Table of Content

01 August 2017, Volume 39 Issue 4

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The Structural Features of Wanjinta and Its Control Effect on Deep Gas Reservoir

LIU Yuhu, CAO Chunhui, LI Ruilei, HAN Shuxia, WANG Xiaofeng

2017, 39(4):  1-12.  DOI: 10.11885/j.issn.16745086.2016.04.14.01

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The Wanjinta area is located in the step fault system at the western edge of the Dehui fault Depression. The structural features and deformation mechanism of the area are closely related with hydrocarbon accumulation. The structure, fault system features, periods of fault activity, and the stress fields of the WanJinta area were analyzed by employing information from regional geology, drilling, and three-dimensional seismic resolution data. Two geological sections that cross the study area were selected for analysis, and 3D move-balanced sectional-structure recovery technology was used to recover and remodel the tectonic evolution history. Based on the fracture reservoir-control function, the average-weighted algorithm was used to establish the corresponding mathematical evaluation model. The "three-parameter evaluation method" was proposed to evaluate quantitatively the activity level of the deep fracture. Research shows that the WanJinta area has been subject to transformation adjustment following polycyclic tectonic movement. The area has a complex superimposed structure and forms two fracture systems, with deep rift-basement faults and a shallow reverse-fault system. The deep fault indicates deep formation change characteristics along segments of the deep part, and the shallow part shows the characteristics of section change. The level of fault activity in the reverse-fault slip region is relatively high, controlling the enrichment of the CO₂ gas reservoir in the plane. However, there could be reservoirs enriched in organic hydrocarbon gas in areas that are located far away from the deep faults subject to high-level activity, with little impact from a second fold structure superimposed fracture adjacent to the Shahezi Formation hydrocarbon source rocks.

Reservoir Characteristics of Organic-rich Mudstone of Niutitang Formation in Northern Guizhou

QIN Chuan, YU Qian, LIU Wei, YAN Jianfei, ZHANG Haiquan

2017, 39(4):  13-24.  DOI: 10.11885/j.issn.16745086.2016.05.12.01

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A complete analysis was conducted of the reservoir characteristics of the organic-rich mudstone of the Niutitang Formation in Northern Guizhou, China. The rock core of seven shale-gas survey wells and four geology shallow wells was sampled and various experimental tests, such as mineral and geochemical analyses, porosity and permeability measurements, and hydrocarbon content analysis were carried out. The results show that the organic-rich mudstone of the Niutitang Formation develops mainly in a deep-water shelf facies. The rocks are mainly silty carboniferous and silty carbonaceous mudstone. The brittle minerals are mainly quartz-feldspar combinations. The content of other authigenic brittle minerals, such as calcite, dolomite, and pyrite is generally low. The clay mineral content, mainly illite, is low but the variation range is large. The overall organic carbon content is high (1.5%～15.7%), and the organic matter has a high level of maturity (1.74%～3.40%), with a higher evolution level. The average density of the reservoir rock is 2.52 g/cm³, and the BET specific surface area is 0.33～33.76 m²/g. The main reservoir spaces are interparticle pores and intraparticle pores. The development of the organic holes is inferior and there is a lack of micro-fractures. The characteristics of the reservoir point to low porosity and low permeability, as well as low porosity and ultra-low permeability. The total gas content is 0.02～2.05 m³/t. Field gas-component analysis shows the N₂ and H₂ content is higher than is the CH₄ content.

Sedimentary Characterization of Paleogene Period, Fan Delta Front, Liaohe Western Depression

WANG Jue

2017, 39(4):  25-35.  DOI: 10.11885/j.issn.16745086.2016.04.01.01

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The sedimentary system of A reservoir in the Liaohe Western Depression was analyzed to clear up the uncertainty about the formation mechanism of the sand body in the study area. The characteristics of sedimentary facies, plane distribution, and the space-time evolution process are described in detail based on abundant data on the rock core logging and granularity. The results indicate that the fan-delta front subfacies reservoir developed in the study area can be divided into five microfacies, namely, the underwater distributary channel, distributary mouth bar, underwater distributary inter-channel sand, underwater distributary inter-channel mud, and frontal sheet sand. The underwater distributary channel is the main sedimentary microfacies and has a banded distribution in a northwest-southeast direction. This microfacies is affected by the water withdrawal cycle, with the base level descending generally. From bottom to top, the y₂₄^I sublayer shows a process of base level rise and fall, the y₂₃^I layer shows a process of base level rise, and the y₁₂^I layer shows a process of base level fall. The heterogeneity of the reservoir is controlled by the distribution of sediment, as well as evolution. Detailed research results of the sedimentary system provide a solid geological foundation for facies-controlled modeling and further study on the distribution of the remaining oil deposit. This information provides a critical basis for the adjustment of the development method.

Microscopic Characteristics of Pore-throat Structures of the Chang 6 Member Tight Oil Layer in the Huaqing Region

XU Liming, NIU Xiaobing, LIANG Xiaowei, CHEN Yajuan, MA Jiye

2017, 39(4):  36-46.  DOI: 10.11885/j.issn.16745086.2016.03.28.03

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Structural characteristics of microscopic pore-throats of the Chang 6 Member tight oil layer of the Triassic Yanchang Formation in the Huaqing Region, Ordos Basin were studied using techniques including constant-speed pumping and microand nano-CT scanning. Results show that the pore spaces in the Chang 6 Member were predominantly microscopic, while nanoscopic pore spaces were less dominant. The mainstream throat radius was 1.07 μm, average throat radius was 0.84 μm, and pore radius was 140.12 μm. The average pore-to-throat ratio was 281.03, representing significant micro/nanoscopic porethroat characteristics. The relationships of pore infiltration with throat radius and pore-to-throat ratio show that pore-throat characteristics are the key factors that influence the physical properties of the tight oil layer. The effects and limitations of porethroat parameters on infiltration rate are more significant than those of porosity. Micro/nano-CT scans on tight oil layers indicate the presence of complex and interconnected micro/nanoscopic pore-throat networks. The three-dimensional reconstruction of micro/nanoscopic pore space reveals that nanoscopic pores are mainly oval, elongated, or irregular, whereas their microscopic counterparts are circular, oval, triangular, and irregular. Tight oil is located in micro-cracks and in the clustered and spherical pore-throat structures with relatively larger pore spaces.

Depositional Characteristics of the Zhaojiazhuang Formation, Changcheng System, in the Middle–Southern Areas of the Taihang Mountains

WANG Qingchun, HE Ping, DU Jiangmin, LIU Rongfang, LI Wei

2017, 39(4):  47-56.  DOI: 10.11885/j.issn.16745086.2016.03.22.04

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As Precambrian sedimentary strata have likely been rigorously transformed due to metamorphism over a long period of geological time, studies on this sedimentary process are often challenging and yield unconvincing results. Employing a detailed field geological profile analysis, combined with an analysis of the regional geological background, we identified rock types and sedimentary structures, and conducted a comprehensive analysis of lithofacies and palaeogeography. Our results showed that the terrestrial→marine sedimentary facies in the Zhaojiazhuang Formation, Changcheng system of the mid-southern areas in the Taihang Mountains were formed mainly by alluvial fan facies→tidal flat facies. The boundary between the top and bottom layers was clearly visible and identifiable. The complete stratigraphic sequence of the Zhaojiazhuang Formation can be divided into four sections:coarse clastic rock, sandstones/shale interbed, shale, and layered limestone reef. The rock types are abundant, and the stratigraphic sedimentary characteristics are prominent. These features are significant indicators of the sedimentary environment and can be used by geologists to study the Precambrian sedimentary environment and early paleontology.

Play and Hydrocarbon Potential of the Khorat Plateau Basin in Thailand

WANG Jun, BAO Zhidong, WU Yiping, YANG Yichun, HE Lingyuan

2017, 39(4):  57-70.  DOI: 10.11885/j.issn.16745086.2016.02.28.02

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Aiming to address the low degree of exploration and lack of data, we have studied play and resource potential using the Khorat Plateau Basin, Thailand, as an example. Under the framework of the progressive control of tectono-sedimentary evolution on hydro-carbon accumulation, several conclusions have been drawn as follows. First, from the Neopaleozoic to the Cenozoic, the basin was divided into two tectonic sequences:TS1 and TS2. Nearshore, shelf, and carbonate platform sediments of shallow water facies are filled within the framework of TS1; alluvial fan, fluvial, and lacustrine sediments of continental facies are filled within the framework of TS2. There are two main source-reservoir-cap rock combinations in the basin, controlled by sedimentary facies distribution. Considering the major trap types and characteristics of the reservoir-cap rock assemblage in the basin, two first order plays and four second order plays are further differentiated. In the future, gas exploration should be directed at the proven Permian first order play. Horizontally, special attention should be paid to the regions favorable for the exploration of petroleum.

Study on Spatial Distribution of Calcite-cemented Strips Under Close Well-spacing Conditions

WU Qiongyuan, WU Shenghe, QIN Guosheng, CHEN Cheng, ZHANG Jiajia

2017, 39(4):  71-80.  DOI: 10.11885/j.issn.16745086.2016.03.24.03

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Using data from casting sheet images and physical property analyses, this study investigates calcite-cemented strips in compact sandstones of low-permeability under close well-spaced conditions, as exemplified by the Chang-8 oil formation in the Huaqing area of the Ordos Basin. Specifically, their basic characteristics are analyzed, the controlling factors for the development of them at different locations are identified, and a spatial distribution model for calcite-cemented strips under close well-spacing conditions is built. The results of the study show that calcite-cemented strips were widely developed in the sandstones of the Chang-8 oil formation in the Huaqing area, and mainly comprise late-stage ferrous calcites. The primary controlling factors for their development vary with the location of development. In particular, their development is primarily controlled by granularity, lateral adjacent mud shales and feldspar corrosion in the middle, and by the dispositional relationship between granularity and degree of mud shale development at the top and bottom. Their vertical distribution is controlled jointly by a combination of sedimentary microfacies and a configurational interface. Horizontally, they mainly take on a crumby structure and have a zonal distribution at the end of the distributary river way, far away from the provenance in the middle of the study area, and in the middle of the nearby estuary dam. Overall, the spatial distribution of calcite-cemented strips under the close well-spacing condition is characterized as follows:(1) in the thick-layer coarse-grained sands, calcite cementation is thick and far-stretching at the bottom and top, but thin and near-stretching in the middle; (2) in the thin-layer sands, calcite cementation is thin and near-stretching.

The In-situ Stress Field Distribution Numerical Simulation of No.3 Coal Seam in the North of Fanzhuang CBM Well Blocks

YIN Shuai, DING Wenlong, GAO Mindong, ZHOU Guangzhao

2017, 39(4):  81-89.  DOI: 10.11885/j.issn.16745086.2015.06.02.02

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To develop effective measures for improving the fracture net of the No. 3 coal seam in the northern Fanzhuang wellblocks, we analyzed Permo-Carboniferous strata tectonic patterns and fault characteristics of the CBM well blocks using 2-D seismic data. Accordingly, the stress field characteristics of the No. 3 coal seam in the Shanxi Formation were analyzed using multiple approaches, including the fracturing method, image logging, and finite element simulation. The results show that values of the three principal stresses follow the order σ_H > σ_v > σ_h; they all increase with depth. σ_v shows the largest stress gradient of about 0.025 MPa/m, followed by σ_H, at about 0.018 MPa/m, and σ_h at about 0.013 MPa/m. The values of σ_H and σ_h of the No. 3 coal seam are slightly higher than those in the sand/shale formation of the roof and floor by about 1.0-2.5 MPa, leading to the formation of a fracturing wear layer and making fracture height control difficult. The values of σ_H-σ_h are mainly distributed in the range of 2.0-6.0 MPa. This is also the main cause for the high fracturing effect and gas production of the relatively shallow No. 3 coal seam. The imaging logging induced fracture analysis shows that the in-situ stress direction in the study area is located between SN and NNE, which is associated with the Himalayan middle-late tectonic compression movement. The complex local structure characteristics and the existence of faults can result in a change in this direction. Results from finite element simulation of the study area show that the stress plane distribution characteristics are dominated by multiple factors such as buried depth, lithology, folds, and faults. The values of σ_H and σ_h of the No. 3 coal seam from simulation are distributed between 15.6-21.0 MPa and 12.5-16.0 MPa, respectively, and are consistent with the measured results.

Study on Simulation Method of Multi-scale Fractures in Low Permeability Reservoirs

LIU Jianjun, WU Mingyang, SONG Rui, HUANG Liuke, DAI Xiaojun

2017, 39(4):  90-103.  DOI: 10.11885/j.issn.16745086.2016.06.25.03

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The flow of fluids in low permeability reservoirs displays typical multi-scale mechanical behavior across dense matrices, natural fractures, hydraulic fractures, and wellbores. Studying the classification of the scale of fractures in reservoirs, establishing an accurate multi-scale fracture model in low permeability reservoirs, and exploring the cascade coupling process and internal correlations between multi-scales are the keys to the study of seepage flow in low permeability reservoirs and provide an important theoretical basis for effective recovery from low permeability reservoirs. Based on the multi-scale correlation method of complex disciplines and the relevant literature on multi-scale fracturing modeling of reservoirs, the scale classification criteria of multi-scale fractures in reservoirs are proposed. In addition, the multi-scale correlation method and multi-scale fracture model in low permeability reservoirs are summarized. The advantages and disadvantages of several representative models constructed from a continuous media perspective and a discrete media perspective were analyzed. Based on the study's results, recommendations for constructing the multi-scale model of reservoir fractures are proposed and the research trend of multi-scale fracture modeling in low permeability reservoirs is pointed out.

The Adsorption and Desorption of Coal Bed Methane: A Review

ZHU Suyang, DU Zhimin, LI Chuanliang, PENG Xiaolong, WANG Chaowen

2017, 39(4):  104-112.  DOI: 10.11885/j.issn.16745086.2015.11.12.05

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In this study, we investigated progress in the following three aspects of coal bed methane(CBM) research:adsorption theory and experiments, the effects of water on adsorption and desorption, and liquid-solid adsorption theory. We summarized the achievements of gas phase adsorption theory in studying CBM adsorption, and, in particular, the influence of water. In addition, existing complications in gas phase adsorption theory with regards to CBM adsorption were analyzed, and the new liquid phase adsorption theory and CBM compound desorption model built on its basis were introduced. Different water phases have a variable impact on CBM adsorption. When water saturation is low in the coal sample, equilibrium moisture in the gas phase can significantly reduce the amount of CBM adsorbed. As water saturation reaches critical levels, water content has little effect on the amount of CBM adsorbed. Upon further increase in water content in the coal sample, liquid-state water in the injected coal sample increases the quantity of CBM adsorbed. The occurrence of CBM is not in agreement with the existing gas phase adsorption model. In contrast, the liquid phase adsorption model explains both the hydrocarbon generation condition of coal in an aqueous environment and the absence of critical desorption pressure in gas phase adsorption. Under the conditions of liquid phase adsorption, the desorption of CBM is a compound desorption process controlled by both gas phase and liquid phase desorption.

Characteristics of Well Test Curves Corresponding to a Combined Reservoir with Linear Faults

LIU Qiguo, JIN Jiyan, GUI Fu, LI Ke, CHENG Xian'an

2017, 39(4):  113-118.  DOI: 10.11885/j.issn.16745086.2016.03.02.01

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There are few reports in the literature on the effects of linear faults on well test interpretations, especially for linear faults located within combined oil and gas reservoirs. To address this gap, we have constructed a mathematical model for interpreting well tests from infinite formation combined oil and gas reservoirs with two radial zones based on seepage mechanics. Solutions for this model were obtained via Laplace transformations; the principles of image theory and superposition were then used to study the dimensionless bottom hole pressure and pressure derivative curves corresponding to combined oil and gas reservoirs that contain linear faults, along with the factors affecting these curves. The results of this study are as follows:when the distance between a fault and well is less than half the radius of the inner area, the dimensionless pressure derivative curve changes from the 0.5 horizontal line, indicative of an infinite-acting radial flow, to the 1.0 horizontal line, indicative of a faultaffected inner area, and the value that shows the fault's impact on the outer area is the M₁₂ horizontal line. When the distance between a fault and well is more than half the radius of the inner area, the dimensionless pressure derivative curve shifts from the 0.5 horizontal line, corresponding to an infinite-acting radial flow, to the M₁₂/2 horizontal line, which shows the characteristics of the outer area, and the value that is indicative of the fault's impact on the outer area is the M₁₂ horizontal line.

Study on Stress Evolution and Failure Mechanism of Mudstone Layer in Thermal Recovery Environment

WANG Haijing, XUE Shifeng, TONG Xinghua, SUN Feng, ZHU Xiuxing

2017, 39(4):  119-126.  DOI: 10.11885/j.issn.16745086.2016.04.27.03

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The in-depth understanding of stress evolution and failure mechanism of the mudstone layer, in a thermal recovery environment, is helpful for scientifically and effectively solving the problem of cap rock cracking and interlayer blocking in the process of steam injection for heavy oil reservoirs. Firstly, the heat transfer and pressurization mechanism of the mudstone layer, in the reservoir thermal recovery process, are studied quantitatively, based on theory. The different stress evolution trends and failure modes of the mudstone layer are analyzed under variable temperature and pressure conditions. Then, using the cap rock fracture event as an example, which occurred in one of the thermal recovery projects in Canada, the heat-water-stress coupling numerical simulation of mudstone cap rock is performed. Thereby, the failure mechanism and key influencing factors of the mudstone layer, in the thermal recovery environment, are revealed. The results show that the stress evolution of the mudstone layer, in the thermal recovery environment, is mainly affected by heat-induced pressurization. Pore pressure is much higher than injection pressure and the peak value is located near the temperature corresponding to the maximum thermal pressure coefficient of pore water. The rock layers adjacent to the peak of the pore pressure are possibly subjected to tensile and shear damage. The development of a reasonable steam injection scheme, for the thermal recovery of reservoirs, must take into account the heat-water-stress coupling effect of the mudstone strata.

A Study on Permeability Curve Based on Polymer Elasticity in Porous Media

TANG Yongqiang, LÜ Chengyuan, HOU Jirui

2017, 39(4):  127-135.  DOI: 10.11885/j.issn.16745086.2015.05.12.03

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The relative permeability curve of polymer flooding is of great significance for a further understanding of the flow characteristics of polymer in porous media. However, existing studies have not fully considered the elasticity of polymer in porous media. An elasticity recovery experiment was performed to measure the pure viscous pressure difference after eliminating elasticity and to calculate the elastic viscosity of the polymer in the core. The elasticity of the polymer was introduced into the calculation of the relative permeability curve in the form of elastic viscosity. The results showed that the calculated relative permeability of the water phase is smaller when the elastic viscosity is not considered. The relative permeability curves of polymers with different molecular weights and at different concentrations were determined. The results showed that the elastic viscosity was greater for a higher molecular weight and concentration. The increase in the elastic viscosity increases the relative permeability of the oil phase, decreases the relative permeability of the water phase, and decreases the residual oil saturation. In addition, compared with the traditional J.B.N. method, the relative permeability curve calculated by this method is not affected by the flow rate and the calculation results are more reliable.

Progress and Trends in Shale Gas Seepage Mechanism Research

DU Dianfa, ZHAO Yanwu, ZHANG Jing, LIU Changli, TANG Jianxin

2017, 39(4):  136-144.  DOI: 10.11885/j.issn.16745086.2015.12.24.04

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The flow of shale gas in complex porous space is a typical case of multi-scale and multi-field coupled flow. Studying shale gas seepage mechanisms helps to reveal the mechanism for fluid migration in shale gas reservoirs, and lays a theoretical foundation for the establishment of a mathematical model, development of numerical simulations, and evaluation and prediction of future yields. Based on a literature review from China and abroad combined with recent work, we summarized current research in the field from two perspectives:1) experimentally, in terms of pore structure characteristics of shale, adsorption desorption patterns, shale gas content tests, stress sensitivity, and reservoir fluid migration and 2) theoretically, in terms of micro-flow mechanism simulations, such as molecular dynamics, direct simulation Monte Carlo, and lattice Boltzmann. The progress in the research on shale gas seepage mechanism are also described. The following areas are envisioned as the future direction of research:the influence of gas adsorption on the percolation pattern, fabrication of experimental devices to evaluate shale gas multi-scale media flow mechanisms, and experimental and theoretical studies of shale gas reservoir gas-water two-phase flow.

Analysis of Factors Influencing the Casing-tubing Annular Pressure of Gas Reservoir Wells

WANG Zhaohui, CHEN Jun, HE Xueliang, YIN Yiyong, LIU Chong

2017, 39(4):  145-151.  DOI: 10.11885/j.issn.16745086.2015.08.20.01

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Due to the problem of sustained casing pressure in the trapped annulus of gas reservoir injection-production wells, we develop a calculation model for sustained casing pressure under changing temperature and pressure of the production tubular column. An analysis model is first established for the trapped annulus according to a site-specific simplified structure of the actual gas reservoir. Next, based on the plane strain theory of elasticity, the thermal expansion and contraction properties of the material and the PVT equation of state for fluids, a mathematical analysis model is constructed. Finally, when the raw well data is considered, it is clear that the temperature change in the gas reservoir injection-production well is the main factor affecting the formation of the annular pressure. When only the effects of temperature and pressure on the protecting fluid are considered, this protocol meets the engineering requirements on site, with an error less than 5% and calculated results in good agreement with field data. This method facilitates the rapid prediction of annulus pressure in the field.

Experimental Study on Penetration Depth of Drilling Fluid into Tight Sandstone in an Area of the East China Sea

ZHANG Haishan, CAI Bin, LIU Yongbing, SHI Xiangchao

2017, 39(4):  152-158.  DOI: 10.11885/j.issn.16745086.2015.11.02.01

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To study the serious formation damage caused by the drilling fluid of drilling operations in the East China Sea area, an intrusion depth study of the drilling fluid into tight sandstone in an area of the East China Sea was performed. The resistance test method and the volume method were used in the study. The resistance of the core sample during the penetration of drilling mud and the maximum penetration volume of the filtrate of the drilling mud were measured. Based on the relationship between the resistance, penetration volume and the time, the relationship between the penetration depth and the time was obtained via fitting and the penetration depth of the drilling fluid was calculated. The penetration depth of the drilling fluid into a core sample of the tight sandstone matrix in the lower section of the Huagang Formation was obtained. The penetration depths of the drilling fluid calculated using resistance measurement and the volume method were 34～49 cm and 30～56 cm, respectively. The degree of formation damage after penetration of the drilling fluid was 78.2%～97.9%, and the damage degree was strong. This experimental study provides a basis for controlling the filtration loss and reducing the formation damage during drilling and completion operations in the East China Sea area.

Experimental Research on Dynamic Responses of Derrick of Offshore Floating Drilling Platform

ZHAO Guanghui, LIANG Zheng, LI Yiping, JIANG Faguang, YE Zhewei

2017, 39(4):  159-167.  DOI: 10.11885/j.issn.16745086.2015.04.28.01

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The drilling rig derrick of floating platform is often subjected to big dynamic loadings from marine environmental pressure and platform movement. In order to evaluate the influence of platform movement on dynamic responses of derrick, a derrick model and six-degree-of-freedom (6-DOF) moving boundaries, which were based on one offshore floating platform and the matching tower-shaped derrick, were designed according to similarity principle and test of the dynamic derrick was carried out by means of large-scale seism simulation test-tables. Acceleration and dynamic strain were measured under vibrational excitations with each single DOF and coupling 6-DOF, which modeled platform movement under operating condition, survival condition and design survival condition. It's found that the derrick is the most impressible to pitch and heave driving, and relatively insensitive to yaw driving. Hook load has little effect on derrick's dynamic responses. Based on the movement of the platform under the combined loads of the wave, current and wind that are from different directions, we find that the dynamic response of the derrick is the strongest with the incidence angle of 180 degree. Experimental results would provide reference for design of the matching derrick for drilling rig of deep-water floating platform.

Characteristics of Self-excited Oscillations in the Stick-slip Vibrations of Drill String Systems

TANG Liping, ZHU Xiaohua, SHI Changshuai, TANG Jian

2017, 39(4):  168-175.  DOI: 10.11885/j.issn.16745086.2015.07.07.03

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Stick-slip vibrations in drill string systems appear to generate effects that increase continuously over time. To address this issue, we studied the characteristics of self-excited oscillations that occur during stick-slip vibrations. By constructing a mechanical model for stick-slip vibrations in drill string systems, we have derived state equations to describe a drill in its sticking and slipping stages and obtained the oscillatory response of the drill during its slipping stage. Based on the characteristics of stick-slip vibrations in drill string systems, we then studied the phase trajectory of the relative rotary motion of a drill in varying initial conditions. The results indicate that a perturbation in the initial phase point of the drill will result in motions that trend toward a stable form of stick-slip vibration. This manifests as a phase trajectory that converges to a stable limit cycle. We also analyzed the reasons underlying the generation of stick-slip vibrations, and found that drill string systems undergoing stickslip vibrations always display a decrease in friction torque during the transition between sticking and slipping states, which is equivalent to the application of an external load in the direction of rotation of the drill. In other words, the negative damping effect that is present during the critical state transition of a drill changes the vibrations of the drill string into self-excited oscillations.

The Selection of Paraffin-degrading Bacteria and Their Effects upon the Rheological Characteristics of Heavy Crude Oil

WANG Weiqiang, LI Jia, WANG Guofu, ZHANG Xiaobo

2017, 39(4):  176-182.  DOI: 10.11885/j.issn.16745086.2016.06.30.03

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The extraction of heavy crude oil presents challenges due to the high viscosity and immobility of the emulsions. Microbes may offer a more economical and environmentally friendly alternative to conventional methods for improving the rheological characteristics of heavy crude oil. A strain of paraffin-degrading bacteria (ZL-7) was screened from activated sludge collected from the Liaohe Oilfield. It was found that the optimal cultivation conditions occurred during a culture period of seven days at 45 ℃. The treatment of heavy crude oil using bacteria-containing enriched culture media (i.e., a bacterial suspension) for seven days resulted in a pronounced reduction of viscosity and paraffin content. Focused beam reflectance measurements were used to monitor the chord length distribution and droplet diameter distribution of the heavy crude oil during treatment. After treatment, it was found that the percentage of small and large diameter droplets had increased and decreased, respectively. This shows that the average diameter of the crude oil droplets decreased after treatment, which is beneficial to the preparation and isolation of heavy crude oil emulsions.