Table of Content

01 August 2018, Volume 40 Issue 4

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Factors Influencing Formation of Heavy Oil in the Shahejie Formation of the Qingdong Sag

MIN Wei, ZHANG Linpu

2018, 40(4):  1-8.  DOI: 10.11885/j.issn.1674-5086.2017.02.06.01

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The Shahejie Formation, located at the western and southwestern margins of the Qingdong Sag, contains extensive reservoirs of heavy oil. At present, there is a lack of in-depth research into and clear understanding of the formation mechanism of this oil. This knowledge gap has affected the evaluation of resources within the study area, as well as oil and gas exploration processes. A comprehensive analysis of the formation of heavy oil in the area was made based on the physical properties of the crude oil and composition characteristics of the biomarkers. The results indicate that the heavy oil contained native immature oil and distributions of heavy oil produced by secondary effects, such as biodegradation. The former was mainly distributed within the Qingdong Well 5 area, located in the southwestern section of the sag; the latter, biodegraded oil, was found at the western slope of the sag and Qingdong-Qingnan transition zone. Degradation of the crude oil was at levels 2-8, and generally had the following characteristics:heavier above and to the south, and lighter below and to the north. The formation and distribution of heavy oil in the study area was mainly affected by factors including the early hydrocarbon generation of organic matter, paleotopography, and tectonic setting.

Fan Delta Sedimentary Characteristics of of the Lower Jurassic Badaowan Formation in the Eastern Margin of the Junggar Basin

XU Lei, WANG Zongjun, WANG Pangen, SHANG Zhilei, GUO Xiao

2018, 40(4):  9-16.  DOI: 10.11885/j.issn.1674-5086.2017.05.18.03

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The Member 1 of the Lower Jurassic Badaowan Formation (J₁b) in the Dishuiquan region at the eastern margin of the Junggar Basin is a newly discovered oil and gas reservoir. In-depth reservoir sedimentation research on this member is important for understanding the favorable conditions for reservoir formation in the region and evaluating the potential reserves. The Di20 site in the eastern Dinan uplift was studied in this project. A detailed core description was integrated with well logging and outcrop measurements to propose the following principle:" Constrained by the marker beds and based on the microfacies, spin echo and reserve determination are conducted at local and region scales, respectively." Spatial comparison of constituent sand bodies was thereby performed to investigate the sedimentation characteristics and modes of the fan delta in the eastern Dinan uplift. The results demonstrate that the sandstone-conglomerate J₁b reservoir in the region is a typical fan delta deposit formed as water from mountains entered lakes. It is characterized by small plains and large fronts. The microfacies can be further divided into underwater distributary channels, braided bars, underwater distributary bays, delta-front sheet sand, delta-fronts sand, and other facies. The J₁b reservoir in the Dishuiquan region is the piedmont deposit of the Kalamaili Mountain with abundant source materials. Near-source delta-fan deposits often result when the fan shapes are mainly controlled by paleo-geomorphological characteristics and the supply of source materials.

Mechanism of Geological Activities of Eastern Tarim Basin Hydrothermal Fluids and Its Significance in Reservoir Transformation

YAN Bo

2018, 40(4):  17-28.  DOI: 10.11885/j.issn.1674-5086.2017.02.09.02

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Faults developed in the Eastern Tarim Basin. Early and late faults underwent processes such as mutual transformation, superposition, and recombination, and deep faults run through multiple strata. Compresso-crushed zones and fracture zones that were formed during fault activities function as the ascending passage for magmatic hydrothermal fluids and provide favorable conditions for reservoir reconstruction by hydrothermal dissolution. As such, identifying the hydrothermal activity stage is crucial for understanding of the mechanism of dolomite reservoir formation in the Eastern Tarim Basin. By observing, identifying, and analyzing drilling cores and fragments, rock thin sections, and casted rock thin sections, albitization and featherlike authigenic illite precipitation were identified and discovered in the Lower Paleozoic carbonate rocks for the first time in the Tarim Basin. By studying geochemical properties such as the homogenization temperature, and oxygen and carbon isotopes of the inclusions, the source of the diagenetic fluids was found to be magmatic hydrothermal fluids. This finding supplements the mineralogical indications of hydrothermal activities in the region. The study further identified the existence of stage-three hydrothermal activity, and by investigating its mechanism and significance in the reservoir space, it was concluded that stage-one hydrothermal activity does not play a key role in reservoir formation. Stage-two hydrothermal activity is mainly developed in constrained sedimentation environments. Stage-three hydrothermal dissolution forms dissolution fissures and cavities that are favorable for the migration and accumulation of natural gas, and therefore, the fissures and cavities serve as the main seepage passage for natural gas accumulation and reservoir formation in the region.

Shale Pore Characteristics of Longmaxi Formation in Wulong Area, Southeastern Chongqing

ZHANG Ying, ZHANG Haitao, HE Xipeng, GAO Yuqiao, ZHANG Peixian

2018, 40(4):  29-39.  DOI: 10.11885/j.issn.1674-5086.2016.08.08.01

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Core observation, argon-ion polishing scanning electron microscopy, and low temperature nitrogen adsorption and desorption methods were used to study the pore characteristics of shale in the Longmaxi Formation, Wulong area, southeastern Chongqing. The shale pore structures and fractal features of the LY-1 Well, Longmaxi Formation, were investigated using image analysis technology and the fractal geometry theory. The results show that the macro shale fractures in the area are mainly interlayer lamellation cracks and diagenetic shrink micro-cracks. The intragranular pores, intergranular pores, and organic matter (OM) holes can be observed by the scanning electron microscope. The OM holes, with dimensions from several to dozens of nanometers, develop well, and the low-temperature N₂ adsorption results show that the shale in the Longmaxi Formation has three significant pore sizes. The pore size of type I ranges from 1.0 to 1.5 nm, indicating that micropores are the most developed pores in the area. The types Ⅱ and Ⅲ pore sizes are from 2.5 to 3.5 nm and from 5.0 to 18.0 nm. When the pore size >4.0 nm, the curve fractal dimension value has a negative relationship with TOC, brittle mineral and gas contents, and has a positive relationship with clay minerals. The study shows that OM pores provide major reservoir spaces for shale gas, and pores with complex structures in clay minerals make a certain contribution to the shale gas reservoir.

Quantitative Characterization of Fault Sealing and Its Control on Hydrocarbon Accumulation

LI Qiang, TIAN Xiaoping, HE Jing, JIA Haisong, ZHANG Guokun

2018, 40(4):  40-50.  DOI: 10.11885/j.issn.1674-5086.2017.03.31.03

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To understand the differences in hydrocarbon accumulation between the lateral and vertical directions of the three fault blocks in eastern LD21 Oilfield, this study established a chart for determining the relationship among embedded depth-shale volume fraction-displacement pressures on the basis of the fault sealing mechanism. The shale volume fraction in fault rocks and reservoir rocks was calculated, and the influence of the compaction diagenetic pressure and time of the displacement pressure of the fault rock were analyzed. The sealing capability of a fault was quantitatively evaluated by calculating the difference in fault-reservoir displacement pressure. The maximum height of the hydrocarbon column height that can be enclosed in different oil-forming faults was predicted, which agrees well with the hydrocarbon column height of various oil fractions in the actual drilling of oilfields, with an average error of 9.5%. This proves the feasibility of this quantitative evaluation of the fault sealing capability, and elucidates the causes of the difference in hydrocarbon accumulation in the lateral and vertical directions. In addition, the orthogonal experiment was performed to quantitatively evaluate selected factors that could affect the displacement pressure of fault rocks. The four factors of fault depth, shale volume fraction, fault dip angle, and effect-time ratio were ranked according to their importance.

Study on the Reservoir Architecture and Remaining Oil Distribution of Braided Channel

LI Weiqiang, YIN Taiju, ZHAO Lun, LI Feng, CHEN Liang

2018, 40(4):  51-60.  DOI: 10.11885/j.issn.1674-5086.2017.03.05.05

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The reservoir architecture is the key factor influencing the outcome of oil and gas production, and controls the distribution of the remaining oil. Based on the theory of architectural-element of fluvial reservoir and the depositional model of braided rivers, this study performed hierarchical analysis and dissected the reservoir architecture elements of sandbodies in detail for the purpose of future production. Structural elements including braided channels, channel bars, abandoned channels, and mud drapes, were identified and confirmed. Contact modes among architectural elements of different levels and periods were recognized to complete the detailed characterization of the reservoir architecture of braided channel. Based on these results, and in conjunction with interpretation of water-flooded layers from logging data of different time periods and dynamic production performance, water-flooding characteristics at various production and adjustment stages were reconstructed. The water-flooding process controlled by reservoir architecture of braided channel was dynamically characterized. The distribution of the remaining oil controlled by the reservoir architecture is summarized and confirmed by the detailed water-flooding results of densely spaced sealed coring wells. The results show that the braided river reservoir has a generally high water content, with three-dimensional inhomogeneous and random water-flooding characteristics that do not occur in segmentations. The non-flooded regions are very scattered. Locations with local deterioration of reservoir physical properties, mud drapes, and low-permeability barrier occlusion directly control the distribution of the remaining oil.

Development and Application of Three End-member Gas δ¹³C₁-R_o Models

CHENG Fuqi, ZHU Yajie, JIN Qiang, HONG Guolang

2018, 40(4):  61-68.  DOI: 10.11885/j.issn.1674-5086.2017.02.21.01

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End-member gas δ¹³C₁-R_o modeling is an important tool for natural gas origin identification, gas-source correlation, and mixed-source gas quantitative processing. In order to identify and evaluate type Ⅱ natural gas generated by organic matter, organic thermogenic gas is broken down into the following three types of end-member gas according to the type of parent material:Type I, Ⅱ, and I. δ¹³C₁ -R_o models for the three types of end-member gas were then developed using 126 endmember gas were then developed using 126 data sets obtained from natural I:δ¹³C₁=27.55 lg R_o -47.2; Type Ⅱ:δ¹³C₁=25.55 lg R_o -40.76; Type Ⅲ:δ¹³C₁=48.77 lg R_o -34.1, if R_o < 0.9%, and δ¹³C₁=22.42 lg R_o -34.8, if R_o > 0.9%. To facilitate application of the models, a chart was generated to identify the δ¹³C₁-δ¹³C₂ values of the three end-member gases and their mixtures using δ¹³C₁ and δ¹³C₂ data obtained from the samples for modeling. from the samples for modeling. According to the chart, the natural gases from Jiyang Depression Tuo 165, Che 571, and Gudong 9 wells are mixture of Type I and Ⅱ end-member gas, Type Ⅱ end-member gas, and Type I end-member gas, respectively.

Numerical Method of Lithostratigraphic Description and Its Realization

ZHAO Gang, CHEN Hu, TAN Xiucheng, XIAO Jindong

2018, 40(4):  69-78.  DOI: 10.11885/j.issn.1674-5086.2017.03.29.01

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To solve problems in conventional mechanical recording and lithostratigraphic description methods, this study investigated the numerical description of rock strata and its digitization using diagenetically related depositional sequences as the fundamental geological unit for lithostratigraphic description. By comparing the self-similarity of stratigraphic units and the basic stratigraphic units, a framework was constructed for rock stratigraphic description. Iterations were performed with abstract rock unit symbols for visualizing the framework content. The measured cross-section profile of Lengshui Creek was taken as an example for demonstrating the feasibility and scientific validity of digitizing stratigraphic descriptions with the system. The results show that stratum description with the self-similarity of the basic stratigraphic units explains the sedimentation process and pattern in a clear and concise manner. This has important theoretical and practical significance in regional geological survey and hydrocarbon resource exploration.

Construction of a Quantitative Geological Knowledge Base for the Braided River of Hanggin Banner and Its Application

ZHANG Guangquan, HU Xiangyang, JIA Chao, LIU Bin, LIU Jiandang

2018, 40(4):  79-89.  DOI: 10.11885/j.issn.1674-5086.2017.03.06.01

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In this study, a quantitative geological knowledge base covering the width of braided channels, as well as the width, length and thickness of river islands in Hanggin Banner was established by determining and analyzing the probability distribution and correlation among various parameters of braided channels and river islands. For this purpose, up-to-date deposition and outcrop data of braided channels as well as drilling data and lateral and in-plane distribution of sedimentation microfacies in the Lower Shihezi Formation of the study area were employed. The knowledge base serves to determine the distribution range of various parameters of the braided river in Hanggin Banner, and to guide the 3D geological modeling and prediction of reservoirs. It also forms the geological foundation for future well positioning. The length, width, and thickness of river islands in Hanggin Banner area were found to be 6 500~8 500 m, 3 000~4 500 m, and mostly 5~14 m, respectively, with a length to width ratio of 1.85~2.30. The width of the braided river is 8 000~18 000 m. The established quantitative parameters of the various microfacies of the braided channels were used to construct a 3D geological sedimentation microfacies model for the Lower Shihezi Formation of the Gin 58 drilling site in Hanggin Banner. This model was used to select favorable drilling areas. Its precision is demonstrated in actual drilling situations.

Application of Continuous Circulation Drilling in Extended Reach Wells in the Panyu Oilfield

ZHANG Jie, WANG Zhiwei, MA Ling

2018, 40(4):  90-96.  DOI: 10.11885/j.issn.1674-5086.2017.01.03.01

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During the drilling of φ215.9 mm extended reach wells in the Panyu Oilfield, technical difficulties are encountered, such as deep well sections, long inclined sections, large inclination angles, rock debris accumulation, faults, abrupt narrowing of the safety density windows of drilling fluids, employment of oil-based drilling fluids throughout sections, high viscosity, and large gel strength. To overcome these difficulties, continuous circulation drilling is introduced, and the application of this technique in the φ215.9 mm section of the PY10-5-A1H Well is evaluated. The results demonstrate that, when the fluctuations of the full-section(1 026 m) equivalent circulating density (ECD) are less than 2.3%, the circulation pressure within the well is stable and the torque fluctuations are steady. The average penetration rate reaches 19.30 m/h, and the hole is well cleaned. Unnecessary repeated reaming and short trips are reduced, and non-drilling time is shortened to improve drilling efficiency and ensure safe and smooth drilling through faults. No complicated downhole situations such as leakage, collapse, and blockage are observed. This paper summarizes the impacts to the drill pipes and hoses at the moment when the circulation channels are switched and problems such as the disrupted transmission of measurement while drilling signals during field application. Specific measurements and improvements are suggested.

Invasion Depth Model for Drilling Fluids in Fractures in Dense Sandstones

LEI Qiang, TANG Hongming, ZHANG Liehui, ZHU Boyu

2018, 40(4):  97-104.  DOI: 10.11885/j.issn.1674-5086.2017.04.25.03

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The pore structure and fractures of the reservoir in the Keshen site of the Kelasu Gas Field cause significant leakage of drilling fluids while drilling. In particular, solid particles block the fractures, leading to low or even zero productivity for some wells after completion. In the field, acid fracturing is mainly employed to remove blockage. The variations in the invasion depth of the drilling fluid in fractures can reflect the performance of acid fracturing. Concerning this issue, according to the flow mechanism of Newtonian fluids in fractures, fracture aperture variations, and filtration losses of fracture walls, a dynamic model for drilling fluid leakage in a single fracture is established. The invasion depth of the drilling fluid is quantitatively described based on the distribution of the drilling fluid pressure inside the fracture. The relationship between the invasion depth and invasion time and that between the pressure difference and fracture aperture are considered and analyzed. Finally, using the established model, the relationship between the invasion depths of the acid and drilling fluid is predicted. The results are compared with the actual values obtained from the Keshen C well, indirectly validating the model.

Study of Quadrilateral Unstructured Grids in a Numerical Simulation of Fractured Reservoirs

LIU Yong, PENG Xiaolong, DU Zhimin

2018, 40(4):  105-115.  DOI: 10.11885/j.issn.1674-5086.2017.03.25.21

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The introduction of unstructured quadrilateral meshes in the numerical simulation of fractured reservoirs is a popular area of research in the field of high-precision numerical simulation. These meshes can be used to create a grid with an unfixed number of connected components under the conditions of complex reservoir boundaries, cross fractures, multiple fractures, multi-directional fractures, complex multilateral wells, etc. Compared to other unstructured meshes, these meshes require fewer grids and have a higher precision. In this study, a mixed percolation model was established by combining the continuous medium model and the discrete fracture model. The numerical solution showed that the introduction of quadrilateral unstructured grids resulted in a significant decrease in the computational efficiency of the numerical simulation. Therefore, three solutions were proposed:morphing of the fracture endpoints with an improved paving algorithm, reordering of two-dimensional and threedimensional mesh numbers, and MPI parallel computing. The computational efficiency was then increased by an average of 70%.

Method for Computing In-plane Streamlines in Cross-sealed Boundaries of Any Angle of Tip and Its Applications

LI Gen, WU Haojun, CAI Hui, SHI Peng, OU Yinhua

2018, 40(4):  116-122.  DOI: 10.11885/j.issn.1674-5086.2017.05.04.01

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At present, to solve the function of complex potentials in cross-sealed boundaries with an analytic method requires the condition that the boundary angle of the dip equals π/n (where n is a positive integer). To extend the application of n to any real number, performing conformal transformation followed by mirror imaging is proposed. Specifically, the idea is to mirror a sealed boundary of any angle of the tip in the original plane onto a target plane by performing conformal transformation, thereby enabling the angle of the tip of the sealed boundary after mirroring to satisfy the condition that n is a positive integer. According to the property that the values of the complex potential and current of the point source (convergence) of a point in the original plane remain unchanged after conformal transformation, the complex-potential function of the corresponding point in the target plane of the point in the original plane is solved, and the resulting value is the value of the complex-potential function of the point in the original plane. An equation for computing the flow velocity of any point in flow fields was also derived. The streamline distribution in flow fields was rendered graphically by employing the contours method. In addition, the shortage of flow fields computed with the analytic method was reviewed and, correspondingly, improvements were proposed. The field application results show that an area closer to the vertex of the boundary angle of the tip has a higher streamline density, lower flow velocity, and higher degree of oil saturation.

Production Prediction for the Volume-Fracturing Horizontal Wells of a Tight Oil Reservoir in the Ordos Basin

WANG Chong, QU Xuefeng, WANG Yongkang, CHEN Daixin, ZHAO Guoxi

2018, 40(4):  123-131.  DOI: 10.11885/j.issn.1674-5086.2017.03.29.05

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The methods currently available for horizontal-well production prediction, whether by numerical simulation or theoretical analysis, are not appropriate for tight oil reservoirs because of their large heterogeneous variations, difficulties in describing their artificial fracture development, etc. To resolve these issues, the production of the horizontal wells of a tight oil reservoir was analyzed from geological and engineering perspectives on the basis of interpreting log data and volume fracturing parameters. The analysis was conducted using the geological parameters of the horizontal section (total hydrocarbon value obtained through standard mud logging, permeability, and brittleness index) as well as the enveloping surface area and lateral heterogeneity index of the oil reservoir as the sensitivity parameters of oil content, permeability, compressibility, and heterogeneity of the oil reservoir. In addition, the quantity of fluid injected into a single well was used as the sensitivity parameter of the well production through gray correlation analysis. The effectiveness of this analysis was then assessed. A fitted-regression-based log model was developed for predicting the production of the horizontal wells of the tight oil reservoir, thereby helping to mitigate the investment risk of large-scale tight oil reservoir development and increasing production efficiency. The method was then applied to six horizontal wells of the tight oil reservoir located in the Ordos Basin. The difference between the predicted and actual production was only 8.0%, thereby demonstrating the effectiveness of the method.

Comparative Study of Microseismic Event Location-fitted Stimulated Reservoir Volume Computation Methods

SHAO Yuanyuan, HUANG Xuri, XING Yang

2018, 40(4):  132-142.  DOI: 10.11885/j.issn.1674-5086.2017.08.01.01

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The microseismic event location-fitted computation of stimulated reservoir volume (SRV) was studied. The current methods available for computing microseismic event location-based SRV are not accurate because of the impact of noise. A three-dimensional hydraulic fracturing model was employed to simulate the development of principal fractures and the diffusion of fracturing fluids into the matrix. The existence of induced microseismic events was then determined by the existence of fractures and critical pressure in the pore space. Consequently, an SRV value was obtained by the hydraulic fracturing simulation. The SRV was then computed with different algorithms by fitting the point set of microseismic events, both with and without the abnormal points removed, and the resulting SRV values were compared with the SRV value obtained from the hydraulic fracturing simulation. The results showed that the removal of abnormal points reduced the impact of noise on the SRV values computed by all three microseismic event location-fitted algorithms. More specifically, the bin algorithm, although conservative, yielded an SRV value similar to that obtained by the hydraulic fracturing simulation and exhibited good stability and anti-noise characteristics. In contrast, both the three-dimensional Delaunay triangulation algorithm and minimum volume enclosing ellipsoid algorithm yielded SRV values quite different from the SRV value obtained by the hydraulic fracturing simulation; however, these algorithms were capable of clearly identifying the active zone of microseismic events mathematically and providing the geometric structure of SRV in a more detailed and quantifiable manner.

Effects of the Degree of Coal Metamorphism on CH₄ Adsorption Behaviors

TANG Jupeng, MA Yuan, TIAN Hunan, SUN Shengjie

2018, 40(4):  143-150.  DOI: 10.11885/j.issn.1674-5086.2017.05.29.02

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Coals with varying degrees of metamorphism (fat, coking, lean, and meagre coals and anthracite) were studied using macromolecular structural models and grand canonical Monte Carlo (GCMC) method, in order to analyze how the degree of coal metamorphism affects their CH₄ adsorption. The results indicate that anthracite has the highest capacity for CH₄ adsorption, followed by meagre coal, lean coal, coking coal, and finally fat coal. Regardless of the degree of metamorphism, the adsorption of CH₄ in coals is mediated by van der Waals and electrostatic interactions, and the stability of their adsorption states varies very little. The different coal samples show a linear decrease of CH₄ adsorption with increasing temperature, and the magnitudes of change are similar. CH₄ adsorption also decreases linearly with increasing water content, and this effect is more pronounced for lower degrees of metamorphism. When CH₄ and H₂O are both present, the adsorption of CH₄ on the coals becomes extremely small, and the adsorption process no longer fits the characteristics of isothermal adsorption described by the Langmuir adsorption model. Therefore, the gas content of coal is related to its degree of metamorphism. The water content also strongly affects the gas content, especially at lower degrees of metamorphism. It was also found that the effects of temperature on coal gas content are virtually independent of the degree of metamorphism.

Multicriteria Decision Making for a Shale Gas Water Resource System with Environmental Constraints Taken into Account

CHEN Yizhong, HE Li, ZHAO Honghai, LIU Jia

2018, 40(4):  151-161.  DOI: 10.11885/j.issn.1674-5086.2017.07.27.02

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A life-cycle-evaluation-based multicriteria planning model was developed for the Marcellus shale gas supply system to examine three of its component factors (economic benefit, water resource allocation, and greenhouse gas emission reduction) in two scenarios (high and low drilling production output). The research results show that gas output in the planned lifecycle for the high and low-production scenarios is 882.31 bcf and 472.40 bcf, respectively; the water to be consumed during the lifecycle is essentially for hydraulic fracturing and terminal electricity generation; wastewater treatment will be essentially realized by the on-site equipment, with the vast majority of the production wastewater to be recycled. The system's average greenhouse gas emission, economic benefit, and water consumption per unit of electric power will be 6.00 kg/MJ, 0.48 MJ^-1, and 0.10 gal/MJ, respectively. As supported by a multicriteria decision analysis using the TOPSIS method, the multicriteria planning avoids the strong bias of traditional single-criterion planning models and provides decision makers with information about the advantages and disadvantages of different planning models.

Simulation of the Effect of Corrosion Performance of Four Types Under CO₂-assisted Steam Flooding Conditions

SHI Shanzhi, DONG Baojun, ZENG Dezhi, YU Huiyong, CHEN Yuxin

2018, 40(4):  162-168.  DOI: 10.11885/j.issn.1674-5086.2017.05.02.02

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CO₂-assisted steam flooding is a novel method for heavy oil production. However, studies on CO₂ corrosion of downhole pipe in a high-temperature steam environment are relatively scarce. Therefore, in this study, the CO₂ corrosion behavior of oilfield pipe casing steel in a high-temperature steam environment was investigated using four types of oilfield pipe casing steel. A high-temperature, high-pressure autoclave was used to simulate the operating conditions during CO₂-assisted wellbore steam flooding. With a CO₂ partial pressure of 2 MPa and an operating temperature of 240℃, a weight loss coupon corrosion test was performed on four types of oilfield pipe casing steel, and the corrosion rates of the pipeline materials under the simulated operating conditions were obtained. Scanning electron microscopy, energy dispersive x-ray spectrometry, and x-ray diffraction were used to determine the morphology and composition of the corrosion products. The results show that under the experimental conditions, the uniform corrosion rates of the four types of materials were lower than the allowable corrosion rate for oil fields (0.076 mm/a). The corrosion morphology of N80 steel was uniform corrosion, while that of 3Cr, 9Cr, and 13CR steel were localized corrosion. The corrosion rates of all four steel materials met the corrosion control requirements for CO₂-assisted wellbore steam flooding.

External Corrosion of Buried Carbon Steel Pipelines in Oilfields

LIU Xiao, CHEN Yanhua, ZHONG Huiyuan, LI Liujun, CHEN Yu

2018, 40(4):  169-176.  DOI: 10.11885/j.issn.1674-5086.2017.05.05.04

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With regard to the relatively severe external corrosion of buried carbon steel pipelines in the Longhupao operation zone of the Daqing Oilfield, soil corrosivity was analyzed by determining the physical properties, chemical properties, and counts of sulfate-reducing bacteria of soil around coating defects on the buried pipelines. The morphology, elemental composition, and mineral content of corrosion products on the external walls of the pipelines were characterized and analyzed using techniques such as scanning electron microscopy-energy dispersive x-ray spectrometry, x-ray diffraction, and infrared spectroscopy. By means of potentiodynamic polarization curves and electrochemical impedance spectroscopy, the corrosion behavior on the external walls of buried carbon steel pipelines in soil extract solution was further investigated. The results indicated that the corrosion of buried carbon steel pipelines in the zone was caused by the wet saline-alkali soil environment as well as complex and varying soil properties. The corrosion products were mainly Fe₃O₄ and FeOOH, located in the inner and outer layers, respectively. The corrosion products contributed to cathodic reactions or acted as large cathodes, further aggravating pipeline corrosion.

Assessment of Particle Size Distribution Models for Black Powders in Natural Gas Pipelines

QIN Yunsong, ZHANG Jijun, AN Jianchuan, HUANG Xin, ZHENG Da

2018, 40(4):  177-186.  DOI: 10.11885/j.issn.1674-5086.2018.03.05.01

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Understanding the particle size distribution (PSD) of black powders in natural gas pipelines is critical to resolving the black powder issue. There are now many PSD models available; however, there is a lack of established methods for assessing them. In this study, seven common PSD models were assessed for their goodness of fit and prediction capacities, on the basis of black powder data of a real natural gas pipeline, by employing assessment indexes such as S_RMSE, R², and I^AIC as well as a confusion matrix and ROC curve. The results showed that the log-normal model not only is capable of both concentrated and even distribution, but also exhibits better goodness of fit. In addition, the log-normal model is capable of effective prediction in the full range of particle sizes (0.30~7.25 μm). Therefore, it is the PSD model with the most comprehensive prediction capability.