development, the middle section was slightly weaker, and the western section was the weakest; the eastern section was developed with a large number of flower-like assemblages, and dominated by hydrocarbon accumulation on traction anticline; the middle section was developed with domino-type assemblages, and dominated by hydrocarbon accumulation on fault-black and fault-nose; in the Es₃ period, the fault activity was the strongest, and the center of activity was located in the middle section, with an average vertical activity rate of 227.5 m/Ma; the fault activity in the Ed period was weaker, the center of activity was located in the east section, with an average vertical activity rate of 7.0 m/Ma; the Ed period was the key period of secondary hydrocarbon generation and accumulation of the Carboniferous-Permian source rocks in Beidagang Buried Hill, and the accumulation elements were matched with each other. The middle section of Beidagang Buried Hill was a favorable area for hydrocarbon accumulation, as well as the key area for further exploration and development. The results of the study can be used as a reference for in-depth exploration of the Carboniferous-Permian primary oil-gas reservoirs in Beidagang Buried Hill.

Pore Structure and Fractal Characteristics of Low-permeability Reservoirs in Songliao Basin

Lyu Tianxue, Zhang Guoyi, Yi Lixin, Li Zhongcheng, Song Peng, Li Siqi

2022, 29(1):  59-65.  DOI: 10.3969/j.issn.1006-6535.2022.01.009

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To address the problem of unclear understanding of the pore structure of low-permeability reservoirs in Quantou Formation, Wangfu Fault Depression, Songliao Basin, the fractal method of reservoir pores was applied in combination with experimental methods such as X-ray diffraction, casting slice, scanning electron microscopy, and mercury intrusion porosimetry, to determine the pore fractal characteristics of the low-permeability sandstone reservoir and discuss the relationship between the fractal dimension and the physical properties and mineral composition of the reservoir. The experimental results showed that: the pores of the reservoir in this area were mainly intergranular pores and intragranular dissolved pores; the mineral components were mainly quartz and feldspar; the pores of the reservoir were of multiple fractal characteristics, and the structure of small pores was better than that of medium and large pores; the fractal dimension was unrelated to the reservoir porosity, but closely correlated with permeability and mineral composition. This study discovers the pore size distribution characteristics of low-permeability reservoirs from the perspective of multiple fractal theory, and provides a basis for the development of low-permeability reservoirs in Quantou Formation, Wangfu Fault Depression, Songliao Basin.

Reservoir Engineering

Experimental Study on Variation Pattern of Enhanced Permeability of Supercritical CO₂ in Shale Reservoirs

Wu Di, Geng Yanyan, Xiao Xiaochun, Miao Feng, Zhai Wenbo

2022, 29(1):  66-72.  DOI: 10.3969/j.issn.1006-6535.2022.01.010

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In order to study the effect of supercritical CO₂ on the enhanced permeability of shale reservoirs, a study was performed on the permeability enhancement pattern of supercritical CO₂ injection into shale reservoirs with the self-developed rock triaxial seepage experimental device. The results of the study showed that the CH₄ permeability of shale reservoirs varied negatively exponentially with increasing pore pressure, and the permeability was significantly enhanced after supercritical CO₂ injection into shale samples; taking sample 3 as an example, the CH₄ permeability could be increased by 103.98% at a maximum; the enhanced permeability of supercritical CO₂ at injection pressure of 9.0 MPa was better than that at 8.0 MPa under the condition of constant temperature; as the injection pressure of supercritical CO₂ gradually increased to 10.0 MPa, the shale started to fracture; when the test temperature was 35 to 55 ℃, the enhanced permeability of supercritical CO₂ in shale was decreased with temperature raise, and the wave velocity attenuation rate and the porosity were decreased by 0.15 percentage points and 0.90 percentage points with temperature raise, respectively, showing a linear decreasing relationship between wave velocity attenuation rate and temperature. The study results provide a theoretical basis for enhancing the permeability of shale gas reservoirs during the development of supercritical CO₂-enhanced shale gas.

Calculation Method of Deviation Factor and Early Reserve Prediction of Shuangyushi Ultra-deep Gas Reservoirs with High Temperature and Pressure

Deng Bo, Lu Zhengyuan, Liu Qilin, Luo Jing, Liu Siqi, Peng Yang, Xiong Yu

2022, 29(1):  73-79.  DOI: 10.3969/j.issn.1006-6535.2022.01.011

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In order to improve the accuracy of early dynamic reserve calculations for ultra-deep gas reservoirs with high temperature and pressure, a new DAR method was used to establish the independent variable functions of dimensionless pseudoreduced temperature and pressure by multiple regressions based on gas reservoirs in Shuangyushi Qixia Formation which was taken as the study object, the equations were continuously revised by comparing and analyzing with experimental data, and a new calculation equation was proposed for the deviation factor of ultra-deep gas reservoirs with high temperature and pressure; the method of determining the comprehensive compression coefficient of the reservoir at high temperature and pressures was given through comparative studies and laboratory tests on full-diameter samples for compression coefficient analysis. The results of the study showed that the established new calculation method for the deviation factor of gas reservoirs could improve the accuracy of early dynamic reserve prediction of ultra-deep gas reservoirs with high temperature and pressure by more than 3.0 percentage points; the rocks in the ultra-deep gas reservoirs with high temperature and pressure were micro-compressible, and the order of magnitude of their compression coefficients was equivalent to that of formation water; the example analysis showed that the prediction of deviation factor and the determination of compression coefficient of ultra-deep gas reservoirs with high temperature and pressure were the key to early accurate reserve calculation. The results of this study can be used as a reference for the early accurate calculation of dynamic reserves in ultra-deep gas reservoirs with high temperature and pressure.

Application of Pre-stack Inversion Technology to Reservoir Prediction of Mahu Oilfield

Lian Guihui, Zhu Yating, Wang Xiaoguang, Li Zhou, Qin Ming

2022, 29(1):  80-84.  DOI: 10.3969/j.issn.1006-6535.2022.01.012

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To address the problem of low accuracy in identifying effective reservoirs in Mahu Sag, Baikouquan Formation in Well Block M2, Mahu Oilfield was taken as the study object. The geostatistical methods in combination with simultaneous and stochastic inversion were employed to predict effective reservoirs of dense glutenite in Mahu Oilfield, complete the reservoir quality and sweet spot classification evaluation, and characterize the reservoir characteristics of Baikouquan Formation, Mahu Oilfield in detail. The results of the study showed that S-wave prototype was established, and waveform factor parameters, quality control of inversion results and multi-attribute volume operations were adjusted, which not only solved the problem of wave impedance overlap in dense reservoirs, but also substantially improved the longitudinal and transverse wave resolution in reservoir prediction, and the objectivity and reliability of the inversion results were further determined. In addition, pre-stack inversion technology was used to improve the lithology probability and the prediction effect of reservoir physical properties, and successfully characterize the sand body boundary, providing reliable spatial constraints for geological and rock mechanics parameter modeling. The study results provide technical support for planning and deployment of reservoir evaluation and development in Baikouquan Formation, Mahu Oilfield.

Optimal Objective Function Processing Method for Oil-Water Relative Permeability of Low-Permeability Reservoirs

Cui Jian, Shen Guihong, Shang Lin, Sun Yanchun, Gong Lirong, An Yi

2022, 29(1):  85-90.  DOI: 10.3969/j.issn.1006-6535.2022.01.013

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For heterogeneous and hydrophilic cores with permeability,the relative permeability curve was obtained from the non-stationary experimental data by the conventional JBN method, and there were some anomalies such as "humps" in the water relative permeability curve caused by calculation failure. In addition to experimental factors such as heterogeneity of rock samples,early water breakthrough at the outlet, inappropriate displacement velocity,and difficulties in determining the oil-water viscosity ratio, the limitation of conventional methods such as JBN in processing the experimental data of unsteady water-oil displacement is the root cause for the abnormal shape of the relative permeability curve.To solve the above problems,an oil-water seepage model of low-permeability reservoir was established considering the effect of capillary force.The experimental data of oil-water relative permeability were processed by automatic historical fitting method, and the effects of different methods on the oil-water relative permeability of low-permeability core were compared and analyzed.The results showed that, compared with the JBN and Jones methods, the automatic history fitting method for oil-water relative permeability in low-permeability reservoir based on the optimal objective function was advantaged by smooth curve and obvious rules,and reduced the errors in the calculation of relative permeability in low-permeability reservoir to the greatest extent; when capillary force was considered,the results obtained by the three methods were that the oil relative permeability was increased,the water relative permeability was basically unchanged, and the automatic history fitting method reflected the trend of oil relative permeability in a more reasonable and uniform way.The automatic history fitting method overcame the limitations of conventional methods such as JBN,with better applicability,providing important parameters for subsequent numerical simulation and other studies.

Experiment on the Effect of Water Saturation of Filling Medium on the Dissolution and Diffusion of CO₂ and N₂ in Fractured-Vuggy Reservoir

Wang Zhixing, Hou Jirui, Yang Yuhao, Zhu Guiliang

2022, 29(1):  91-98.  DOI: 10.3969/j.issn.1006-6535.2022.01.014

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In order to gain a deeper understanding of the mechanism of the effect of CO₂ and N₂ injection in fractured-vuggy reservoirs on the residual oil in the filling medium inside the fractures and pores, pressure depletion experiment was conducted to determine the gas diffusion coefficient and solubility, for the purpose of studying the effect of water saturation on the diffusion pressure, solubility and diffusion coefficient of the two injected gases and analyzing the mechanism of the effect of water saturation of the filling medium on the dissolution and diffusion of CO₂ and N₂ under real reservoir conditions. The experimental results indicated that the water inside the filling medium was increased from being bound water to being fully saturated, the pressure drop of CO₂ and N₂ was 41.62% and 43.22% and the solubility drop was 40.16% and 42.50% respectively, and the diffusion coefficients were both reduced by an order of magnitude; the existence of formation water in the filling medium reduces the dissolution, diffusion and mass transfer capacities of the injected gas; the physical properties of the formation fluid and the difference between the formation fluid and the porous medium were the main influencing factors. This study provides a reference for the gas injection development of residual oil in fractured-vuggy reservoir in high water-cut stage and the optimization of relevant parameters.

Experimental Study with Orthogonal Numerical Simulation on Reservoir Deformation and Failure Induced by Depressurization and Decomposition of Natural Gas Hydrate

Zhai Cheng, Sun Keming

2022, 29(1):  99-106.  DOI: 10.3969/j.issn.1006-6535.2022.01.015

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In the exploitation of natural gas hydrate with depressurization method,the instability and failure of hydrate bearing sediment is a key factor restricting its effective development.Taking into account the effect of the simultaneous changes in hydrate saturation and effective stress on the main mechanical parameters of hydrate bearing sediment,a fluid-solid coupled elastoplastic model was established for the reservoir deformation and failure induced by depressurization and decomposition of natural gas hydrate,USDFLD subroutine was programmed,and a experimental method with orthogonal numerical simulation was adopted to study the sensitivity of initial hydrate saturation,downhole pressure and effective principal stress difference to the deformation and failure of hydrate bearing sediment near the well.The results demonstrated that the downhole pressure and the effective principal stress difference were two significant factors affecting the deformation and failure of the near-well reservoir,the effect on the maximum equivalent plastic strain of the near-well reservoir was descending in order of downhole pressure, effective principal stress difference and initial hydrate saturation,and the effect on the plastic range was descending in order of effective principal stress difference,downhole pressure and initial hydrate saturation.It is recommended to optimize the downhole pressure design according to the geological conditions of hydrate reservoir. The study results are of great significance to the safety and controllability of natural gas hydrate exploitation with depressurization method.

Evaluation of the Effect of Acid Erosion on Stress Sensitivity of Carbonate Rocks Based on Fracture Reconstruction

Pang Ming, Chen Huaxing, Tang Hongming, Lu Hao, Wang Zhao

2022, 29(1):  107-113.  DOI: 10.3969/j.issn.1006-6535.2022.01.016

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Carbonate rocks are highly brittle and stress sensitivity test will lead irreversible damage to the cores, resulting in the inability to evaluate the difference in stress sensitivity before and after acid erosion with the same cores and the poor comparability of evaluation results. To address this challenge, a method to evaluate the effect of acid erosion on stress sensitivity of carbonate rocks was established based on fracture reconstruction technology, the fracture surfaces before and after acid erosion were characteristically scanned and reconstructed, the finite element method was applied to study the fracture closure pattern under effective stress, and the effect of acid erosion on fracture seepage under effective stress was simulated to evaluate the variation in fracture permeability. It was found in the study that the difference in mineral components on the fracture face leads to non-uniform acid erosion on the fracture surface, making the fracture surface more rough and resulting in greater difference in the fractal dimension between the fracture surfaces; under the effective stress of 40 MPa, before the acid erosion, the fracture width decreased by more than 55% and the permeability damage rate was greater than 90%; however, after the acid erosion, the fracture width decreased by less than 40%, the permeability damage rate was less than 75%, and the stress sensitive damage decreased. In other words, acid fracturing and pickling are conducive to improving the permeability of artificial fractures and prevent fracture closure. This study can provide a reference for the evaluation of stress sensitivity damage of fractured tight sandstone and shale reservoirs.

Experiment on Enhanced Oil Recovery by In-situ Catalytic Reforming of Super-heavy Oil

Tang Xiaodong, Chen Tingbing, Guo Erpeng, Guan Wenlong, Jiang Youwei, Li Jingjing

2022, 29(1):  114-120.  DOI: 10.3969/j.issn.1006-6535.2022.01.017

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In order to solve the problems of high flow resistance and difficulties in production of super-heavy oil in low-permeability area during steam stimulation and steam flooding, in-situ catalytic reforming and viscosity reduction technology for super-heavy oil in low-permeability area was proposed. Reactor method and physical model experiment method were used to select high-efficiency in-situ reforming catalysts, and the catalyst injection method was studied. Five types of catalysts and their reforming conditions were selected. The study showed that: when organozinc was used as catalyst, the amount of catalyst was 0.1%, the water content of heavy oil was 50%, the super-heavy oil showed a better performance in reforming and viscosity reduction; the effect of physical model experiment method on in-situ catalytic reforming and viscosity reduction was better than that of reactor method; under conditions that the water content of heavy oil was 50%, the amount of catalyst was 0.1%, the reaction temperature was 240 ℃, the back pressure of sand filling pipe was 8 to 10 MPa and the reaction time was 24 h, the viscosity of heavy oil was decreased to 54 260 mPa·s from 145 000 mPa·s, and the viscosity reduction rate was 62.58%; the density and acid value of the heavy oil reformed by physical model experiment were decreased, the content of heavy components (colloid and asphaltene) was decreased by 10.85 %, and the fractions before 300 ℃ and 500 ℃ were increased by 6.75% and 17.29%, respectively. At 240℃ and 10 MPa, the self-made biomass-based profile control agent was used to block the dominant seepage channel, and the catalyst was injected into the low-permeability sand-packed pipe and then water flooded. The viscosity of the reformed heavy oil was reduced to 68 450 mPa·s, the viscosity reduction rate was 52.79%, the process flow resistance was reduced by 19.74%, the recovery rate reached 95.22%, and the comprehensive recovery rate of heavy oil was increased from 46.94% to 85.13%. The method provides references for in-situ catalytic reforming and viscosity reduction of heavy oil in low-permeability areas to enhance the oil recovery in the process of steam stimulation and steam flooding of super-heavy oil.

Experimental Study on Development Mechanism and Production Characteristics of CO₂-Pure Water Stimulation of Inter-Salt Shale Oil

Zhang Chong, Xiao Hanmin, Xiao Pufu, Cui Maolei, Zhao Qingmin, Zhao Ruiming

2022, 29(1):  121-127.  DOI: 10.3969/j.issn.1006-6535.2022.01.018

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The shale oil reservoirs in Qianjiang Sag are of abundant reserves, and their pores are rich in a variety of soluble minerals. In the development with CO₂ - pure water stimulation, the supercritical characteristics of CO₂ can be utilized for oil displacement and the CO₂ sequestration in the target reservoirs also can be improved, which effectively reduce CO₂ emissions. However, the key to implementation of this method lies in whether the CO₂- pure water system can enter the shale micro-nano pores and effectively drive the crude oil in the pores. Experiments of stimulation with CO₂, CO₂- pure water and CO₂- formation water were designed, the stimulation characteristics of CO₂ injection, CO₂- pure water injection and CO₂- formation water injection were clarified based on NMR method, and the law and mechanism of crude oil production in different pores were summarized. The experimental results showed that the stimulation efficiency of CO₂- pure water injection was 8.62 percentage points higher than that of CO₂ injection, and 12.66 percentage points higher than that of CO₂- formation water injection, and the acidic fluid formed by the combination of CO₂- pure water could dissolve the soluble minerals on the pore surface, improve the pore connectivity and enhance the reservoir permeability, and effectively increase the production of crude oil in pores smaller than 0.01 μm and 0.01 to 0.10 μm. After multiple cycles of stimulation, the volumetric solubility of ionic substances in the produced liquid was decreased significantly, indicating that the subsequent injection of pure water failed to expand the swept volume, but only improved the production of crude oil. This study proves that the CO₂- pure water injection can effectively improve the recovery rate of shale reservoirs, and provides a new idea for the effective development of onshore shale oil.

Study and Test on the Mechanism of In-situ Combustion with Combination of Vertical and Horizontal Wells in Thick Heavy Oil Reservoirs

Liu Qian

2022, 29(1):  128-133.  DOI: 10.3969/j.issn.1006-6535.2022.01.019

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In order to solve the problems of serious overlap in combustion range, poor longitudinal development degree and low recovery percent of in-situ combustion in conventional straight wells in thick heavy oil reservoirs, numerical simulation method was applied to determine the combustion spreading law of in-situ combustion with combination of vertical and horizontal wells, analyze the production characteristics of vertical and horizontal wells at different stages, and figure out the action mechanism of straight and horizontal wells. The results of the study showed that the well network pattern of in-situ combustion with combination of vertical and horizontal wells changed the original in-situ combustion field of vertical wells, guided the combustion downward, and improved the longitudinal swept volume; the application of in-situ combustion with combination of vertical and horizontal wells to in the reservoir development could realize the dual effects of displacement and pol drainage, effective expansion of the swept volume of the in-situ combustion, and higher recovery percent of the in-situ combustion. This study provides a theoretical reference for the development mode transition for thick heavy oil reservoirs after steam stimulation.

Experiment on the Mechanism of Enhancing Oil Recovery by CO₂ Flooding with Gas-soluble Demixing Agent

Sun Dalong, Zhang Guangdong, Peng Xu, Jing Hao, Lyu Hua, Ren Chaofeng, Wang Ning

2022, 29(1):  134-140.  DOI: 10.3969/j.issn.1006-6535.2022.01.020

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In view of unclear understanding of the mechanism of reducing mixed-phase pressure and enhancing oil recovery by CO₂ flooding with soluble demixing agent, the demixing mechanism of gas-soluble demixing agent was clarified by analyzing the changes in crude oil composition, crude oil viscosity, oil-gas interfacial tension and crude oil phase characteristics in combination with laboratory experiment, numerical simulation and weight analysis. The experimental results showed that after injecting the gas-soluble demixing agents of 3% and 20% volume fraction, the content of heavy components in crude oil was reduced by 25.07% and 48.21%, the average crude oil viscosity reduced by 61.51% and 96.88%, the interfacial tension between crude oil and CO₂ reduced by 30.36% and 91.16% at the same pressure, and the saturation pressure reduced by 6.22% and 15.90%, respectively. The dominant demixing mechanism of gas-soluble demixing agent was to crack the heavy components of crude oil, reduce the crude oil viscosity and decrease the oil-gas interfacial tension. The study also analyzed the oil displacement mechanism for oil recovery enhancement by mixed injection and plug injection of gas-soluble demixing agent and CO₂, and clarified the seepage characteristics of different affected areas in the formation, namely there was two-phase seepage of CO₂ or gas-soluble demixing agent and formation crude oil in the mixed injection area, and three-phase seepage of CO₂, gas-soluble demixing agent and formation crude oil in the mixed-phased area. This study provides a basis for the optimization and field practice of the gas-soluble demixing agent for CO₂ flooding.

On Factors Influencing Fracture Damage in Tight Gas Reservoirs, Linxing Block

Yu Cuipei, Zhang Binhai, Li Zihan, Huang Jing, Dong Zibiao, Yue Qiansheng

2022, 29(1):  141-146.  DOI: 10.3969/j.issn.1006-6535.2022.01.021

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In order to identify the factors influencing the fracturing damage in tight gas reservoirs, Linxing block, laboratory experiments were performed with core permeability damage rate as an evaluation index to analyze the damage of reservoir sensitivity, water block effect, guar gum fracturing fluid residue after gel breaking and residue content to the reservoirs. The results showed that the reservoirs were moderately to weakly water-sensitive, and weakly to moderately weakly acid-sensitive and alkali-sensitive; the reservoir water block index was 85% to 100%, and the damage was strong to extremely strong; when single ammonium persulfate is used as gel breaker, the gel breaking viscosity was high at a temperature of lower than 30 ℃ and the damage to reservoirs was strong; the damage rate of fracturing fluid residue to reservoir permeability was low. The extent of damage to Linxing fractured tight gas reservoir in descending order was water block damage, fracturing fluid residue damage after gel breaking, reservoir sensitivity damage, and fracturing fluid residue damage. This study plays a guiding role in the selection of fracturing fluid systems and reservoir protection measures for Linxing tight gas reservoirs.

Mechanism and Community Distribution of Microbially Activated Water Flooding in Sai 169 Low-permeability Reservoir

Zhuang Jian, Lu Canyang, Zhang Xu, Yang Wenjun, Yuan Weibin

2022, 29(1):  147-153.  DOI: 10.3969/j.issn.1006-6535.2022.01.022

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In view of the poor adaptability of microbial activated water flooding to Sai169 low-permeability reservoir, Changqing Oilfield, the types and contents of metabolites such as biosurfactants, organic acids and biogas in microbially activated water in the on-field expanding culture insert were quantified by gas chromatography-mass spectrometry and other testing methods, the mechanism of micro oil displacement was clarified, and the microbial community distribution in the produced fluid of 22 wells was sequenced by the paired-end sequencing method. The results of the study showed that the mass concentration of esters in the microbial metabolites ranged from 1.57 to 4.98 mg/L, the mass concentration of organic acids ranged from 108.34 to 133.61 mg/L, the volume concentration of biogas ranged from 18 to 38 mL/L, the oil-water interfacial tension was reduced by 30% to 70%, the pH value was reduced by 3% to 5%, and the oil displacement efficiency was increased by 5.46 to 11.93 percentage points; the biosurfactants, organic acids, biogas and biopolymers in the microbially activated water metabolites played an important role in oil displacement; the gene numbers of bacteria, archaea and methanogens in the fluids of production wells generally increased in the process of microbial flooding, and the differences in distribution between wells gradually decreased; the differences in microbial communities between wells were mainly related to permeability and injection-production horizons. This study provides an important theoretical basis for microbial flooding, formation and well selection, and optimization of injection-production scheme in low-permeability heterogeneous reservoir.

Drilling & Production Engineering

Study and Application of Temporary Plugging Agent for Turnaround Fracturing in Deep Well

Li Wei, Xiao Yang, Chen Mingxin, Liu Huifeng, Fan Wentong, Huang Longzang, Peng Fen, Cao Kexue

2022, 29(1):  154-159.  DOI: 10.3969/j.issn.1006-6535.2022.01.023

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Turnaround fracturing can widen the swept scope of hydraulic fractures and the contact area between crude oil and fractures, and it is an effective method to develop low-porosity and low-permeability reservoirs. To address the problems of low plugging pressure of turnaround fracturing and less study on temporary plugging agent suitable for turnaround fracturing of deep wells, the formulation of temporary plugging agent was optimized by laboratory experiment, the fracturing effect of the formulation was analyzed by numerical simulation, and the simulation results were compared with the field microseismic monitoring data. The results showed that no plugging was effective by using temporary plugging particles alone, and the optimized compound temporary plugging agent formulation was 0.8% fiber plus 0.5% temporary plugging particles, with the maximum pressure for borehole plugging of 8.7 MPa; the lateral swept width of the optimized turnaround fracturing system was very similar to the microseismic monitoring results, which clearly demonstrates the effectiveness of the compound temporary plugging agent. There is much for reference of the study results for improving the turnaround fracturing effect of deep wells.

Analysis of Causes and Countermeasures for Sidewall Instability in Deep Shale Wells, Longmaxi Formation, Southern Sichuan

Zhang Zhen, Wan Xiumei, Wu Pengcheng, Li Zhengtao, Wen Li

2022, 29(1):  160-168.  DOI: 10.3969/j.issn.1006-6535.2022.01.024

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The deep shale reservoirs in Longmaxi Formation, Southern Sichuan are subject to severe wall instability during drilling and frequent downhole obstruction, which increases the construction cost and difficulties of later operations. Therefore, an experimental study was conducted on the physical, chemical and mechanical properties of downhole core water, the experimental results were employed to comprehensively analyze the root reasons for dynamic and static sidewall instability in Longmaxi Formation, and the technical countermeasures were developed for sidewall stability. The results showed that the shale in Longmaxi Formation, Southern Sichuan was of high content of brittle minerals, dense lithology, and high mechanical strength; the frequent alternation of small layers was found in the collapsed interval, with well developed bedding fractures at the intersection of small layers, leading to significant anisotropy in rock pore permeability parameters and mechanical parameters, especially the sidewall stability in the inclined interval was greatly affected by the mechanically weak surface effect of the bedding fracture; the downhole differential pressure during the drilling process exacerbated the seepage of the drilling fluid along the bedding fracture, resulting in an increase in the effective stress due to the pressure penetration effect, which reduced the effective support of the drilling fluid to the sidewall and further induced sidewall collapse; for formations developed with bedding fractures, it was necessary to further strengthen the plugging performance of drilling fluid, optimize well trajectory and strengthen the flushing and carrying of downhole cuttings. There is much guiding significance of the study results for safe drilling in deep shale reservoirs.

Development and Application of an Experimental Device for Evaluating the Scouring Resistance of Resin-coated Particles for Artificial Sidewall Sanding Control

Zhang Yunfei, Su Yanhui, Jiang An, Zheng Xiaobin, Huang Yuxiang, Cui Guoliang, Wang Kangnan

2022, 29(1):  169-174.  DOI: 10.3969/j.issn.1006-6535.2022.01.025

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The scouring resistance of resin-coated particles is an important indicator of the effectiveness of the artificial sidewall sanding control technology, but there is no professional equipment available to evaluate this indicator. Therefore, an experimental device for evaluating the erosion resistance of resin-coated particles for artificial sidewall sanding control was developed, which can simulate the effects of such factors as scouring displacement, linear scouring velocity, scouring pressure difference and scouring time on the scouring resistance of resin-coated particles for artificial sidewall. The device was applied to evaluate the scouring resistance of two types of resin-coated particles (FQ-1 and HY) used in an offshore oilfield. The experimental results were in general agreement with the actual sanding in the field, and the resin-coated particle HY was more resistant to scouring. According to the experimental results, the resin-coated particle HY was used in the subsequent sanding control of the artificial sidewall, achieving outstanding sanding control effect. The experimental device successfully developed provides a new evaluation method for the selection of resin-coated particles for artificial sidewall sanding control, and provides technical support for the large-scale promotion and application of artificial sidewall sanding control technology in offshore oilfields.