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Table of Content

    01 December 2017, Volume 39 Issue 6
    A Well Logging Study for Preliminary Geological and Physical Shale Gas Horizontal Well Modeling
    XIA Hongquan, WANG Hanwei, ZHAO Hao
    2017, 39(6):  1-9.  DOI: 10.11885/j.issn.1674-5086.2016.07.15.03
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    The subject of this study was shale gas of the Cambrian Qiongzhusi Group located in the Weiyuan County of southern Sichuan. First, the reservoir evaluation results from adjacent logging wells were used to select the "geological sweet spot" as the geosteering target layer; next, we analyzed the LWD (Logging while drilling) response characteristics, such as varying lithology, physical properties, lubricity, and brittleness, of stratigraphic cross sections encountered along the well hole trajectory. Based on the LWD and MWD (Measuring while drilling) data, we determined the stratum's dip angle and thickness in real time and accurately established a preliminary geological and physical model. Finally, using well hole trajectory-LWD curve-reservoir lithology cross-section integrated mapping technology, we promptly identified the location, extension length, and incline/decline direction of the horizontal well hole trajectory within the shale reservoir and optimized the well hole trajectory in real time and revised the preliminary geological and physical model to guide the drilling bit to a high-quality shale reservoir, thus improving the reservoir encounter rate. Applying this method to geologically guided drilling of horizontal shale gas wells within the study area led to significant results.
    Study on Microscopic Pore Structure of Shale Oil Reservoir in Shahejie Formation in the Dongying Sag
    TIAN Tonghui, LU Zhengyuan, QI Minghui, LIU Yi, FENG Mingshi
    2017, 39(6):  10-18.  DOI: 10.11885/j.issn.1674-5086.2016.05.20.01
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    The microscopic pore structure of reservoirs provides the basis for shale oil exploration and development. In our analyses of the pore development characteristics of shale oil reservoirs, we integrated the argon-ion scanning electron microscopy test and a statistical method to determine the parameters of the pore images of the reservoir. Eighteen rock-core samples of the Shahejie Formation in the Dongying Sag, Bohai Bay Basin, China, were used in the analyses. The results indicate that the main pore types of the shale oil layer in this formation are inter-particle pores, dissolved pores, inter-crystal pores, and intra-crystal pores. The largest rate of contribution to plane porosity is from the argillaceous inter-particle pores, with the dissolved pores making a high contribution, and the inter-crystal pores and intra-crystal pores making the lowest contributions. The pore diameter in the shale oil reservoir is of nano and micron scale, with the nano-scale pores being the dominant quantity. However, the non-dominant micron-scale pores provide porosity to the reservoir plane. Therefore, the micron-scale pores provide the main reservoir space to the shale oil layer in the Shahejie Formation.
    Study on Trachyte Prediction Methods in the Oulitouzi Area
    DONG Wenbo
    2017, 39(6):  19-24.  DOI: 10.11885/j.issn.1674-5086.2016.06.29.04
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    Research on trachyte distribution prediction methods can provide a basis for the exploration and exploitation of trachyte gas and oil reservoirs. Trachyte predictions were made based on the analysis of its geophysical response characteristics using multi-parameter lithology inversion combined with a wave impedance inversion technique. The results of well logging showed that the trachyte in this area is characterized by low-time lag, high-density, and high natural gamma logging response characteristics, which can be used to distinguish trachyte from the surrounding lithology. The combination of the multiparameter lithology with longitudinal wave impedance inversion technology allows the prediction of the distribution of trachyte rock mass. The inversion results showing that the natural gamma and wave impedance exceed 95 API and 1.0×107 kg/(m2·s), respectively, confirm that a tracheae development area exists. The reliability of the prediction results was confirmed through drilling.
    Reservoir Genetic Model Analysis of Pai 66 Carboniferous Igneous Rocks in Chunfeng Oilfield
    WANG Wei
    2017, 39(6):  25-33.  DOI: 10.11885/j.issn.1674-5086.2016.06.28.01
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    The Pai 66 Carboniferous igneous rocks have a complex lithology and various reservoir space types. There is still uncertainty about favorable reservoir-development characteristics and distribution patterns. However, for effective development of oil reservoirs, it is necessary to analyze the genetic model of the reservoir for guidance on the exploitation of the oil reserves. We used a geostatistical method for analyses, based on the data of the rock cores, imaging, and analysis assays. The results indicate that andesite and tuff are the main rock types in the area. The main reservoir spaces include dissolution holes and cracks. The primary pores alone are unable to form an effective reservoir owing to weak development. After studying the distribution characteristics of three types of reservoirs, an igneous reservoir genetic model was established for this area. This is because the lithology controls the scope of the reservoir, tectonic movement is the immediate cause of crack formation, and diagenesis shapes the development of the present reservoir. A fracture-cavern reservoir is generally distributed along the fault zone, controlled by a favorable diagenetic facies running lengthwise.
    Study on the Displacement Characteristics and Microscopic Properties of the Reef Limestone Reservoir in Liuhua Oilfield
    NING Yuping, WANG Junfeng, LUO Donghong, ZHANG Wei, DAI Zong
    2017, 39(6):  34-44.  DOI: 10.11885/j.issn.1674-5086.2017.09.04.02
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    Compared to clastic reservoirs, reef limestone reservoirs are oil wettable and their microscopic porous structure is complex. Furthermore, the main factors controlling their water flooding mechanism are still not clear. This study combined static data from rock core SEM, casted rock thin sections, mercury injection, wettability tests, and dynamic experiments, such as microscopic displacement and core displacement, in analysis and established a new method to study the seepage mechanism that involves the dynamic usage of static data. The microscopic properties and fluid displacement characteristics of the Liuhua reef limestone reservoir, and the distribution pattern of residual oil in different porous structures were also identified. The microscopic porous structure of the reservoir was classified into four types, of which type I, Ⅱ and Ⅲ are favorable reservoirs with low displacement pressure, low mercury withdrawal efficiency, and strong heterogeneity. Percolation in the reservoir is closely related to the reservoir type, and controlled by three key factors:reservoir type, pore throat radius ratio, and wettability. The distribution characteristics of oil and water before and after water flooding, and the displacement dynamics and effects in different cores are controlled and directly linked to the microscopic properties of reservoirs.
    The Differentiation Oil-beanring Characteristic and Control Factors of Continental Tight Oil
    YAN Lin, RAN Qiquan, GAO Yang, CHEN Fuli, WANG Shaojun
    2017, 39(6):  45-54.  DOI: 10.11885/j.issn.1674-5086.2016.06.21.04
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    This study discusses the different oil-bearing characteristic and control factors of continental tight oil to support reservoir evaluation and increase knowledge on such reservoirs. Contrast analyses were conducted, such as macro distribution of tight oil, rock core observation, reservoir conditions, and single well productivity. The results reveal that different oilbearing phenomena exist both in marine and continental tight oil. In continental tight oil, the oil-bearing characteristics are decentralized distribution, a wide range, and large differences. Techniques such as core observation, slice identification, logging interpretation, and development area anatomy analysis clarified the oil-bearing characteristic of the Lucaogou tight oil in the Jimusaer Depression, Junggar Basin, showing obvious differences at different scales. Increasingly, attention is being paid to the optimization of well location, well trajectory, and fracturing design. In addition, the controlling factors of tight oil differentiation and oil bearing distribution were discussed. The factors that determine the macroscopic distribution of oil-bearing are the sedimentary facies, diagenetic facies, fault system, and the source storage configuration. Factors such as the reservoir beds, physical properties, pore structures, micro fractures, and others control the local distribution of the tight oil differentiation oil-bearing.
    Prestack Gather Optimization Technique and Application
    XIONG Xiaojun, JIAN Shikai, LI Xiang, LIU Yang
    2017, 39(6):  55-62.  DOI: 10.11885/j.issn.1674-5086.2016.07.21.02
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    This research presents an optimization technique involving the combination of prestack gather "noise suppression-equalization-reduction" with the improvement of the AVO (Amplitude variation with offset) analysis accuracy as the starting point. First, the characteristics of the effective and interference waves of the target formation of the prestack gather were analyzed; next, random noise from the target formation was removed using a fourth-order polynomial fit; then, the phase-axis target formation gathers were equalized using non-surface-consistent residual static correction. Finally, the effective incidence angle selection range for the target formation was optimized to reduce big-offset, stretched or distorted gathers. The practical application in some areas of the Bohai Sea shows that the signal-to-noise ratio between the optimized gathers and stacked profile improved significantly. Moreover, the AVO-curve characteristics of the target formation became more apparent and the obtained AVO-characteristics prediction results conformed more closely with the actual drilling conditions.
    Study on the Effects of Changes in Pore Pressure on Wellbore Stability
    LI Wei, LIANG Lixi, LIU Xiangjun, WANG Tao
    2017, 39(6):  63-68.  DOI: 10.11885/j.issn.1674-5086.2016.07.09.05
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    Long-term injection/projection operations cause large fluctuations in pore pressure, which in turn cause relatively large changes in rock strength and ground stress in the strata compared to the initial stage of mining. Furthermore, the complexity in the horizontal distribution of pore pressure induces and exacerbates wellbore instability and results in frequent occurrences of complex downhole situations. We carried out a combined theoretical and experimental study to understand the effects of pore pressure changes on the rock strength and stratum collapse pressure of sandstone strata. The results show that:(1) Rock strength is negatively correlated with pore pressure. As pore pressure increases, stratum rock strength and elastic modulus show a linear decrease. (2) Changes in pore pressure affect wellbore stability. Fractures, water injection, and polymer injection operations result in increased pore pressure around the well, which exacerbates wellbore instability. In addition, pressure-attenuated mining of oil and gas within a certain range decreases the stratum collapse pressure, which is beneficial for maintaining wellbore stability.
    Hierarchy Characterization of Intercalations in Branching-based and Wandering-based Braided River Reservoirs
    WANG Min, ZHAO Guoliang, SUN Tianjian, HUANG Tongfei
    2017, 39(6):  69-77.  DOI: 10.11885/j.issn.1674-5086.2016.07.12.02
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    Currently, research focus is on one general braided river model, disregarding other types of braided river models. Based on the literature survey and outcrop observations, the FN Oilfield, Sudan, was taken as an example. We analyzed the characteristics of the intercalation-hierarchy structures of the two types of braided river systems, namely, branching and meandering rivers, taking into account the hierarchical-constraint idea and the interface configuration of fluvial hierarchy proposed by Miall and others. The results show that the intercalation of branching braided-river systems includes flood plain deposits, mud filled or semi-filled channels, sedimentation layers, and fine sediment channels. The intercalation of wandering braidedriver systems only includes flood plain deposits and small amounts of mud semi-filled channels. Intercalations in different hierarchies and at various scales vary with respect to lithofacies type, development degree, thickness and horizontal character. This differs from each other in bottom-water reservoir method and the distribution of the remaining oil. In conclusion, we present a summary of five intercalation models that affect the distribution of the remaining oil in vertical configuration and the corresponding remaining oil-rich areas.
    A Method for Calculating the Compressibility Coefficient of the Rock Matrix
    WEI Xian, YIN Hanjun
    2017, 39(6):  78-84.  DOI: 10.11885/j.issn.1674-5086.2016.06.01.02
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    Establishing the relationship between the physical properties, fluid properties, and elasticity parameters of a reservoir is the basis for reservoir prediction and hydrocarbon detection. When using the Biot-Gassman equations to study the elastic properties of rocks with different fluids, the speed and accuracy of deriving the compressibility coefficient of the rock matrix are critical. Based on reasonable assumptions, the Biot-Gassmann equation was used to derive a calculation method for the rock matrix compressibility coefficient; the method was verified by using dehydrated and saturated and high-and low-frequency experimental data for identical tight sandstone. The results showed that this method is effective and reliable for rocks with identical matrix compressibility coefficients. Due to the influence of the microscopic pore structure and hydrophilic minerals, the derived rock matrix compressibility coefficients display small variations with different frequency and saturation fluids. Therefore, the frequency and pore fluid influence should be considered during the application.
    Surface Fractal Characteristics and Their Influence on Shale Nanopores
    SUN Lei, GAO Yuqiong, PAN Yi, OU Chenghua
    2017, 39(6):  85-91.  DOI: 10.11885/j.issn.1674-5086.2016.11.03.30
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    To further understand shale adsorption mechanism and adsorption model, the surface fractal characteristics of shale nanopores must be clarified. Using the shales from the Wufeng group/Longmaxi group and the Yanchang group as study objects, the surface fractal dimension of the shale nanopores was calculated based on the pore structure parameters obtained from the nitrogen adsorption method. It is then compared with the fractal dimension of mudstone and dense sandstone to reveal the effect of organic matter, clay mineral, and other factors on the fractal characteristics of the pore surface. The results show that the surface fractal dimension of shale nanopores in the Wufeng group/Longmaxi group varies between 2.7~2.9, and that for the Yanchang group shale varies between 2.3~2.7, and the fractal dimension depends on the cumulative surface area of pores below 10 nm. The value of the fractal dimensions of the shale has a positive correlation with the developmental stages of the maturation of organic matter. Its relation with organic matter content is however complicated. The surfaces of pores having organic matter and clay mineral crystal contents have obvious fractal characteristics. The development of these pores is the fundamental reason behind the shale having surface fractal characteristics; however, the fractal dimension value of the former is significantly greater than that of the latter. The surface of organic matter and clay minerals is highly rough and seriously deviates from smoothness. Therefore, the adsorption of methane in shale nanopores is a typical heterogeneous solid adsorption process.
    Application of Top Gas Injection-Assisted Gravity Drainage in Bottom Water Reservoirs
    ZHOU Wei, ZHANG Jiandong, TANG Yongliang, CHAI Xiaoying, ZHOU Yan
    2017, 39(6):  92-100.  DOI: 10.11885/j.issn.1674-5086.2016.05.10.03
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    The application of top gas injection-assisted gravity drainage technology in domestic oilfields is still in the exploratory stage. Based on the comprehensive consideration of the formation's geological parameters, development parameters, and other factors, preliminary qualitative and quantitative screening conditions for identifying reservoirs suitable for gas-assisted gravity drainage were established. A bottom water sandstone oil reservoir in the western part of China, where top gas injection-assisted gravity drainage technology was implemented for the first time first in China, was used as an example. Combined with the reservoir engineering method, numerical simulation method, field dynamics, and monitoring results, studies on the key points of gas-assisted gravity drainage reservoirs, estimation of gas injection scale, optimization of well opening timing, and stability of the gas-liquid interface were carried out. Key technologies of gas-assisted gravity drainage technology were discussed, including injection optimization control technology, production well switch and control technology, injection and production optimization perforation technology, dynamic tracking, and adjustment technology. Finally, for the control of gas-liquid interface stability, the technology policy was proposed that recommended focusing on both injection and production, zoning to control pressure and limit production, and tracking interface adjustment.
    SAGD Productivity Prediction Model Considering the Injector-Producer Pressure Difference
    HUANG Shijun, XIONG Hao, YE Heng, GU Hao, YANG Yang
    2017, 39(6):  101-108.  DOI: 10.11885/j.issn.1674-5086.2016.02.01.02
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    In addition to gravity drainage in the Steam Assisted Gravity Drainage (SAGD) production process, the existence of injector-producer pressure difference also significantly affects oil production. Therefore, using the Butler method for forecasting productivity can easily lead to errors. This study investigates the effect of injector-producer pressure difference on SAGD productivity. A double horizontal well is established as a prediction model of the SAGD productivity, and the mathematical model is solved. The following results were obtained:(1) The results of the SAGD numerical simulation calculation program show that the injector-producer pressure difference has a certain effect on the SAGD productivity. (2) The oil recovery effect will be better if there is a certain injector-producer pressure difference in the SAGD recovery process. However, a higher injector-producer pressure difference is not preferable. Because steam channeling has a negative impact on production when the injector-producer pressure difference is greater than the critical pressure value of steam channeling, there is an optimal value of injector-producer pressure difference. (3) The existence of the injector-producer pressure difference increases the oil recovery rate and reduces the time required by the steam to reach the top of the oil reservoir. With the increase in the injector-producer pressure difference, the rate of increase in the magnitude of oil recovery rate and the rate of decrease in the time required for the steam to reach the top of the reservoir decreases gradually.
    Mathematical Simulation Study on Interlayer Interference in Commingled Production
    ZHOU Wensheng, LI Qianru, GENG Zhanli, WANG Shoulei
    2017, 39(6):  109-116.  DOI: 10.11885/j.issn.1674-5086.2016.08.20.01
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    Offshore oil fields usually adopt a large-interval commingled production strategy. Interlayer interference is relatively serious. To evaluate this phenomenon correctly, based on the theory of seepage mechanics, an interlayer interference evaluation numerical model considering the number of layers, thickness, permeability, porosity, viscosity, formation pressure, supply radius, and so on, was established. The model considered four aspects of the interlayer interference mechanism, including the interlayer heterogeneity, initial pressure gradient, wellbore connectivity, and liquid transfer. Based on the mathematical model, the productivity of each layer and the entire well during commingled production, the productivity of each layer and the entire well during separate production, and the interlayer interference coefficient were calculated. The productivity under the conditions of commingled and separate production was analyzed. The results show that the contributions of the permeability difference ratio, test operation procedures, water saturation stage, hydrostatic pressure, and other parameters to the interlayer interference were 50%, 25%, 15%, and 10%, respectively. Interlayer interference is mainly controlled by interlayer heterogeneity. During the commingling of multiple layers, if the permeability ratio is less than 4, the interlayer interference coefficient can be guaranteed to be less than 0.6. If the permeability ratio is less than 10, the interference coefficient can be guaranteed to be less than 0.6.
    Deformation of Five-spot Well Patterns for Anisotropic Oil Reservoir
    XIE Weiwei, WANG Xiaodong, NIU Lijuan
    2017, 39(6):  117-123.  DOI: 10.11885/j.issn.1674-5086.2016.05.01.01
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    Existing researches of anisotropic oil reservoir mainly focus on two special types of well patterns in which the injectorto-producer or producer-to-producer direction is parallel to the principal permeability direction, and the studies concentrating on other types of well patterns are rare. The primary reason is that other types of well patterns might increase the uncertainty of development effect. Based on coordinate transformation method, the real anisotropic oil reservoir can be converted into a equivalent isotropic oil reservoir, and the flow field and development indexes of well pattern can be also calculated, and on this basis, the impact of the degree and direction of anisotropy on the development effect can be analyzed. Research results indicate that the existence of directional permeability can lead to deformation, even destruction and reconstruction, of original well pattern flooding units, when the principal permeability direction is neither parallel nor perpendicular to the injector-producer direction and injector-injector direction. The destruction and reconstruction of well patterns is a consequence of the anisotropic degree and the relative angel between anisotropic direction and well line direction, thereby resulting in the uncertainty of well patterns development effect. In anisotropic reservoir, the degree of anisotropy is greater, the deformation effect of well patterns is severer, and the development effect of well pattern is more sensitive to the change of angle between maximum principal permeability direction and well line direction. To avoid the occurrence of destruction and reconstruction effectively, the best choice for pattern configuration is making sure that the injector-to-producer or producer-to-producer direction is parallel to the principal permeability direction.
    MDT Pressure Measurement History Match Method and Its Application in Adjustment of Reservoir Recovery
    GU Wenhuan, YANG Li, YANG Baoquan, YUAN Zhiwang, ZHANG Xin
    2017, 39(6):  124-130.  DOI: 10.11885/j.issn.1674-5086.2016.07.27.02
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    It is difficult to analyze the degree of utilization of each layer of sand stored in deep water turbid water channels. This aggravates the uncertainty and multi-solution characteristics of reservoir history match fitting and brings great challenges to the study of remaining oil distribution and to the adjustment of the recovery plan. In this paper, a new method is presented for the correction of the deep-water turbid water channel sedimentary reservoir model, based on the history match of MDT pressure measurements of later wells. The main idea is to use later well MDT pressure measurement data as fitting parameters. Through the fitting of newly drilled well MDT pressure measurement data, at the same location and corresponding time and pressure of each layer at a certain time in the model, the horizontal and vertical connectivity of the sand bodies in the model are continuously corrected. Consequently, the superposition and connectivity of the sand bodies in the model gradually approximate those of the actual reservoir. Further, combined with conventional recovery index fitting, the research of the remaining oil distribution trend and the study of the recovery program adjustment are finally completed. Practical application results show that the history match method of MDT pressure measurement on later wells is an effective method to fit the connectivity of the sand body of multi-stage channels.
    CO2 Channeling Sealing In Ultra-low-permeability Reservoirs
    ZHAO Xisen, SHI Lihua, WANG Weibo, BAI Yuan, TIAN Feng
    2017, 39(6):  131-139.  DOI: 10.11885/j.issn.1674-5086.2016.05.01.61
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    To effectively seal preferential breakthrough channels and to increase the sweep volume and flooding efficiency of carbon dioxide, in-situ experimental models with different permeabilities were established, including a heterogeneous core model, a high permeability zone gas-channeling model, and a fracture model. CO2 was measured at different stages of breakthrough time, channeling time, and degrees of recovery. On this basis, a high-strength, high-performance channeling sealing agent, ethylene diamine, containing formation water, starch, unsaturated monomer, a cross-linking agent, and a gel control agent was selected to carry out the carbon dioxide sealing control experiments in various permeability core models. The channeling sealing agent reacted with the carbon dioxide and formed carbamate. The experimental results showed that the carbon dioxide sweep volume was seriously affected by the gas channeling, and the gas channeling effect became stronger with increasing core heterogeneity. As the gas flooding effect weakened in the low-variation permeability conditions, the degree of recovery decreased with the increasing permeability. When the variation exceeded 100, the degree of recovery decreased rapidly and the gas channeling became severe. The low permeability reservoirs became difficult to reach, and as a result, the degree of recovery decreased. By using the high-strength, high-performance channeling sealing agent, ethylenediamine, the relatively high permeable channeling layers in the matrix were sealed in a stable manner and the carbon dioxide flooding sweep volume was expanded effectively. The in-situ degree of recovery increased by over 30%.

    A Study on the Correlation between the Pressure Drawdown Gradient and Reservoir Pressure and the Coal Deformation
    SHI Yingshuang, LIANG Bing, SUN Weiji, SHI Zhaobin, XUE Lu
    2017, 39(6):  140-146.  DOI: 10.11885/j.issn.1674-5086.2016.03.25.04
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    In order to further understand the deformation mechanism of desorbed gas (coalbed methane) coal and to investigate the effects of pressure drawdown gradient and original reservoir pressure on the desorbed gas from coal (coalbed methane), desorbed gas (coalbed methane) coal deformation tests were performed using raw coal samples under different original reservoir pressures and pressure drawdown gradients. The results showed that the time required for desorption to reach equilibrium has a positive correlation with the original reservoir pressure and the pressure drawdown gradient. When the pressure is reduced to 0.3 MPa, the shrinkage strain decreases with an increase in the original reservoir pressure. When the pressure is reduced to atmospheric pressure, the variation in the shrinkage strain with the original reservoir pressure follows the trend that under 0.9 MPa it is higher than that under the 0.6 MPa, which is higher than that under 0.3 MPa. When the original reservoir pressure is the same, the shrinkage strain increases with an increase in the pressure drawdown gradient. When the original reservoir pressure is relatively small, the desorption capacity of the high pressure gradient reservoir is larger than that of the low pressure gradient reservoir.
    Simulation Study on Microscopic Transport of Suspended Particles in Pore Throats
    CHEN Xianchao, ZHAO Fengkun, WANG Lei
    2017, 39(6):  147-153.  DOI: 10.11885/j.issn.1674-5086.2016.05.25.01
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    The microscopic transport simulation of suspended particles in porous media is becoming a fast and effective method to study the particle clogging mechanism and to evaluate formation damage. Based on the particle discretization method, a micro-pore simulation method describing the transport of suspended particles in porous media was established. The Antler natural sandstone skeleton particle model was established by fitting the macroscopic mechanical parameters. A simulation method for coupled skeleton particle fluid and pore network was developed. The accuracy of the skeleton particle fluid coupling model was verified by Darcy linear flow. Then, based on the analysis of the force of the suspended particles, the coupled model for the injection of the suspended particles was established considering the interaction between the injected particles and the fluid, injected particles, and skeleton particles. The simulation results show that when complete clogging occurred, the smaller suspended particles cause greater formation damage, and the permeability of the core with large initial porosity decreases greatly. Therefore, during the design of the water injection scheme, the particle size and concentration of suspended particles should be strictly controlled based on the porosity and permeability of the specific formation.
    Study on Rubber Bushing Failure of PDM having Uniform Wall Thickness
    TONG Hua, CHEN Guoyin, ZHU Xiaohua
    2017, 39(6):  154-161.  DOI: 10.11885/j.issn.1674-5086.2016.08.09.02
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    With the gradual promotion and application of positive displacement motors (PDM) having uniform wall thickness, the rubber bushing failure has become increasingly prominent. Following the failure of the rubber bushing, the PDM exhibits decreasing output performance and lifespan, which may impact the drilling speed and efficiency substantially. A numerical model consisting of a stator and rotor is established based on thermodynamic principles and the principal strain fatigue algorithm. The impact of well depth, magnitude of interference, and roughness on the thermo-mechanical behavior of a rubber bushing is studied, and the lifespan of the rubber bushing is predicted using Fe-safe fatigue analysis software. According to the research results, the main failure form of uniform-wall-thickness rubber bushings is fatigue failure. Comparison of rubber bushings having conventional and uniform wall thicknesses shows that the uniform wall thickness bushing has better sealing performance and lower sensitivity to static liquid head. While designing a rubber bushing of uniform wall thickness, attention should be paid to the magnitude of interference on the abrasion of the rubber bushing to guarantee the safe and efficient operation of a PDM.
    Analysis of Factors Influencing Hermetic Seal Performance with a Spherical-conical Surface and Special Screw Threads
    XU Honglin, YANG Bin, SHI Taihe, ZHANG Zhi
    2017, 39(6):  162-166.  DOI: 10.11885/j.issn.1674-5086.2016.07.25.04
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    A theoretical model for calculating the hermetic seal pressure of special screw threads subjected to an applied torque from a spherical-conical sealing surface is established. The influence of the spherical radius, cone and taper, and applied torque of the sealing face are studied. According to the results, the hermetic seal pressure of the screw threads declines according to a power law with increasing spherical radius; the hermetic seal pressure declines gradually with an increase in the cone and taper; and the hermetic seal pressure increases parabolically with increase in the make-up torque added to the sealing surface. Generally, the spherical radius and make-up torque added to the sealing surface have a significant impact on the hermetic seal performance of the special screw threads. It is suggested to optimize the spherical radius while properly controlling the applied torque such that the stress distribution of the sealed contacts can simultaneously satisfy the hermetic seal performance and stop the yielding of the sealing surface.
    A Study of Gel-plugging Agents Used in Gas Channels with Multi-thermal Fluids in Offshore Oilfields
    HAN Xiaodong, ZOU Jian, WANG Qiuxia, LIU Yigang, MENG Xianghai
    2017, 39(6):  167-172.  DOI: 10.11885/j.issn.1674-5086.2016.03.25.02
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    With respect to the problems arising from the application of multi-thermal fluids in the Bohai Bay field, such as the formation of gas channels and the low plugging strength of the existing foaming system, a series of experiments were conducted to evaluate and select the rheological property and blocking efficiency of gel-plugging agents with relatively high plugging strength. At the same time, the effects of oil content and wastewater on the performance of the plugging system, as well as those of the plugging effects on the demulsification of the recovered liquid, were evaluated. The experimental results proved that the preferred gel-plugging agent demonstrated high blocking efficiency at high temperatures and could be prepared with waste water of the platform. The tolerance of the gel-plugging agents was relatively high, and their application did not significantly affect the demulsification output. Based on a large number of experiments, a selection chart for the gel formulation based on different oil reservoirs and various gas channeling conditions was established to guide the on-site design and implementation of the scheme.