Table of Content

01 April 2017, Volume 39 Issue 2

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An Analysis of the Causes of Chang 2 Low Resistivity in Middle-western Ordos Basin

CHEN Shijia, GAO Xingjun, YU Jian, MA Jie, HUANG Hai

2017, 39(2):  1-8.  DOI: 10.11885/j.issn.1674-5086.2015.06.11.01

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The reservoir resistivity of the Chang 2 Oil Formation is too low to be identified in the midwestern Ordos Basin. The causes for the low resistivity are the basis for the analysis and identification of low-resistivity reservoirs. The study of the low resistivity of the Chang 2 Formation is therefore important. The relationships between each geological factor and the reservoir resistivity were analyzed in the study area to determine the main reasons for the low resistivity using the analysis of data from rock physics experiments and logging and oil test data. The results show that the low resistivity is mainly controlled by the highly irreducible water saturation, high salinity of the stratum water, and complex pore structure; however, it is not governed by the content of conductive minerals in the reservoir. Moreover, the clay mineral content and the small clastic grain size have a small effect on the reservoir resistivity.

Patterns of Barrier-interlayers in Mouth Bar of Fluvial Dominated Delta Reservoirs

WANG Xuying, JIANG Zaixing, YUE Dali, WANG Xin, WANG Xiabin

2017, 39(2):  9-17.  DOI: 10.11885/j.issn.1674-5086.2015.07.07.04

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Taking the Es₂⁸ Formation in the second district of Shengtuo Oilfield as an example, using the cores, dynamic monitoring data and dense well condition, this paper adopts the research methods of hierarchical control and dynamic fit. It analyzes the types, causes and architecture hierarchy of barrier-interlayers in the mouth bar. It also deeply discusses the position, shape, extending scale and distribution patterns of barrier-interlayers in different orders. The research shows that barrier-interlayers can be divided into three types:muddy barrier-interlayers, calcareous interlayers and physical barrier-interlayers. Five-order barrier-interlayers limit the composite mouth bar. Five-order barrier-interlayers which contain muddy barrier-interlayers, calcareous interlayers and physical barrier-interlayers are located at the top of the composite mouth bar. Muddy barrier-interlayers and physical barrier-interlayers are lateral stitching, sheet size and easy to form a stable barrier. Calcareous interlayers are discrete and non-continuous. Four-order interlayers limit a single bar. Four-order interlayers are located at the stacking part of single bars. Muddy, physical and calcareous interlayers of four-order extends along the top of the single bar. They are oblique crossing laterally and discrete. There-order interlayers limit the proliferative bodies. Muddy, physical and calcareous interlayers of there-order are multiple, thin, discrete and non-continuous. These interlayers distribute between the proliferative bodies and incline to the lake.

Boundary Fault of the Steep Slope Zone and Its Control on Hydrocarbon Accumulation of the Lower Minghuazhen Formation of Neogene in the Miaoxibei Sag of Bohai Basin

ZHAO Ye, WANG Xin, GUAN Dayong, LIU Pengbo, WANG Guangyuan

2017, 39(2):  18-26.  DOI: 10.11885/j.issn.1674-5086.2015.11.17.01

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Focusing on differential deposition and oil and gas accumulation in the shallow water delta of the Neogene Minghuazhen Formation in the steep slope zone of the Miaoxibei Sag, Bohai Bay Basin, this study analyzes the boundary fault combination style, fault activity, and influence of datum level change on sandstone stacking style and differences in oil and gas accumulation. The analysis showed that the formation is a favorable combination of hydrocarbon accumulation near source rocks, fault transporting, reservoir development, and late accumulation in the Neogene of the Miaoxibei Sag. The structures of the depression boundary faults, from the southern to the northern blocks, are seat, horsetail, and cascade types. Differences in paleogeomorphology and intensity of fault activity determine horizontal reservoir development in the northern blocks, and datum level change determines sandstone stacking style in the vertical direction. "Isolated" underwater distributary channel sand bodies developed in the early stage of the lower Minghuazhen Formation, sandstone reservoir with "laminations" developed widely in the middle stage, and in the late stage, the sand body is distributed with "similar sheets". The formation factors for integrated oil and gas accumulation are highest in the north blocks, good in the middle blocks, and poorest in the southern blocks.

Characteristics and Controlling Factors of Dolomite Reservoirs in the Ma5₅ Sub-member of Ordovician in Majiagou, Ordos Basin

YANG Xiyan, BAO Hongping, REN Junfeng, MA Zhanrong

2017, 39(2):  27-34.  DOI: 10.11885/j.issn.1674-5086.2015.09.07.01

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In this study, we analyzed the characteristics and controlling factors of dolomite reservoirs based on core survey, well logging analysis, thin section and SEM identification, and analysis of physical properties. We found that the reservoir rock mainly consists of siltcrystal dolomites and siltcrystal-micrite dolomites. The main types of reservoir spaces are intercrystalline pores and dissolution pores-caves with low porosity and low permeability. At Ma5₅, dolomitic flat, lime mud depression, dolomitic depression, and dolomitic-lime mud depression facies are developed. The dolomite belt of the dolomitic flat is controlled by sedimentary microfacies, with the thickest dolomites of the best physical properties widely distributed in Jinbian County. The location of high quality reservoir is controlled by dolomitization:some intercrystal micro-pores were formed by penecontemporaneous dolomitization; intercrystal pores were formed after penecontemporaneous dolomitization; and intercrystal pores and emposieu by buried dolomitization. Reservoir effectiveness is controlled by dolomitization subfacies; the best subfacies are boundary-interim subfacies. Most reservoir spaces formed by epigenic karstification and burial dissolution are filled by vadose silt, calcite, and quartz.

Pore Structure Characteristics of Taiyuan Formation(C₃t) in Eastern Uplift of Liaohe Depression

ZHANG Qin, PANG Zhenglian, LIU Renhe, MAO Junli

2017, 39(2):  35-42.  DOI: 10.11885/j.issn.1674-5086.2015.10.22.02

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This paper uses the Taiyuan Formation in the Eastern Uplift of the Liaohe Depression as an example to study transitional shale reservoir structures. Cores from well T2905 were observed and described, and samples systematically collected. XRD, high pressure mercury injection, nitrogen adsorption, and field emission scanning electronic microscope(FESEM) methods were adopted for analysis of pore characteristics including pore size, pore shape, and pore structure. Data from high pressure mercury injection showed a bimodal pore size distribution with a higher peak in the 10-100 nm range and a lower peak in the 10 000-100 000 nm range. Nitrogen adsorption can provide information on much smaller pores, so can help to precisely partition pores under 100 nm. Results from nitrogen adsorption indicated that mesopores(10-50 nm) provided the largest pore volume, accounting for 33.48%-43.96%. According to BET analysis, smaller pores provide less specific surface area. Extremely small pores(<2 nm) and micro-pores(2-10 nm) were the major contributors to specific surface area, accounting for 82.92%-91.58%, with an average 87.36%. The results from single factor analysis demonstrate that the specific surface area is not controlled by organic matter content but by clay mineral content, and the coefficient index is 0.901. The hysteresis loop from nitrogen adsorption and desorption can be used to determine pore shape; parallel-plated pores and slit-shaped pores were the main types identified. Such open nano-pores can improve shale gas desorbing efficiency and reservoir permeability, and enhance shale gas production.

The Forming Mechanism and Geological Significance of Diagenesis Tight Zone in Saline Lacustrine Basin

LI Jilin, ZHANG Tingshan, LIU Peng

2017, 39(2):  43-52.  DOI: 10.11885/j.issn.1674-5086.2015.12.26.02

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Under the influence of an alkali(first)-acid(second) diagenetic environment, tight reservoirs developed in the early stage of diagenesis in the saline lacustrine basin. To reveal the formation mechanism and petroleum geological significance of the diagenesis tight zone, we took Es₄^s of Bonan Sag as an example and analyzed physical property, casting sheet, and mineral X-ray diffraction data. Research has shown that the cementation rate of carbonate cements in the diagenesis tight zone of E₄^s is greater than 0.5 and the compaction rate is less than 0.2. Primary cores were lost due to strong cementation in the early stage of the alkaline diagenetic environment; the contact area between organic acid and soluble minerals reduced later, and the occurrence of large-scale dissolution lagged in time. Therefore, the diagenesis tight zone was formed at 2 800-3 200 m. Reservoir porosity in the diagenesis tight zone was generally less than 12%, hence, the more favorable reservoir is located above and below the diagenesis tight zone, in delta front, mixed sedimentary rock beach-bar, and near-shore subaqueous fan front environments. The inferior physical properties of the diagenesis tight zone not only stops secondary migration of hydrocarbons, so that hydrocarbon accumulation modes differ above and below the zone, but also causes the degree of hydrocarbon enrichment and deliverability to differ in the inner and outer parts of the zone.

An Experimental Study on the Effects of Shale Stratification and Hydration on the Transmission of Acoustic Waves

FAN Xiangyu, DUAN Meiheng, ZHANG Qiangui, ZHAO Pengfei, JIA Guangsheng

2017, 39(2):  53-61.  DOI: 10.11885/j.issn.1674-5086.2016.01.08.01

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Studies on the acoustic characteristics of rocks are the groundwork for geomechanical logging and the evaluation of reservoir properties, and the acoustic properties of shale are especially affected by its stratification and hydration. Hence, we have performed ultrasonic transmission tests on stratified shale with varying levels of hydration, to study the characteristics of the soundwaves' transmission speed and attenuation of energy under these circumstances. We have found that as the stratification angle increases, the P- and S-wave velocities and the P-to-S-wave velocity ratio decrease, while the attenuation coefficients of the P- and S-waves increase. Furthermore, as hydration increases, the velocity of P-waves first increase, then decrease at higher levels of hydration, whereas its attenuation coefficients are inversely correlated. The velocity of S-waves decreases slightly at lower levels of hydration, and basically remains unchanged at higher levels of hydration, while its attenuation coefficients are not significantly affected by hydration. A comparative analysis of the normalized results demonstrates that stratification angles have a greater effect on P- and S-wave velocities than hydration.

Innovative Geosteering Strategies for the Sustainable Development of Mature Oilfields

TAO Bin, LI Lin, CHANG Botao, WANG Chao, HE Jingbo

2017, 39(2):  62-70.  DOI: 10.11885/j.issn.1674-5086.2015.08.30.01

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Based on the overall goals of geosteering in horizontal wells, we have proposed an innovative strategy for geosteering based on boundary detection and borehole imaging techniques, to resolve uncertainties in subsurface structure, and stratum and fluid characteristics. This strategy allows highly-effective control of the bore to produce smooth bore trajectories, and accurate steering of the bore within a stratum to allow horizontally-drilled bore trajectories to proceed as closely as possible to the ceiling of a reservoir (within 0-1 m of the ceiling). Successful implementations of this strategy in various types of oil reservoirs have shown that this innovative geosteering strategy can improve well-drilling effectiveness, decrease costs, and contribute significantly towards the production optimization; with this strategy, 2000-6000 barrels were produced a day from an oil well, with extremely low levels of water content, and the cumulative production of the oil field has exceeded initial estimates by 17%. Hence, we have effectively optimized the recovery rate of oil fields by avoiding "attic" oil and postponing the use of waterflooding in a well. Furthermore, the updating of post-drilling geological oil reserve models may provide an effective form of guidance for the optimization of well deployment and development strategies in the future.

The Technology and Application about the Complex Reservoir Horizontal Well Geological Steering in the Sichuan and Chongqing Basin

ZHANG Shudong, WU Baoyu, TIAN Yunying, GUO Tingliang, LI Honggang

2017, 39(2):  71-80.  DOI: 10.11885/j.issn.1674-5086.2015.07.23.01

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With the development of oil field in the late stage of oil and gas exploration and development, more and more complex geological conditions, mining difficulty is increasing, the complex reservoir conditions especially in low porosity and permeability, thin, fractured and cave reservoir of oil and gas exploration and mining is subject to the attention of the educators in Sichuan and Chongqing oil, become a hot and difficult research. Practice has proved that the horizontal well geological guiding technology is an effective means to solve the above problems. This paper aiming at the complex geological objects in Sichuan and Chongqing area, starting from the complex reservoir geological characteristics, geological orientation difficulty, sort out the common problems in the field geological guidance work. The technical countermeasures are summarized in the view of the nature. In order to solve the technical key points of this kind of reservoir in actual geological guidance. Taking Chongqing gas field in East Sichuan steep and complex structure with Yunan XX well as an example, Comprehensive use of logging, drilling, geology, earthquake and other information, Implementate Pre-drilling design, geological modeling, real time guidance, Close combination of Geology and Engineering, Greatly improve the reservoir drilling rate, Final oil production gas up to 55×10⁴ m³/d, it is the highest production wells in the region, the field application effect is obvious.

A Review on Wellbore Stability with Multi-Field Coupling Analysis

HE Shiming, CHEN Yulin, MA Dexin, ZHOU Jun, WANG Wei

2017, 39(2):  81-92.  DOI: 10.11885/j.issn.1674-5086.2015.05.16.03

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Wellbore stability is greatly affected by various factors, such as the formation fluid, and the properties and temperature of the drilling fluid, and is critical to petroleum drilling. By summarizing the actuality of wellbore instability analytical studies both locally and globally, the influencing factors of wellbore instability during drilling were divided into five fields:rock structure, stress, seepage, temperature, and chemical. Their detailed contents and causes were analyzed, based on which the impact of multi-coupling on wellbore instability was systematically analyzed. The stress and rock structure fields were the basis for the research on multi-coupling analysis of wellbore instability. The rock structure types and rock deformation mechanism were classified by summarizing the research methods of wellbore instability both locally and globally. The three-field, fourfield, and five-field coupling wellbore stability studies was based on the rock structure and stress fields combined with three other fields; pore medium-thermoelastic theory and pore medium-chemical-thermoelastic theory have been widely used in fourfield and five-field coupling analysis. To conclude, the development tendency of multi-coupling analysis was explored with the features of different fields and the development of drilling technology considered.

Comprehensive Limits of the Movable Reserves in Fractured-Porous Sandstone

FANG Jianlong, PENG Xian, LIU Linqing, YE Liyou

2017, 39(2):  93-98.  DOI: 10.11885/j.issn.1674-5086.2015.06.23.03

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The identification of limits in the movable reserves of a reservoir is the basis for the determination of its movable and immovable reserves. Through the simulation of fractured gas reservoirs using equivalent medium models, and evaluations based on the economic limits of single wells, we have identified the comprehensive limits of movable reserves in reservoirs with varying levels of fracture development in the XU-2 fractured porous sandstone gas reservoir of Pingluoba in Western Sichuan. The results of this study have shown that the properties of the matrix have the most significant impact on the cumulative gas production of a single well, whereas the fracture angle only has a very small effect. The influence of fracture density on the cumulative gas production of a single well was found to increase with decreases in matrix permeability. The comprehensive limits of the movable reserves also appear to decrease as natural fractures develop further. In reservoirs without developed fractures, the comprehensive limits of its movable reserves are as follows:its lower limit of permeability under overburden pressure is 0.008 mD, while its upper limit of water saturation is 42%, and its lower limit of porosity is 6.4%.

Impact of Temperature on Relative Permeability of Heavy Oil/Hot Water

SUN Baoquan

2017, 39(2):  99-104.  DOI: 10.11885/j.issn.1674-5086.2015.08.04.02

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There is limited research on the oil-water relative permeability of thermally recovered heavy crude oil reservoirs, and the oil-water movement rules have not been clarified. Thus, this experimental study focuses on the relative permeability of heavy oil/hot water at different temperatures (100~280℃); explores the rules concerning the impact of end-point saturation of the relative permeability curve and the corresponding single-phase permeability of heavy oil/hot water; and analyses the mechanism of the variation law from a microscopic perspective. Experimental results suggest that the bound water saturation increased initially and then decreased as the temperature rose; the oil-phase permeability gradually declined under the bound water condition; and the residual oil saturation was observed to decrease, whereas the corresponding water-phase relative permeability gradually increased. The water-phase permeability was lower under the residual oil condition with different temperatures. Studies on the variation laws of relative permeability of heavy oil/hot water at various temperatures will deepen the understanding of fluid flow characteristics of thermal recovery reservoirs.

Fractal Model of Carbonate Acidized Wormhole and Acidizing Parameter Optimization

ZHANG Hewen, ZOU Honglan, YAN Xuemei, CUI Mingyue, JIANG Weidong

2017, 39(2):  105-110.  DOI: 10.11885/j.issn.1674-5086.2015.08.06.02

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Carbonate matrix acidizing mainly features acid wormholes that are deemed to be unordered and disorganized in Euclidean space. It is difficult to conduct experimental simulations and mathematically describe wormholes using classic mathematical methods; however, an accurate description of the penetration depth of acid wormholes is of significance for construction parameter optimization and acidizing effect improvement. To this end, multiple groups of core acidification flow simulation tests were carried out first in this study, using CT scanning to determine the true forms of the wormholes. The term "box dimension" was introduced to verify the wormhole fractal properties and to calculate the Hausdorff fractal dimensions of different wormhole forms. Test results indicate that the wormholes in this test featured favorable fractal properties; fractal dimensions varied from 1.19-1.98 as solution features changed; and the dimension corresponding to the optimum capacity was approximately 1.6. Moreover, a combined method of laboratory testing and fractal geometry was employed to establish a fractal calculation model for the equivalent length of acidized wormholes, and to conduct a sensitivity analysis of factors affecting the wormhole's equivalent length.

Research on Enhancing Heavy Oil Recovery by Steam-Air Flooding

YANG Jian, LI Xiangfang, YAN Yiqun, ZHANG Baorui, ZHANG Xiaolin

2017, 39(2):  111-117.  DOI: 10.11885/j.issn.1674-5086.2015.09.22.01

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In order to enhance the recovery of heavy oil, we carry out in-door experiment and numerical simulation of steam-air flooding. In-door experiment with different mass ratios of air and steam revealed that heavy oil recovery increased first then decreased as the mass ratio of air and steam increased. When the mass ratio of air and steam was 2:32, the oil recovery was the highest, which was up to 68.67%, and temperature at the end of the sand packed tube was almost the same as it was with steam flooding. When the mass ratio of air and steam exceeded 3:32, the oil recovery of steam-air flooding was less than that of steam flooding, and the temperature at the end of the sand packed tube was much lower than that with steam flooding. Numerical simulation of X Block in Liaohe Oilfield by CMG produced similar results as in in-door experiment, although there existed differences in the results due to differences between in-door experiment and the oil field simulation. Through the field pilot in X Block of Liaohe Oilfield, we found that when the mass ratio of air and steam is 1:16.1, oil production rate increased while the water cut decreased with steam-air flooding which proves the accuracy of the in-door experiment and numerical simulation results.

Imbibition Mechanism of Hydraulic Fracturing in Shale Gas Reservoir

LEI Zhengdong, QIN Bin, LIU Shuangshuang, YU Tao

2017, 39(2):  118-124.  DOI: 10.11885/j.issn.1674-5086.2015.03.11.05

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To better understand the imbibition behavior in shale reservoirs during production and hydraulic fracturing operations, we investigated the imbibition mechanism and evaluated the formation damage resulting from imbibition. This paper first presents a hydro-mechanical model for a shale gas reservoir with consideration for multiple flow regimes, gas diffusion and desorption, stress sensitive effect, and capillary pressure. Then the formation damage caused by the imbibition mechanism is evaluated by quantifying facture face skin evolution during fracture cleanup and subsequent production. The simulation results indicate that (1) the imbibition has a huge influence on reservoir performance in well tests and production periods, and a high capillary pressure is the main cause behind the imbibition phenomenon and water blockage around hydraulic fractures; (2) it is possible to obtain the original gas pressure by detecting the fracture pressure of new wells with hydraulic fracturing stimulation; (3) formation damage caused by wetting phase trapping is one of the main causes impairing well productivity hydraulic fracturing of tight gas reservoirs, which should not be neglected. This research provides a theoretical foundation for a better understanding of reservoir performance of shale gas, especially for optimizing production by reducing formation damage caused by imbibition at an early period.

Experiments of CO₂ Huff-n-puff Process in Staged Fracturing Horizontal Wells for Developing Tight Oil Reservoirs

ZHOU Tuo, LIU Xuewei, WANG Yanli, QIN Chunguang, GAI Changcheng

2017, 39(2):  125-131.  DOI: 10.11885/j.issn.1674-5086.2015.11.23.03

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Using large physical simulation equipment and outcrop plate model, an experimental study on CO₂ huff-n-puff in staged fracturing of horizontal wells was carried out. The experiment results show that CO₂ huff-n-puff can effectively improve oil recovery of tight oil reservoir. In addition, the injection pressure is an important factor affecting CO₂ huff-n-puff effect. By analyzing pressures at different model locations, production and other key parameters, we clearly define the stratigraphic energy supplement features of CO₂ huff-n-puff. The result of the research has great significance in the effective development of tight oil reservoirs.

Study on the Establishment of Material Balance Model for Fractured Horizontal Well in Shale Gas Reservoir

GUO Xiaozhe, LI Jing, ZHANG Xin

2017, 39(2):  132-138.  DOI: 10.11885/j.issn.1674-5086.2015.08.05.02

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To solve the difficulty in calculating the contributions of fractures, matrix, free gas, and desorbed gas to gas output, the establishment of and an analytical study on static seepage sub-regions were carried out in this work. A dynamic seepage sub-region was designed. A calculation method for reservoir water saturation under the effect of imbibition after fracturing fluid flow back was determined by calculating the propagation speed of the pressure drop and the average formation pressure in different seepage zones, with an integrated material balance equation consisting of 12 items. A total of seven seepage stages of gas well production was obtained by means of model case application and analysis. This revealed the variation law of air supply structure and proportion during different production times and the fact that it was the free gas of the matrix reservoir; and that desorbed gas was the major factor that maintained long-term production of gas wells. Establishment of the model simplified the description and solution procedure of the complex multi-dimensional seepage equation. This provides a new approach for theoretical analysis and field application, as well as a basis for in-depth understanding of and study on seepage characteristics of shale gas reservoirs.

Application of Slip Lag Time in Discrete to the Calculation of Gas upward Velocity in Deep Well

HUANG Zhiqiang, OU Biao, KONG Xiangwei, WANG Xingyu, LIN Yuanhua

2017, 39(2):  139-144.  DOI: 10.11885/j.issn.1674-5086.2015.06.24.01

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Considering the gas slipping velocity, the flow characteristics at the stopping and starting pumps are studied, and the method of slipping lag time in discrete is proposed for calculating the speed of gas slip in deep wells. The time sliding mesh algorithm is proposed in a multiphase flow, which is used to solve for the gas slipping velocity, and the data are calculated using a cubic spline function. Corresponding calculations are performed, and a control software is designed. We applied this mathematical model to a well in Yuanba Sichuan, and obtained results that met operational requirements. The gas remains in a supercritical state in the deep well, and the gas slippage is not serious and has little effect on the gas upward velocity. When the state of the gas changes from a supercritical state into a gas near the wellhead, substantial gas slippage occurs. If the gas slippage is not considered, the error rate is up to 76.36%. It was observed that with an increase in the pump stopping time and gas monitoring time, as well as a decrease in lag time, and that of the reservoir top boundary, upward velocities of the gas tend to decrease.

Finite Element Simulation of Drilling Processes Using Hybrid Bits

DENG Rong, AN Mei, TANG Dong

2017, 39(2):  145-152.  DOI: 10.11885/j.issn.1674-5086.2015.12.19.35

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This paper discloses a novel design of a roller cone and a polycrystalline diamond compact (PDC) hybrid drill bit constructed with two roller cones plus two blades. The design includes two types of constructions:cone-leading(CL) and blade-leading(BL) types. The physical processes for drilling tuff and granite using two differently constructed hybrid bits are numerically simulated using an explicit dynamic finite element analysis method. The experimental results demonstrate that the CL-type drill bit proves suitable for drilling hard formations, whereas the BL type can be used for soft earth formations. The axial forces exerted on the roller cone cutting teeth of the two bit types when drilling hard rocks remain larger than those in the case of drilling soft rocks. Furthermore, the CL-type bit exhibits larger imposed axial forces than the BL type when drilling the same rocks. The distribution pattern of the axial loads imposed on the PDC cutters along the radial direction of the bit is very similar to the shape of the blade crown segment, where the maximum axial load occurs at the conic crest. Moreover, the allocation of the weight-on-bit (WOB) on the roller cones and blades is related to the types of bit constructions. The above conclusion deepens the understanding of drilling processes using hybrid bits, which provides useful guidance for the design and on-site selection of drill bits.

Development and Application of Large Scale Visible Physical Simulation Device for Acid Fingering

LI Xiaogang, SONG Rui, ZHANG Rusheng, YANG Zhaozhong, LIN Xin

2017, 39(2):  153-162.  DOI: 10.11885/j.issn.1674-5086.2015.09.29.30

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In pad acid fracturing, low viscosity acid leads to a fingering phenomenon in the high viscosity pad fluid and has a great influence on the result of acid fracturing; judging from former studies, it can only be analyzed through simulation. However, present models differ significantly from the real acid fracturing process. In view of this, a large-scale physical visible acid fingering simulation device was designed and manufactured based on similarity criterions and similarity scales presented by the author. Compared with the physical acid fingering simulation devices of p_redecessors, this simulation device structure featured three plates and double slits capable of studying fingering behaviors under different viscosities, densities, and temperatures and performed acid rock reaction, acid fluid loss as well as low-and medium-pressure acid fingering visual simulation under different fracture inclinations. Simulations of acid fingering were performed, and it was found that acid fingering forms were so complex that quantitative description based on qualitative research was required to reveal the evolution rule.

Simulation and Error Analysis for Pipeline Internal Positioning Based on Integrated Navigation

WANG Zegen, TAN Jing, WANG Jinzhu

2017, 39(2):  163-171.  DOI: 10.11885/j.issn.1674-5086.2015.03.06.06

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The separate application of the current pipeline internal positioning methods may fail to satisfy the actual accuracy requirements of pipeline works. Therefore, an effective method of improving the positioning accuracy is to employ the integrated navigation technology. The existing research on the pipeline internal inspection and positioning technology in China and abroad mainly focuses on a multi-theory and multi-method combination. Hence, this study proposes a positioning method based on integrated navigation, which consists of odometers and the Strap-down Inertial Navigation System (SINS), according to the internal positioning characteristics of the oil and gas pipeline. Furthermore, the simulation models of the trajectory simulator, the SINS device, and the odometer based on the Kalman filter data fusion technology are established. In addition, a simulation experiment is conducted, and the error analysis for the integrated positioning technology is performed, thereby achieving a 0.03% positioning accuracy without auxiliary equipment, such as land marks and fixed-point magnetic markers.

Influence of Production Parameters on Asphaltene Precipitation Regularity in Wellbore

WU Chuan, ZHANG Rusheng, ZHANG Zuguo, SHE Yuehui, WU Zhengbin

2017, 39(2):  172-178.  DOI: 10.11885/j.issn.1674-5086.2015.05.19.04

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This study investigated the influence of production parameters on asphaltene precipitation regularity under the conditions of high temperature and high pressure and evaluated the influence of temperature, GOR (gas-oil ratio), and flow velocity on asphaltene precipitation in wellbores. The results showed that the pressure of asphaltene precipitation (p_onset) was inverse to temperature and the pressure of asphaltene redissolving (p_red) was proportionate to temperature. p_onset was proportionate to the GOR at a certain temperature because the dissolved gases in the oil increased when the GOR improved and may have caused asphaltene precipitation. p_onset and p_red were not influenced by flow velocity in certain GOR and temperature ranges. This study demonstrated how asphaltene precipitation regularity can be controlled to decrease the risk of asphaltene precipitation in wellbores.

Synthesis and Performance of Partially Branched and Partially Crosslinked Polyacrylamides

ZHU Yangwen

2017, 39(2):  179-184.  DOI: 10.11885/j.issn.1674-5086.2017.01.13.02

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Mechanical degradation during polymer flooding causes the viscosity of the polymers to decrease, thus affecting the ultimate recovery rate. To improve the working viscosity and shear stability of the polymers, we rationally designed molecular structures by introducing dimethylaminoethyl methacrylate (DMAEMA) as a functional monomer into these molecular structures. A series of partially branched and partially crosslinked polyacrylamides were prepared via conventional free radical polymerization in aqueous solution, using ammonium persulfate and DMAEMA as the initiating system. In particular, the secondary carbons adjacent to the nitrogen of DMAEMA can act as active points to induce the formation of side chains through free radical polymerization. The terminal free radicals of these chains then terminate through coupling to form crosslinked structures. Infrared spectroscopy was used to study the structure of the partially branched and partially crosslinked polyacrylamides, and the effects of different initiator ratios on the performance of the polymers were investigated by examining the polymer properties. The results indicated that the use of DMAEMA as a reducing agent in the polymerization causes partial crosslinking within the polymer structure, and consequently, the viscosity and shear stability of the polymers increase with the DMAEMA content. However, as DMAEMA also acts as a chain transfer agent, the molecular weight of the polymerization products decreases when excessive quantities of DMAEMA are used, which in turn decreases the shear stability.