Table of Content

01 April 2018, Volume 40 Issue 2

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Comparison of Dolomite Reservoir Characteristics Between the Northern Tarim Basin and Lower Qiulitage Group of Bachu

PENG Jun, CAO Junjiao, LI Bin, XIA Qingsong, LIU Xinyu

2018, 40(2):  1-14.  DOI: 10.11885/j.issn.1674-5086.2017.01.23.04

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This study investigated differences in dolomite reservoir characteristics between the Northern Tarim Basin and the Lower Qiulitage Group of Bachu in terms of petrology, reservoir space, physical properties, porous structure, etc., through rock core description, rock thin section assessment, cathodoluminescence, scanning electron microscopy, physical property analysis, and mercury injection data. The results show that dolomite in the Northern Tarim Basin is mainly crystalline or granular. The reservoir space primarily comprises intercrystalline pores, intercrystalline dissolution pores, dissolution cavities, and fissures in various combinations. The pore throat is mainly of the coarse-fine type with good sorting and even distribution. The connectivity and physical properties are both good. Tectonic fractures and karstification also developed in this area, improving the physical properties of the reservoir. The Bachu area is dominated by crystalline dolomite and silty dolomicrite. The development and scope of dissolution cavities and fissures are much lower than in the Northern Tarim Basin, with less varied reservoir space combinations. The pore throat is mainly of the fine-micro type with poor sorting and uneven distribution. The connectivity and physical properties are also poor. The physical properties of the dolomite reservoir in Northern Tarim Basin are better than that in the Bachu area. This difference is mainly caused by the combined effects of sedimentation and diagenesis.

Paleogene Source-constrained Seismic Reservoir Predictions in A Oilfield of the Southern Bohai Sea

ZHANG Jianmin, QIAN Geng, ZHU Jianmin, ZHANG Lan, YUE Honglin

2018, 40(2):  15-24.  DOI: 10.11885/j.issn.1674-5086.2016.11.18.01

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Seismic reflections from the upper Oil Layer I (within the Middle of the Sha 3 Member in Shahejie Formation) in A Oilfield in the Southern Bohai Sea only returned a single peak, while the reservoir's thickness changed rapidly in the horizontal direction, and the accuracy of well-seismic ties were also very poor in this region. These issues posed difficulties for further improvements in the accuracy of reservoir predictions. The analysis of heavy mineral data indicated that the main source of the upper Oil Layer I in middle-western plates of the oilfield is the southwestern Kendong Uplift. The eastern plate on the other hand, is controlled by sources in the northeastern Laibei Uplift, and multi-source sedimentation is the key factor underlying the low accuracy of well-seismic ties. Based on previous well-seismic data analyses, optimal seismic amplitude attributes were selected for the analysis of amplitude attributes and sandstone content in 38 drilled wells in the upper Oil Layer I, and it was found that these quantities were linearly correlated when source-constrained. The smaller the amplitude attributes of sediments from the same source, the higher is the sandstone content, and vice versa. Source-constrained seismic reservoir predictions quantified the relationship between amplitude attributes and sandstone content, improving the accuracy of reservoir predictions in the upper Oil Layer I of the Middle of the Sha 3 Member in Shahejie Formation.

Sedimentary Characteristics of Upper Paleozoic Mixed Deposits in Southeastern Ordos

LIU Guizhen, ZHANG Dandan, LI Pan

2018, 40(2):  25-34.  DOI: 10.11885/j.issn.1674-5086.2016.10.20.02

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To identify the mixed sedimentary characteristics of the Benxi and Taiyuan formations in the southeastern part of the Ordos Basin, research was performed on mixed carbonate-clastic sedimentary layers using information derived from the region's geology, and data obtained from well logging/drilling and rock cores. Results indicate that the Benxi-Taiyuan formations are land and sea transitional facies deposits with barrier coasts, as well as mixed sediments. Lithologically, the mixed sediments in the Benxi Formation are mainly comprised of clastic rocks and limestone sandwiched between coal seams, while the Taiyuan Formation is mainly composed of argillaceous limestone and coal seams. The mixed sediment facies developed in the Benxi Formation include mixed tidal flat sediments, mixed lagoon sediments, and mixed barrier bar sediments, whereas mixed platform sediments were developed in the Taiyuan Formation. It was concluded on the basis of the region's sedimentation data that there were two modes of assemblage for the sedimentary facies:mixed barrier coast facies (developed in the Benxi Formation) and mixed continental shelf sedimentary facies (developed in the Taiyuan Formation). The main factors affecting the development of mixed sediments and their mode of assemblage were found to be tectonics, changes in sea level, and climate; in particular, mixed sedimentation was mainly caused by Multi level marine transgressions. Rich-source rocks, high-quality reservoirs, and multiple reservoir-caprock assemblages within the Benxi Formation were developed from these mixed sedimentary layers, and are highly favorable strata containing rich reserves of oil and gas.

Study on Hydrocarbon Source Rock of Late Paleozoic-early Cenozoic in the Southwestern Dongpu Sag

LIU Zhongliang, ZHANG Chengfu, LI Qingchen, LIU Jun, AN Hailing

2018, 40(2):  35-45.  DOI: 10.11885/j.issn.1674-5086.2016.12.19.03

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There are two main groups of hydrocarbon source rocks in southwest Dongpu Sag, the Carboniferous-Permian and the third member of Shahejie Formation. Their hydrocarbon generation capacity and their contribution to hydrocarbon accumulation affect the evaluation of the exploration strata and targets in this area, especially since there have been differing opinions regarding whether the third member of the Shahejie Formation can produce gas, and whether the gas reservoirs are mixed-source. By using geochemical methods, such as carbon isotopes, rare gas isotopes, natural gas compositions, and chromatography-mass spectrometry, oil and gas sources were compared and the geochemical characteristics of the source rocks were studied. The study shows that the organic matter in the source rocks of the third member of Shahejie Formation are dominated by Type Ⅱ₂ to Type Ⅲ Kerogen with high abundance and are poor-moderate grade source rocks, while the organic matter in the Carboniferous-Permian source rocks are dominated by Type Ⅲ Kerogen with high abundance and are good quality source rocks. The crude oil in the area comes from the third member of Shahejie Formation, where the source rocks are characterized by generating oil first and then entering a high-maturity stage to produce coal-type gas with heavy carbon isotopes. The Carboniferous-Permian source rocks mainly produce coal-formed gas. These two types of gas mix and become important natural gas sources in the area. The two groups of hydrocarbon source rocks are the most favorable exploration sources in this area.

Fault Tectonics and Petroleum Entrapment in the Laoyemiao Region of the Nanpu Depression

CHEN Weichang, YAN Jingjing, SUN Guanyu, YANG Rongchao, ZHOU Wei

2018, 40(2):  46-56.  DOI: 10.11885/j.issn.1674-5086.2016.11.04.02

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To elucidate the tectonic characteristics of faults in the Laoyemiao region of the Nanpu Depression and their role in the control of petroleum entrapment, the principles of tectonic analysis were used to perform an in-depth analysis of the coupling between petroleum entrapment factors and the evolution of faults. It was found in this study that fault systems in the Laoyemiao region may be broadly divided into basement and superficial faults, and the vertically developed igneous rocks constitute the ductile transfer zone of deep and shallow fault systems. The NNE and NE striking basement faults formed by forwards-extending Eocene rifts were present as multi-periodic listric fan faults, while the NEE-striking superficial fault systems formed by obliquely extended Oligocene rifts mainly occurred as horst and graben faults, and domino faults. In the Neogene layer, flower-and Y-shaped styles of deformation formed along the main fault line in the cap rocks due to the sliding deformation of deep faults, in the midst of thermal precipitation. The multi-periodic rifting of the main fault line in the Laoyemiao region was favorable for the sedimentation and maturation of source rocks, and the main fault line was also the primary channel for the vertical migration of petroleum within this region. Hence, anticlines and sloped zones are favorable areas for hydrocarbon accumulation.

Evaluation of 3D Hydrocarbon Migration System in Baodao Sag in the Deep-water Area of Qiongdongnan Basin

JIANG Rufeng, GUO Minggang, ZHU Jitian, ZHOU Jie, XIANG Yuangao

2018, 40(2):  57-66.  DOI: 10.11885/j.issn.1674-5086.2016.06.26.03

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The dominant migration path and migration direction of the Baodao Sag, were identified based on drilling and seismic data using seismic interpretation, paleostructure restoration, oil and gas migration and accumulation modeling, 3D geological modeling, and other technological approaches. Four faults with four levels, four sets of framework sand bodies, and four main structure ridges were identified in the Baodao Sag. The system of source rock-migration system-reservoir-trap in the Baodao Sag has been established, and the migration systems have been classified into three types:the T type, ladder type, and restricted meshwork-carpet type. The southern and northeastern step-fault zones are characterized by ladder type migration systems, while the Songnan low uplift and northern slope zones are characterized by restricted meshwork-carpet type systems. The migration simulation shows that the southern step-fault zone, northeastern step-fault zone, and Songnan low uplift front tectonic zone are the dominant migration directions, which are mainly characterized by vertical and lateral migration. The selection of favorable zones indicates that the southern step-fault zone is the most favorable zone, which has the configuration features of "near source, fault-sand-ridge ladder type migration, submarine fan and fan delta reservoir, and fault block-lithologic trap."

Discovery and Application of the Condensed Layer in Lacustrine Facies

WU Guohai

2018, 40(2):  67-74.  DOI: 10.11885/j.issn.1674-5086.2016.11.17.02

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The aim of this study was to determine potential reservoirs in the Jurassic Akshabulak Formation, located in Oil Field A in Kazakhstan. Logging curves for multiple wells were compared during a comprehensive reservoir study. A shale layer was discovered, which is distributed almost throughout the entire region, and has electrical characteristics of high natural gamma values and low resistivity. After comprehensive analysis of the shale layer via sequence stratigraphy, the condensed layer was confirmed to have been deposited during extensive flooding. The lateral distributional characteristics of the condensed layer were combined with geological and seismic data to establish a sedimentation model. Analysis of the sedimentation characteristics of the upper stratum of the condensed layer indicates substantial variations in sedimentation thickness at the eastern slope of the oil field. A prograding delta developed on this slope, which indicates the high-water mark period. Its leading edge was very favorable for the development of lithological and stratigraphic reservoirs. This observation was validated during the subsequent drilling and exploration process. A method for locating the condensed layer in lacustrine facies is proposed and provides new insights into the detection of lithological reservoirs.

Methods for Detailed Characterization of Geothermal Fields in Complex Rift Basin

LIU Jun, YANG Xipu, XU Wei, YANG Xiaoli, FANG Lei

2018, 40(2):  75-82.  DOI: 10.11885/j.issn.1674-5086.2016.08.26.02

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The geothermal field of rift basins in East Africa are highly complex. In particular, the geothermal gradients within Oilfield E are highly variable, which affects the characteristics of reservoir fluids and the oilfield's development strategies. A detailed analysis of the measurements taken from the oilfield revealed that the geothermal gradients displayed zoning in the horizontal plane and segmentation in the vertical axis, which is difficult to visualize using conventional research methods. Hence, we explored a method termed as zoned-segmented regression that is capable of characterizing the geothermal field variations of an oilfield in detail. This method was used to perform a detailed characterization of complex geothermals in Oilfield E. The results indicate that the average geothermal gradients in shallow ground layers is 6.10℃/(100 m), and the geothermal gradients decreases towards the northern part of the research area. The average geothermal gradients in deep-lying sections is 1.53℃/(100 m), and increases towards the north instead. Sedimentation was found to be the main factor determining the development of variations in geothermal gradients in the horizontal plane.

Experimental Study on Ultrasonic Anisotropic Characteristics of Nitrogen-containing Coal Bodies

ZHAO Yu, ZHANG Yugui, WANG Songling

2018, 40(2):  83-90.  DOI: 10.11885/j.issn.1674-5086.2016.12.15.02

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The investigation of ultrasonic response characteristics of gas-bearing coal bodies can provide the basis for application of ultrasonic technologies for testing the physical properties of coal. An independently developed coal reservoir physical property experimental system was used. Coal samples were selected from the 16-17 Coal Seam of the Pingdingshan Eighth Mine to prepare three kinds of coal samples:the parallel bedding and surface cleat (x), parallel bedding and vertical surface cleat (y), and vertical bedding (z), in which the ultrasonic characteristics of nitrogen-containing coal bodies under different gas pressures and at different adsorption times were tested. The experimental results show that (1) when the axial pressure is constant, as the confining pressure increases, the elastic wave velocity of coal increases continuously, and the P-and S-wave velocity of the coal in a vacuum is slightly greater than that under normal pressure; (2) from the negative pressure state to the gas injection and adsorption state, the velocity of the coal column first decreases and then slowly increases with the extension of the gas injection duration; (3) when the axial pressure and confining pressure are constant, the P-and S-wave velocity increases with the change of gas pressure; (4) the anisotropy among the parallel bedding and surface cleat, parallel bedding and vertical surface cleat, and direction of the vertical bedding is significant, but the ultrasonic response patterns are basically the same; and (5) the relationship between the anisotropy degree of the vertical and parallel bedding and the adsorption time is significantly different between the P-and S-waves of the coal samples. The anisotropy degree of the P-wave is more sensitive to the adsorption time.

Study on Influence of Permeability and Distribution of Pore Diameters in Rock Cores on Measurement of Mobile Fluid Saturation

NING Ning, LI Yichao, LIU Honglin, ZHOU Shangwen

2018, 40(2):  91-97.  DOI: 10.11885/j.issn.1674-5086.2016.04.25.01

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To understand why the calculation of original oil saturation through mercury injection capillary pressure (MICP) curves always yields excessively large values, we measured the mobile fluid saturation of a set of selected samples using nuclear magnetic resonance (NMR), high-speed centrifugation, and conventional mercury injection experiments. Comparisons were then performed to analyze how the mobile fluid saturation in different pore diameter ranges differed from the MICP-measured value of mobile fluid saturation, to reveal the underlying causes of these differences. The experimental results indicated that, in ultralow-permeability reservoir cores (0.1 mD < K < 1.0 mD), MICP-measured saturation was significantly larger than NMRmeasured mobile fluid saturation. Our analysis revealed that this was caused by differences in the quantity of injected mercury and the volume of mobile fluids (as measured by NMR), because of the restrictions imposed by small pore throats. In lowpermeability reservoir cores (1.0 mD < K < 10.0 mD), MICP measurements of mobile fluid saturation may still result in larger values than the corresponding NMR measurements. In reservoir cores with high levels of permeability (K > 10.0 mD), the difference between the MICP-measured saturation and the total mobile fluid saturation was relatively small. In this case, it was found that both the injected mercury and mobile fluids were mainly distributed within pore throats with radii greater than 1.00 μm.

Development Program of Flooding After Huff and Puff in Offshore M Heavy Oilfield

LI Yanjie, LI Na, TAN Xianhong, ZHU Guojin, ZHANG Lijun

2018, 40(2):  98-106.  DOI: 10.11885/j.issn.1674-5086.2016.10.27.02

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This research focused on low recovery efficiency of water flooding of unconventional underground heavy oil at Bohai, which had a crude viscosity greater than 350 mPa·s. Additionally, the research was based on the cycle of huff and puff and platform life. Offshore M heavy Oilfield was taken as an example. A numerical reservoir simulation was performed to optimize the design. This was used to compare and analyze the flooding time, as well as the injection and production parameters of four flooding patterns, namely, steam huff and puff turning into steam flooding, hot water flooding turning into multiple thermal fluid flooding, multiple thermal fluid huff and puff turning into multiple thermal fluid flooding, and hot water flooding. The results indicate that steam huff and puff turning into steam flooding has the best effect, and multiple thermal fluid huff and puff turning into multiple thermal fluid flooding has a relatively unfavorable effect. Optimal parameters of steam flooding are as follows:the flooding turning pressure is approximately 5 MPa, production-injection ratio is 1.3, steam quality at the bottom of the oil well is 0.4, injection temperature is 340℃, and steam injection speed is 240 m³/d.

Analysis of the Influence of the Development of Fracture-porosity-type Volcanic Gas Reservoir

SHANG Kejian, FENG Dongmei, YE Liyou, LIU Huaxun

2018, 40(2):  107-114.  DOI: 10.11885/j.issn.1674-5086.2017.01.08.02

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This study involved building a seepage model of a gas reservoir under the presumption of stress sensitivity, gas slippage effect, and high-speed non-Darcy flow to study the development characteristics of the fracture-porosity-type dual media gas reservoir. The influence of different seepage effects on the gas well was analyzed through numerical simulation methods. The results indicated that the production features of the dual media gas reservoir were basically in line with those of the equivalent homogeneous gas reservoir when the interporosity flow coefficient greater than or equal to 10^-6, Consequently, the theories and methods of analyzing the equivalent homogeneous gas reservoir can be applied to study the dual media gas reservoir. At the early stage of gas well production, the stress sensitivity had little influence on the pressure drop of the pit bottom and variation in the production plateau. The effect of the stress sensitivity is enhanced at the mid-late stage. Theoretically, the influence of the stress sensitivity on the pressure drop of the pit bottom was observed earlier with the decrease in the interporosity flow coefficient, and its effect on the production plateau became greater. The high-speed non-Darcy flow had little influence on the pressure drop of the pit bottom at the early stage of the gas well production, and the influence increased at the mid-late stage. Theoretically, the influence of the high-speed non-Darcy flow on the pressure drop of the pit bottom was observed earlier with the decrease in the interporosity flow coefficient. Its effect on the production plateau also became greater. The slippage gas effect existed in the fracture-porosity-type dual media gas reservoir, but was only slight because of the reservoir and the bottom hole pressure that was comparatively higher.

Optimization Study of the Horizontal Well and Injection-production Pattern in an Ultra-low-permeability Reservoir

FAN Jianming, QU Xuefeng, WANG Chong, ZHANG Qingzhou, WANG Xuanru

2018, 40(2):  115-128.  DOI: 10.11885/j.issn.1674-5086.2017.03.22.03

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This study forms the basic principle of an injection-production well design for a horizontal well in an ultra-lowpermeable reservoir based on the understanding of the breakthrough, effect analysis, and mining rule of the horizontal well in an ultra-low-permeable reservoir to solve the problem of stable production. The following points are discussed:(1) the expanding range of the three-dimensional space of an artificial pressure crack is the foundation of the injection-production pattern design; (2) the direction of the well arrangement is perpendicular to the maximum principal stress; (3) the lateral displacement of the well network is the main supplementary energy; (4) the well spacing, distance, and horizontal section length should be optimized to improve the water flooding control area ratio; and (5) spacing must be reduced and an effective use of the gap between the reserves must be realized to improve the intensity of the single section. The technical direction and policy of further improving the development effect of horizontal wells are put forward based on the basic principles of the injection-production well pattern design along with the combination of field statistics and theoretical analysis to (1) improve the level of the single segment output; (2) improve the level of the well network pressure and optimize the well spacing for 500-600 m (the horizontal segment lengths of the reservoirs of the I, Ⅱ, and Ⅲ classes were 500-550 m, 450-500 m, and 400-450 m, respectively, and the reservoir row spacings of the I, Ⅱ, and Ⅲ classes were 150, 120~130, and 100~120 m, respectively; (3) optimize the water injection intensity of the single well using the technique of low and moderate water injection; and (4) determine the reasonable initial output of the horizontal well using the relationship between the amount of deposit, distance, and velocity of the water line. The reasonable production flow pressure injection was slightly greater than the saturation pressure before the water injection was effective, and was not less than 2/3 of the saturated pressure.

Flow Field Calculation Model and Verification by Physical Model in Different Injection Production Well Pattern

YE Shuangjiang, JIANG Hanqiao

2018, 40(2):  129-134.  DOI: 10.11885/j.issn.1674-5086.2016.12.23.03

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Regarding the problems on flow field distribution with different injection-production schemes (single injection and single production of vertical well and horizontal well, single injection and single production of horizontal well, and dual injection and single production of horizontal well), the study on mathematical calculation models of flow field and physical simulation with different injection-production schemes is carried out. Both conformal transformation and the mirror image reflection theory are applied to establish the mathematical model of the flow field for different injection-production schemes and deduce the mathematical expressions for the potential function and stream function. Thus, the inner equipotential lines and streamline distribution diagrams of different injection-production schemes are obtained. The computational results of the calculation model of the flow field for different injection-production schemes are compared and verified through a physical simulation experiment of water-oil displacement with different injection-production schemes. The results show that the calculation results of the calculation model for flow fields in different injection-production schemes are basically consistent with the physical simulation experiment results. The inner flow field for the single injection and single production of vertical and horizontal wells shows a certain linear flow, and both the water-driving wave and area are increased compared to the conventional injection-production schemes of vertical well. The inner injection-production cross-well streamline of single injection and single production in horizontal wells presents a linear flow. The area controlled by the streamline is larger; the displacement effect is better in comparison with single injection and single production of vertical and horizontal wells. The streamline near the heel end and toe end of the inner horizontal production well in dual injection and single production of a horizontal well is relatively dense; that near the mid position of the horizontal production well is relatively sparse, being easy to form a lag oil zone.

Derivation and Practice of the New Gas Flooding Characteristic Curve of Reservoir with Gas Injection Flooding

YUAN Zhiwang, YANG Baoquan, YANG Li, GU Wenhuan, SHANG Fanjie

2018, 40(2):  135-141.  DOI: 10.11885/j.issn.1674-5086.2017.04.23.01

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Because the conventional gas flooding characteristic curve fails to effectively describe actual seepage fluid (injected gas) in stratum and displaced crude oil, displacement process of gas injection development is analyzed and oil and gas seepage characteristics of miscible flooding and immiscible flooding are represented, respectively. Based on the steady seepage theory, the relational expression of the actual seepage fluid (injected gas) in stratum and displaced crude oil is derived, which is a semi-logarithmic function of cumulative gas injection volume and cumulative oil output; this is termed the new gas flooding characteristic curve, and its applicable conditions are analyzed. Relevant examples are applied to study the effect of changes to the injection production pattern and displacement mechanism on the characteristics of gas flooding curve. Under the condition of a given volume of gas injection, this curve can be used to accomplish long-term and stage-yield prediction, and can be applied at the initial stages of gas injection development.

Testing and Calculation of the Pore Volume of Bottom Deposits in the Salt Rock Reservoir

REN Zhongxin, YANG Haijun, LI Jianjun, LIU Jianyi, FAN Zhou

2018, 40(2):  142-150.  DOI: 10.11885/j.issn.1674-5086.2016.12.06.02

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Affected by the geological characteristics of the reservoir-building salt layer, generally, after cavern creation is completed, a large amount of insoluble matter will accumulate at the bottom. The existence of deposits not only reduces the effective storage volume but also results in many unfavorable factors for the overall construction of the reservoir. In order to scientifically evaluate the deposits and effectively handle them or transform them to increase the storage volume, their distribution characteristics and deposition pore volume were studied. Samples of deposited insoluble matter were obtained by indoor cavern experiments. The particle size distribution was determined by sieving. The distribution characteristics of particle size were analyzed according to the fractal distribution theory, and numerical modeling studies of the deposition pore volume were conducted based on the compressible packing model. Real cases were used in the calculations, and a comparison with field data was conducted. The results show that in the double-logarithmic coordinate system, the particle size of the bottom deposits is linearly correlated with the cumulative amount; that is, the distribution of particle size can be characterized by the fractal distribution function. The fractal dimension can be used as a distribution characteristic parameter to describe the uniformity and centrality of particles.

3D Well-path Design for Horizontal Wells in Heterogeneous Reservoirs

YAN Jizeng

2018, 40(2):  151-158.  DOI: 10.11885/j.issn.1674-5086.2017.05.28.01

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The design and practical application of three-dimensional horizontal wells were studied to solve the problem of three-dimensional horizontal well construction using conventional directional equipment and to reduce costs. After analyzing the three-dimensional trajectory design models and comparing their advantages and disadvantages, the optimal constant tool face angle model was selected. Focusing on the six sections that make up a typical well-path (vertical, increasing, twisting, stable, increasing, and horizontal) and starting with the basic equation with the A target as the constraint, an initial azimuth model was established. After obtaining the build-up rate and the tool face angle based on the initial azimuth, the remaining trajectory parameters were calculated to complete the design of the three-dimensional horizontal well trajectory. This method was applied in the JH17P36 well using conventional equipment to obtain the following results:the vector hit the target, the coincidence rate of the actual trajectory and the design trajectory were high, and the technical indicators were excellent. The drilling example results indicated that the calculation model and the design method were accurate, reasonable, and feasible and also demonstrated the superiority of the constant tool face angle model.

Assessing the Strength of Casing Pipes that Contain Corrosion Pit Defects

LIAN Zhanghua, LUO Zeli, YU Hao, LIU Yang, HE Yong

2018, 40(2):  159-168.  DOI: 10.11885/j.issn.1674-5086.2017.06.21.01

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The corrosion of oilfield casing pipes has become increasingly severe in recent years. According to statistical data, the corrosion of the casing pipes in each block of the Dagang Oilfield has reached 34.4%, leading to problems such as severe casing damage and leakage. To address this problem, this study constructed three-dimensional finite element models for casing pipe-concrete sheath-ground strata systems with casings that contain either one or two corrosion pit defects based on elasticity theory and mechanical models of casing pipes with spherical corrosion pits. By studying the patterns of stress distribution in casing pipes using the model with two corrosion pit defects, it was found that the model with a single corrosion pit defect could be used to evaluate the casing strength if the gap between the corrosion pits exceeds 0.65 mm. The effects of the corrosion pit radius and depth on the strength of the casing pipes and the variational patterns of a casing pipe's stress concentration factors were studied herein using the single-corrosion pit defect model. The effects of steel grade on the loading of casing pipes with corrosion pitting in hydrated strata were also investigated. Consequently, the maximum stress and stress concentration factors in the casing pipes with corrosion pitting both increased with the increasing corrosion pit depth. The stress concentration factor also increased as the radius of the corrosion pit decreased. The stress concentration factor may exceed 2.0 in certain cases. Furthermore, the strata hydration accelerated the damage of casing pipes that contained corrosion pits.

Working Fluid Level Extraction Technique Using Time-frequency Peak Filtering Based on Voice Activity Detection

LIU Yanping, WU Jie, CHEN Yanjun, LIU Li

2018, 40(2):  169-175.  DOI: 10.11885/j.issn.1674-5086.2017.04.04.01

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Several types of noises are generated in an oil jacket annulus, which complicates the fluid level reflection signal measurement. The real position of the fluid level reflection wave could not be accurately identified owing to interference. Thus, the time-frequency peak filtering (TFPF) method is applied, along with the voice activity detection (VAD) method in voice signal processing, to perform effective extraction of the working fluid level wave. The VAD-TFPF method first uses the short-time energy and double-threshold VAD method with a zero-crossing rate, divides the oil well signal measured using the acoustic method, and determines the effective signal data segment and data segment close to zero; then, TFPF with different window lengths is applied to perform filtering to two types of data segments. The filtering experiment and analysis are carried out to measured data under different noise intensities; the results indicate that this method has a stronger identification ability toward working fluid level waves than the wavelet threshold filtering method and is preferable for background noise suppression or for effective wave extraction.

Effects of Hydrocarbon Composition on the Stability of Diesel Storage

REN Lianling, XU Peng, GE Rui

2018, 40(2):  176-182.  DOI: 10.11885/j.issn.1674-5086.2016.07.16.03

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Diesel is known to deteriorate in quality during storage. To examine this issue, the effects of various hydrocarbon compounds on the stability of processed diesel fuels from China were investigated using typical samples of diesel. The results of the hydrocarbon composition analyses indicated that the total aromatics and paraffin contents of diesel are strongly correlated with each other (with a correlation coefficient of 0.90). Through the combined use of correlation analysis and stepwise linear regression, a correlation model was constructed for stability indicators with respect to hydrocarbon composition, and validated through fitting experiments. It was shown that this model can be used to accurately predict the stability of diesel storage. Hydrocarbon compounds with the largest impact on diesel storage stability were determined and were divided into three grades according to their influence on storage stability. It was found that acenaphthylene and tricyclic aromatic hydrocarbons have the largest influence on the deterioration of diesel quality, whereas paraffin has the greatest effect on the cetane number and calorific value of the diesel.