Table of Content

25 August 2007, Volume 28 Issue 4

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A discussion on gas sources in deep gas fields,Daqing oil field

Guo Zhanqian, Liu Junfeng, Li Guishun

2007, 28(4):  441-448.  doi:10.11743/ogg20070401

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Sanzhao sag in Daqing oil region is found to contain many small-to middle-sized gas fields.Analysis shows that there are gas of both inorganic and organic origins and the natural gas is composed of both alkane and non-hydrocarbon gases.The later contains not only conventional non-hydrocarbon gases like carbon dioxide,hydrogen,nitrogen,and etc,but also rare gases,such as helium,argon,and even neon.Rare gases,rare-earth elements,and isotopes of some elements,can be used to determine the sources,migration mechanisms,and enrichment patterns of hydrocarbons and metal deposits.By applying this principle to the analysis of hydrocarbon gases,non-hydrocarbon gases and the isotopes of rare gases,both the hydrocarbon gases and non-hydrocarbon gases are determined to be mainly inorganic gases sourced from the deep of the earth.They occur both in the exploration area and in the developed oilfields.In recent years,Daqing Oilfield has witnessed an increase of non-hydrocarbon gases with higher production of associated gas.In addition,the geochemical behaviors of these gases also indicate an inorganic origin,quite different from that of crude.

Quantitative resource assessment methodology for play and its application to Zhentong depressions in Subei basin

Xu Xuhui, Zhu Jianhui, Jiang Xingge

2007, 28(4):  449-457.  doi:10.11743/ogg20070402

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Based on calculation of hydrocarbon generation and expulsion through TSM basin simulation,potential field analysis,and oil/gas migration channel modeling,dynamic and quantitative resources calculation methods are proposed for plays and structural zones in an attempt to determine the migrated and accumulated resources and pin down their locations,so as to realize dynamic assessment of resources.Zhentong depression in Subei basin is divided into four plays(fault terrace,inner slope,hinge belt,and outer slope) and 11 structural zones according to hydrocarbon expulsion potentials and structural features,and these methods are used to quantitatively calculate the total volume of oil and gas migrated and accumulated in the main targets across all the plays since the deposition of the Sanduo Formation(35.2Ma).The calculation indicates that the main period of hydrocarbon migration and accumulation is from the deposition of the Yancheng Formation(23.2Ma) till present.During this period,about 9.98Mt of hydrocarbons migrated from Sanduo Formation and accumulated in fault terrace play,accounting for 52.0% of the total hydrocarbons generated by this formation.While in the inner slope,the volume from the Dainan Formation is about 21.59 Mt(51.0% of the total),from the 3^rd member of Funin Formation is 145.19Mt(48.5% of its total).The absolute magnitudes of hydrocarbons migrated and accumulated in the fault terrace play are about 61.70 Mt from the 3^rd member of Funin,25.39 Mt from Taizhou Formation,and 19.70 Mt from the 1^st member of Funin Formation.Therefore,in respect of quantitive resource assessment,the exploration focus shall be put on the 3^rd member of Funing Formation and Sanduo Formation in the fault terrace play,while in the inner slope play,it shall be put on the Taizhou Formation,1^st and 3^rd members of Funin Formation,and Dainan Formation.

Sedimentary framework of Changxing-Feixianguan Formations and its control on reservoiring in northeastern Sichuan basin

Ni Xinfeng, Chen Hongde, Tian Jingchun, Wei Dongxiao

2007, 28(4):  458-465.  doi:10.11743/ogg20070403

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As a whole,the Upper Permian Changxing Formation-Lower Triassic Feixianguan Formation in northeastern Sichuan basin were deposited as carbonate platform.The extensive platform was cut into platform-highs and platform basins by fossil fractures.Being controlled by Kaijiang-Liangping platform-basin,relatively symmetrical carbonate sedimentary systems,similar to Wilson's facies pattern,were developed on both sides of the basin in late Changxing to early Feixianguan age,but shelf facies were poorly developed.It is possibly due to the shallow depth,narrow width and rapid facies change of the platform-basin.A second-order eustatic sea level change during this period resulted in the widely development of transgressive bioherm in Changxing Formation and regressive oolitic shoals in Feixianguan Formation,both of which would be the most favorable reservoir facies belts.The bioherm and oolitic shoal facies belts continuously migrated laterally and upraised vertically.They were mainly developed on both sides of the platform-basins and characterized by discontinuous zonary distribution,which were obviously controlled by sedimentary facies belts.

Compartments in the Upper Paleozoic of northern Ordos basin and their relationship with gas enrichment

Li Zhongdong, Hao Shumin, Li Liang, Hui Kuanyang, Guo Min, Wang Zhen

2007, 28(4):  466-472.  doi:10.11743/ogg20070404

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Based on previous research data of sedimentary burial history,hydrocarbon-generating history,geothermal history,inclusion analysis,denuded thickness,deposition velocity,and etc.,in the northern Ordos basin,a study is initiated with the aim of understanding the pattern of abnormal pressure distribution,and the genesis and evolution of pressure in the Upper Paleozoic of the basin by combining its characteristics of universal low abnormal pressure,tight reservoirs,simple sturctures,dispersed gas pays,and compartment-controlled gas accumulation.The results confirm the existence of favorable geological conditions for developing compartments in the Upper Paleozoic of the basin.The Upper Paleozoic is recognized to be a complicate gas system with high and low pressure coexisted in compartments,i.e.the reservoir belongs to normal pressure-abnormal low pressure,while the mudstone still maintains high pressure.The study also suggests that the shape,inner structure,and sandstone-mudstone relationship of the compartment decide the distribution of gas reservoirs in the compartment.

Factors influencing natural gas enrichment in Jingbian gas field,Ordos basin

Wu Yongping, Wang Yuncheng

2007, 28(4):  473-478.  doi:10.11743/ogg20070405

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Single control factor analyses of the palaeogeomorphology unit,structural feature,and mudstone compaction characteristics of Jingbian gas field show that these three factors individually have some impacts upon the gas enrichment in the field.However,a comprehensive and accurate understanding of gas enrichment pattern and recognition of prospect need further analyses of the combinations of the influential factors.Taking the karst paleogeomorphology and the present structures as the predominant control factors,four types of combinations are recognized:1)structural high combinations;2)combinations of platform edge slope and structural nose uplift with different parts of palaeostructure;3)combinations of platform edge slope and structural nose flank with different parts of palaeostructure;and 4) combinations of lowland and structural nose hollow combined with different parts of palaeostructure.Among the four types,the first one is considered to be the most favorable.By integrating various factors that control or affect the natural gas enrichment with geological characteristics of high productivity wells,five necessary conditions for gas accumulation in Jingbian oilfield are proposed to reveal the pattern of gas enrichment.

Sequence characteristics and genesis of the Eogene salt-bearing formation in Dongpu depression

Huang Jianjun, Ji Youliang, Wang Gaiwei, Chen Faliang, Han Fumin

2007, 28(4):  479-484.  doi:10.11743/ogg20070406

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A sequence demarcation that integrates the conventional markers of sequence boundaries and characteristics of salt rhythmite is more practical for the salt-bearing Eogene in Dongpu depression.According to this demarcation,the Eogene can be divided into 2 second-order sequences and 10 third-order sequences,ensuring the unity of salt rhythmite development and sequence growth.The saltbearing third-order sequence(low-frequency) composition displays a peculiar regularity: the horizontal distribution area of salt-bearing formation in Transgressive Systems Tract(TST) and Highstand Systems Tract(HST) is larger than that in Lowstand Systems Tract(LST).Within the high-frequency sequences,especially the subsequence units corresponding with the fifth-order sequences,salt and sandstone layers are synchronous deposits of different faces,and they form a complete unit of subsequences together with the underlying dark mudstone layers. The developing mode of high-frequency salt-bearing sequence is believed to be formed by high-stand evaporation.

Indosinian progressive deformation and its chronogenesis in Longmengshan structural belt

Deng Kangling

2007, 28(4):  485-490.  doi:10.11743/ogg20070407

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According to the coupling relationship between basins and mountains and the depositional characteristics of the Xujiahe Formation in the western margin of Sichuan basin,it is deduced that Indosinian deformation occurred in three episodes in Longmengshan,with episodic progressive overthrust nappes developed from north to south.The early Indosinian episode happened during the deposition of the second member of Xujiahe Formation when the deformation was confined to the north section of Longmengshan;the middle Indosinian episode was within the deposition of the fourth member of Xujiahe Formation when the deformation was limited on the north part of the middle section of the mountain;and the late Indosinian episode was at the end of deposition of the Xujiahe Formation when the deformation was the strongest and most extensive in the study area.It is also noted that the Rhaetian deposits are generally absent in the western Sichuan basin with deformation of various degrees.

Characteristics and genesis of the Upper Paleozoic tight sandstone reservoirs in the northern Ordos basin

Ding Xiaoqi, Zhang Shaonan, Zhou Wen, Deng Lizheng, Li xiuhua

2007, 28(4):  491-496.  doi:10.11743/ogg20070408

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The Upper Paleozoic tight sandstone reservoirs in the northern Ordos basin have the following three characteristics:1)low porosity and low permeability(sensitive to pressure changes),small pores,and slim throats;2) high irreducible water saturation and complex gas-water relationship;and 3)reservoir space being dominated by dissolved pores and micropores and fractures being relatively well developed.Analysis of these reservoirs suggests that the tightness of the Upper Paleozoic sandstone reservoirs is mainly caused by large palaeoburial depth,high thermal evolution,strong destructive diagenesis during its late phase,and further compaction of reservoirs evoked by hydrothermal activities.

Prediction of beach bar sand reservoirs of shore-shallow lake facies in Boxing subsag of Dongying sag

Yuan Hongjun

2007, 28(4):  497-503.  doi:10.11743/ogg20070409

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Featuring in deep burial depth,large lateral variation,and small single layer thickness,the beach bar sand reservoirs in the 4th member of Shahejie Formation is an important type of lithological deposits.Integrated utilization of seismic,geology and drilling data is important to the prediction of this kind of reservoirs.After years of exploration practices,a series of techniques,including palaeosedimentary environment reconstruction,forward modeling,seismic attribute analysis and geologic target-oriented inversion,has taken a complete shape in Shengli Oilfield by aiming at beach bar reservoir prediction.These techniques are employed in the exploration of the beach bar sand reservoirs in Boxing subsag of Dongying sag and the results are quite encouraging.

Depositional characteristics and reservoir properties of turbidites in the 3^rd member of Shahejie Formation in Wuhaozhuang area,eastern Zhanhua depression

Meng Wanbin, Zhang Shanwen, Tian Bo, Feng Bin

2007, 28(4):  504-510.  doi:10.11743/ogg20070410

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Two-phase proximal turbidite fans are developed in the 3^rd member of Shahejie Formation in Wuhaozhuang area,eastern Zhanhua depression,and they form the major pay zones in the lower sub-member of the 3^rd member.The fan is mainly composed of conglomerate,glutenite and dark mudstone and shale,in which the sandsone is low in both compositional maturity and textural maturity.The major genetic types of sandbodies are gravel-rich main channel sandstone,sand-rich midfan braided channel fill,and midfan front lakes with mixed sand/mud.Sandbodies of different types are different depositional and textural characteristics and reservoir properties.The braided channel sandbodies have a better poroperm characteristics than that of the gravel-rich main channel sandstone and midfan front lobes,and serve as the main reservoirs with its mean porosity of 13.1%～13.5% and mean permeability of 0.87×10^-3～5.95×10^-3μm².The reservoir properties also vary tremendously between the upper and lower fans with the former having better poroperm characteristics but thinner net sands and smaller oil-bearing area while the latter having average physical properties but thicker net sands and bigger oil-bearing area.It is suggested that better gradation of sandbodies means better poroperm characteristics,sandbody architectures control the reservoir properties through exerting influences upon the lateral continuity and vertical connectivity of the reservoirs,and diagenesis influences the growth of reservoir pores.

A new type of fracturing fluid

Luo Pingya, Guo Yongjun, Liu Tongyi

2007, 28(4):  511-515.  doi:10.11743/ogg20070411

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Guided by the concept of 'reversible structure solution fracturing fluid',a new type of thickening agent as well as a new fracturing fluid system with the agent is developed.Tests show that the system is a typical viscoelastic structure solution with it's G′ constantly bigger than G″;with a viscosity of merely 30mPa·s(170s^-1),the system has a proppant-carrying capacity strong enough for operation;this viscosity can be easily achieved at the temperature of 150℃ by adding only 0.8wt% of the agent into the system and is not sensitive to the shearing time;the system has a "clean" viscosity break fluid,high proppant flow conductivity(more than 90%),and slight damage to core(lower than 10%).The application of the system in five wells,namely P65-9,Wen 99-20,Wei1 0-40,Weiche-214,and Wei 355-4,in Zhongyuan oil field,yields positive stimulation results and causes lesser damage to formations than that of conventional fracturing fluids.It is also user-friendly because of its simpler composition and preparation procedure,excellent proppant carrying capacity,small friction drag(22% of that of fresh water),and higher working temperature(up to 120℃).

Evaluation of factors affecting productivity of low-permeability coalbed methane reservoirs

Du Zhimin, Fu Yu, Wu Yong

2007, 28(4):  516-519,527.  doi:10.11743/ogg20070412

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Beginning with flow mechanism of coal methane,a mathematical model for coalbed methane reservoirs is established by utilizing the Langmuir isothermal adsorption equation to describe the desorption of methane from coal surface and the Fick's law to delineate the diffusion of methane in coal matrix and micropores.The calculated results of this model show that the productivity steps up with the increase of fracture half length and flow conductivity.However,when the fracture half length is increased to 100m and the fracture flow conductivity is beyond 20 μm²·cm,their effects upon the productivity becomes less evident.Molecular diffusion coefficient has no big influence over the productivity,while the permeability of cleaps,on the contrary,is closely connected with productivity changes.A rational selection of fracture half length and flow conductivity is strongly recommended when put a coalbed methane gas well into production.

Experiment of low-tension system for improving percolation behaviors in waterflooding low-permeability reservoirs

Lai Nanjun, Ye Zhongbin, Chen Hong, Luo Pingya

2007, 28(4):  520-522.  doi:10.11743/ogg20070413

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Waterflooding performs poorly in low-permeability reservoirs due to their low absorbing capacity and gradually increasing injection pressure.This paper takes T oilfield,a low-permeability oilfield under waterflood development program,as an example to illustrate the development of a Low-Tension System for improving waterflooding effectiveness in low-permeability reservoirs.Lab experimental result shows that the system can effectively reduce residual oil saturation,increase oil/water two-phase common seepage intervals,improve percolation behaviors,and thus enhance waterflooding efficiency in low-permeability reservoirs.

Influence of different stress sensitivities on bottom hole pressure behaviors during well opening and shut-in

Liu Qiguo, Tang Fuping, Feng Guoqing, Yan Zejiang, Hu Xinping, Chen Jun, Li Yun

2007, 28(4):  523-527,519.  doi:10.11743/ogg20070414

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Basing on the empirical equation of pressure stress-sensitive effect and the basic theory of fluid flowing mechanics in porous medium,a mathematical model for well testing analysis in homogeneous gas reservoirs is established with considerations of permeability sensitivity to stress,effect of wellbore storage,and skin effect.The perturbation technique and Laplace transform method are employed in the calculation of the model to study the features of percolation in stress-sensitive gas reservoirs and the influence of different stress sensitivities on bottom hole pressure behaviors during well opening and shut-in.The results show that along with the stress sensitivity of permeability increases,derivative curve may upturning higher during drawdown test,while the derivative curve may drop down during pressure build-up test.If the damage to permeability caused by stress sensitivity during drawdown/build-up test is irreversible,the permeability can not be fully recovered through pressure boosting and the degree of pressure buildup is still lower.

Application of boundary element theory to the study of percolation in heterogeneous fractured-vuggy reservoirs

Zhang Liehui, Li Chengyong, Liu qiguo, Li Yun

2007, 28(4):  528-534.  doi:10.11743/ogg20070415

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A mathematic model for describing percolation in vertical well in heterogeneous fractured-vuggy reservoirs was built based on fluid flow mechanics in porous medium.By using Lord Kelvin point source solution,Bassel integration and superimpose formula,boundary element fundamental solutions(BEFS) are derived for percolation in vertical wells in heterogeous fractured-vuggy reservoirs and a BEFS model of bottom hole pressure in vertical wells in heterogeneous fractured-vuggy reservoirs is also established.Log-log type curves of dimensionless pressures and pressure derivatives vs.dimensionless time are plotted through theoretical calculation;and the effects of wells located in different areas of high-permeability zones and high-permeability zones located in different parts of the reservoirs upon the bottom hole pressure behaviors are analyzed in detail.

Influences of effective pressure on petrophysics of low-permeability reservoirs with deformable medium

Huang Hui

2007, 28(4):  535-538.  doi:10.11743/ogg20070416

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A systematic study of percolation behaviors and deformation characteristics in low-permeability reservoirs with deformable medium is carried out through simulated test on core samples from Wendong area.The result suggests that pressure magnitude,pressurizing velocity and manner,and etc.,play important roles in percolation and deformation of reservoirs.The regular permeability is a variable relating to driving pressure difference and effective pressure.The initial pressure difference and its amplitude exert tremendous influence upon the reservoir deformation.Therefore the enhancement of oil recovery relies heavily upon a rational control of the initial pressure difference.

Influence of start-up pressure gradient and stress sensitivity on productivity of low-permeability gas reservoirs

Guo Xiao, Wu Yong

2007, 28(4):  539-543.  doi:10.11743/ogg20070417

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Water/gas percolation in low-permeability gas reservoirs is rather complicate because of the reservoirs'low porosity and low permeability.As a result,the traditional percolation law is inapplicable to this kind of reservoirs.Laboratory research and field application testify the nonlinearity of percolation and multi-variability of fluidal in the low permeability gas reservoirs.The percolation of fluid will not only need to overcome start-up pressure gradient,but will also be enslaved to stress sensitivity.Horizontal well technique is an effective method in low permeability gas reservoirs development.However,prediction of productivity of horizontal well in low permeability gas reservoirs is mostly carried out within the framework of traditional percolation theory and therefore fails to take start-up pressure gradient and stress sensitivity into consideration.In an attempt to fix the flaw,the conformal transformation is introduced in the establishment of a productivity model for horizontal wells in low-permeability reservoirs and the effects of start-up pressure gradient and stress sensitivity is studied based on the real data from a low-permeability gas reservoir.Results indicate that productivity of horizontal wells in low-permeability gas reservoirs decreases with start-up pressure gradient and stress sensitivity at a linear and an exponential relationship respectively.The stress sensitivity has a stronger impact on the productivity compared with start-up pressure gradient.When start-up pressure gradient reaches 0.00025 MPa/m,the productivity will be reduced to 77%;while when rock elastic distortion coefficient exceeds 0.15 MPa^-1,the horizontal gas well will have to be shut down.It suggests that start-up pressure gradient and stress-sensitivity effect should be considered to correctly predict the productivity of horizontal gas well in low permeability reservoirs.