Table of Content

28 December 2012, Volume 33 Issue 6

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Quasi-continuous hydrocarbon accumulation:a new pattern for large tight sand oilfields in the Ordos Basin

Zhao Jingzhou, Bai Yubin, Cao Qing, Er Chuang

2012, 33(6):  811-827.  doi:10.11743/ogg20120601

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Tight oil reservoirs are low-permeability oil accumulations in non-source rock reservoirs that can produce economic flow only through massive fracturing or other special measures. Their absolute permeability is less than 2×10^-3 μm² and porosity less than 12%. Tight oil reservoirs are widely accepted as the continuous-type accumulations of unconventional hydrocarbons. Based on our studies on the Ordos Basin in China, we found that oil reservoirs in the lower and middle plays of the Triassic Yanchang Formation on the Yi-Shan slope are mostly tight or near-tight sand oil accumulations. Their oil accumulation patterns are neither the conventional lithologic accumulations nor the continuous unconventional accumulations, instead a transitional type that we refer to as the quasi-continuous accumulation. The quasi-continuous hydrocarbon accumulation can be defined as extensive accumulations in tight reservoirs with neither defined boundaries nor water legs and controlled by unconventional trapping mechanisms. Studies indicate that the quasi-continuous tight sand oil accumulations in the Ordos Basin have the following characteristics:(1)oil distribution is extensive geographically in quasi-continuous form without a defined boundary;(2)the hydrocarbon generation covers vast area and the charging is highly intensive;(3)the reservoirs have unfavorable physical properties and are highly heterogeneous;(4)traps are unconventional that exist in the transition form between conventional shaped traps such as the structural and stratigraphic traps and the shapeless trap or no trap at all;(5)the oil-water contact is unclear and no obvious edge/bottom water present;(6)the formation pressure system is complex and underpressure is predominant;(7)the migration and accumulation of hydrocarbons are short-distanced and driven principally by non-buoyant forces;(8)the accumulation and distribution of oil pools are rarely controlled by local structures, but primarily by source rock and reservoir conditions;(9)the preservation conditions are excellent and reservoir variations are unobvious; and(10)the resource potential is significant. The quasi-continuous accumulation model indicated that the exploration potential of this kind of reservoirs is large, but the traditional thinking of geological studies and exploration must be adjusted accordingly to fully tap their potentials.

Geochemical characteristics and origin of hydrocarbons in the Well Shun-7 of western Shuntuoguole Block,Tarim Basin

Ma Anlai, Jin Zhijun, Li Jingjing, Zhu Xiuxiang

2012, 33(6):  828-835.  doi:10.11743/ogg20120602

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Condensate oil and natural gas were tested in the Yingshan Formation of the Lower-Mid Ordovician in well Shun-7 of western Shuntuoguole Block in Tazhong-Ⅰ faulted-slope-break zone,Tarim Basin.The condensate oil features in high content of saturate hydrocarbon and low content of aromatic hydrocarbon and non-hydrocarbon asphaltene.The ratio of saturate hydrocarbon to aromatic hydrocarbon and non-hydrocarbon to asphaltene are 13.51 and 18.50,respectively.The Pr/Ph value is about 1.16 and the heptane and isoheptane values are 33% and 3.8 respectively,indicating high maturity of the condensate oil.As the maturity is high,hopanes series basically disappear and tricyclic terpanes is dominated by C₁₉-tricyclic terpanes.The carbon isotope of the condensate oil from well Shun-7 is -29.6‰ and is similar to that of the Cambrian-sourced oil from well TD-2,indicating the condensate oil of well Shun-7 might originated from the Cambrian source rocks.The wet gas of the well Shun-7 has a methane carbon isotope value of -51.7‰,which is similar to that of the gas from the Ordovician in well TZ-45.The distribution of carbon isotopes of the gas components shows a normal carbon isotope sequence (i e,δ¹³C₁<δ¹³C₂<δ¹³C₃) indicating the gas may come from the Upper Ordovician source rocks.

Geochemical characteristics of bitumen in the Upper Permian reef limestone in Liuqiao(Chongzuo)area,Guangxi province

Qiu Zhen, Wang Qingchen, Jia Wanglu, Wei Hengye, Peng Pingan

2012, 33(6):  836-844.  doi:10.11743/ogg20120603

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Bitumen are well developed in the Upper Permian reef limestone in Liuqiao area,Chongzuo,Guangxi province,and they mainly occur in the cavities and fractures of bioherms,followed by biological coelomata.The bitumen samples show a high maturity and an atomic ratio of H/C generally less than 0.1.Based on the gas chromatography-mass spectrometry analyses of saturated hydrocarbon extracted from the bitumen in the Upper Permian as well as from the silicite and and siliceous mudstones in the Dalong Formation in the Liuqiao(Chongzuo)area,we correlated their n-alkane,terpane,hopane,sterane and biomarkers.The result shows that the possibility of the bitumen being sourced from the Dalong Formation of the Upper Permian is slime.Instead,it could come from the source rocks of the Devonian in the study area.

Hydrocarbon accumulation process of Chang-8 low-permeability lithological reservoirs in Zhenjing area,Ordos Basin

Li Weilian, Liu Zhen, Wang Wei, Pan Gaofeng, Zhao Shu, Zhang Jian

2012, 33(6):  845-852.  doi:10.11743/ogg20120604

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In order to better understand the characteristics and main controlling factors of hydrocarbon accumulation in low-permeability lithological reservoirs of Member 8 of Yanchang Formation(Chang-8)in Zhenjing area,Ordos Basin,we performed dynamic analysis of the Chang-8 lithological reservoirs based on study of burial history,thermal history,hydrocarbon generation history and diagenetic process.The evolution features of various hydrocarbon accumulation factors were clarified such as porosity,timing of traps,formation pressure of source rocks and the window of hydrocarbon charging kinetics.Finally,a comprehensive analysis of the dynamic process of hydrocarbon accumulation was performed.The results indicate that porosity of Chang-8 reservoir was of 20% during hydrocarbon accumulation in the middle Early Cretaceous and lowered progressively to current 12% due to reservoir tightening of the later evolution stage.The lithological traps of the Chang-8 were formed at the Late Jurassic and kept on developing till now,thus they are characterized by early formation and long process of development.The formation pressure of Chang-7 source rocks was 22 MPa during the period of hydrocarbon accumulation.It increased up to 35 MPa with a pressure coefficient of 1.5 at the end of Early Cretaceous and later lower progressively to normal pressure in the Neogene.The charging pressure of oil was 15 MPa higher than the pressure of resistance,and the window of hydrocarbon accumulation lasted for a long time from the Early Cretaceous to the end of Neogene.The formation of the low-permeability lithological reservoirs in the Chang-8 is controlled by the favorable temporal and spatial matching of between the factors and process of hydrocarbon accumulation.Physical properties of sandstone and charging kinetics are the main factors controlling hydrocarbon accumulation.

Upper Ordovician source rocks in northern Tarim Basin

Huang Jiwen, Gu Yi, Ding Yong, Jia Cunshan

2012, 33(6):  853-858,866.  doi:10.11743/ogg20120605

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Many wells have encountered the Qrebake,Lianglitaga and Sangtamu formations in Upper Ordovician of northern Tarim Basin.A systematic analysis of source rock samples from the sequence indicates that it was characterized by low abundance of organic matters and mixed organic types.In southern Tahe area,the Upper Ordovician source rock is poorly developed with a vitrinite reflectance (R_o) of about 1%,thus could not be the source of commercial oil and gas reservoirs.In contrast,in northern Tarim Basin,the Upper Ordovician carbo-nates are both source rocks and reservoirs,making conventional oil-source correlation impossible.The study adopted two oil-source correlation methods to compare the properties of direct extracts from source rocks and their pyrolysis yields with that of crude oil from the reservoirs.The result shows that there is no source-product relationship between the crude from Tahe oilfield and source rocks from the Upper Ordovician.The geochemical characteristics of direct extracts from the Upper Ordovician source rocks are something between that of crude oil and pyrolysis yield,while the geochemical characteristics of crude oil and the pyrolysis yield are quite different.The new method of oil-source correlation solved the problem of the conventional oil-source correlation method not applicable to the Upper Ordovician as it is both source rocks and reservoirs at the same time.

Activity of Shinan fault and its control on hydrocarbon accumulation in the Paleogene in Bozhong Depression

Wang Guanmin, Pang Xiaojun, Zhang Xuefang, Liu Zhanbo, Liu Boao

2012, 33(6):  859-866.  doi:10.11743/ogg20120606

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The Paleogene in Shinan area of Bozhong depression is encountered by only a few wells,constraining the understanding of its source rock distribution,reservoir type and distribution,and hydrocarbon migration as well as reservoir types.Based on integration of 3D seismic,drilling,logging and laboratory data and in combination with sedimentary settings,we studied the features of acitivity of Shinan fault in the Paleogene in Bozhong Depression and discussed the relationship between fault activity and hydrocarbon accumulation by using fault slip rate method.The activity of the Shinan fault features in regular change pattern of"weak-strong-weak-strong-strong-weakest".It was the strongest during the deposition of the third member of Shahejie Fm and the 2^nd and 3^rd members of Dongying Fm when source rocks were developed.It was weak during the deposition of the 1^st and 2^nd members of Shahejie Fm and the 1^st member of Dongying Fm when reservoirs were developed.In addition,strong depression occurred during the deposition of the 2^nd member of Dongying Fm when relatively good reservoirs were developed in the large distal delta front deposits.Fault-anticline reservoirs and fault-lithologic reservoirs occur in locations with strong fault activity,while fault-lithologic reservoirs and lithologic reservoirs occur in places with weak fault activity.

Geochemical tracing of the Cambrian-Ordovician reservoir fluid in Well Yingdong-2,eastern Tarim Basin

Wang Dong, Wang Guozhi, Liu Shugen, Ding Daishu

2012, 33(6):  867-876.  doi:10.11743/ogg20120607

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The isotopes of carbon,oxygen and strontium,REE,and the geochemical features of the fluid inclusions in the Middle-Upper Cambrian and Middle-Lower Ordovician side-beds and fracture fillings of the well Yingdong-2 were systematically analyzed.Based on the results,we traced phases and sources of the reservoir fluids,determined chronogenesis of fluid charging,explored the impact of the fluids upon reservoir modification and discussed the dynamic preserving conditions for oil and gas in such settings.The geochemical study of the fluids suggests that there are three charging phases of allogenic fluids with abundant Sr from the crust in the Middle-Upper Cambrian and Middle-Lower Ordovician .The fluids charged in the first phase have a ⁸⁷Sr/⁸⁶Sr ratio of 0.710 8,the fluids charged in the second phase have a ⁸⁷Sr/⁸⁶Sr ratio in the range of 0.709 8-0.710 0,and the fluids charged in the last phase are hydrothermal fluid from the deep crust and have a ⁸⁷Sr/⁸⁶Sr ratio of 0.713 8.The fluids charged in these three phases all played a destructive role in the forming of reservoirs,but the hydrothermal fluids charged in the last phase were constructive to the reservoir formation.The Middle-Upper Cambrian reservoirs had relatively better hydrocarbon preserving conditions during the first phase of fluid charging,but lost them when the second and the third phase occurred.Geochemical tracing of fluids provides a new method for dynamic evaluation of deep fluid activities in the Yingdong tectonic belt as well as the whole eastern Tarim Basin.

Preliminary application of the frame flexibility factor in Puguang gas field

Zhang Hanrong, Sun Yuefeng, Dou Qifeng, Zhang Tingting

2012, 33(6):  877-882.  doi:10.11743/ogg20120608

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In addition to rock porosity,pore structure (pore size and geometry) also affects velocity of seismic waves,lowering the accuracy of reservoir prediction of velocity-porosity model.This paper introduces the Sun Model named by Shell for advanced carbonate reservoir characterization,a new rock-physics-based parameter called frame flexibility factor,and the method of how to calculate this parameter.The frame flexibility factor can be used as a pore type indicator to quantify the effects of pore structure on velocity and permeability and establishes relations among porosity-sonic velocity-frame flexibility factor instead of porosity-sonic velocity.It also can be used to describe permeability heterogeneity,and establish relations among porosity-permeability-frame flexibility factor instead of porosity-permeability.Theoretical model has been built for porosity and permeability prediction and makes it possible to characterize reservoirs with porosity,permeability and pore structure on seismic.Good results have been achieved through the utilization of the model in the Puguang gas field.

Distribution and genetic mechanism of carbonate cements in the Zhuhai Formation reservoirs in Wenchang-A sag,Pear River Mouth Basin

You Li, Li Cai, Zhang Yingzhao, Gan Jun, Liu Jinghuan

2012, 33(6):  883-889,899.  doi:10.11743/ogg20120609

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Sedimentary mineralogical and petrological methods in combination with oxygen-carbon isotope and electron probe were used to analyze the distribution,forming environment and material sources of the carbonate cements in the Zhuhai Formation in Wenchang A sag,Pear River Mouth Basin.The result indicates that the carbonate cements significantly affect reservoir quality.When the content of carbonate cements is greater than 5%,the reservoir quality significantly deteriorates.Furthermore,the distribution of carbonate cements show obvious regularity both vertically and horizontally.The content of carbonate cements in the 1^st and 2^nd members of Zhuhai Formation are higher than that of the 3^rd member,and it is higher in the southern fault zone than the center of the sag.The carbonate cements are dominated by middle ferric calcite and late ferric dolomite,followed by siderite and dolomite as well as local early crystal stock calcite.The forming of carbonate cements is resulted from decarboxylation of organic acid and influenced obviously by freshwater.In addition to dissolution of calcareous foraminifer,feldspar dissolution and clay mineral transformation provide abundant calcium for the carbonate cements development.

Influences of petrologic features on diagenesis and pore development:an example from the Triassic Yanchang Formation in Longdong area,Ordos Basin

Zhong Dakang, Zhou Lijian, Sun Haitao, Yao Jinli, Ma Shiyu, Zhu Haihua

2012, 33(6):  890-899.  doi:10.11743/ogg20120610

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Various data including common thin sections,cast thin section,cathode luminescent thin sections and poroperm characteristics were used to analyze the influence of petrologic characteristics on diagenesis and pores development of the Yanchang Formation in Longdong area,Ordos Basin.The results show that the types and contents of debris,the contents of minor minerals(such as mica)and the size of the clastic particle are the main factors affecting diagenesis.The existence of large amount of plastic debris from rocks such as mudstone and phyllite reduces the strength of rock.These plastic debris are vulnerable toe compressional deformation during burial and can form pore-filling pseudo matrix.The existence of soluble debris from rocks such as carbonates and intermediate and acidic volcanic debris can enhance solubility of rocks,being favorable for the formation of dissolution pores.With the pore water participation,the hydration of minor minerals can release potassium ion which causes alkaline environment.This can reduce quartz stability,and promote the pressure solution of quartz and the formation of the sutures.On the other hand,the silicon-aluminum-iron composition being released from mica alteration could form chlorite membrane cement around quartz and feldspar particles,and kaolinite and iron bearing mineral cement in intergranular pores.These cements can reduce porosity and permeability.Grain size mainly controls the compaction resistance and bond strength of sandstone.

Identification of small fracture-vugs and their fillings through log interpretation in fractured-vuggy Ordovician reservoirs in Tahe oilfield

Tian Fei, Jin Qiang, Li Yang, Zhang Hongfang, Zhang Wenbo

2012, 33(6):  900-908.  doi:10.11743/ogg20120611

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Tahe oilfield is a famous marine Paleozoic giant oilfield dominated by paleokarstic reservoirs of the Ordovician carbonates.Caverns and fractures around them are the main pore space.Numerous small fracture-vugs act as important pore space and and flowpaths.Although the small fracture-vugs can be recognized on outcrops,cores and image logs,they are not readily recognizable in intervals without core and image logging data,seriously influencing the study of fractured-vuggy reservoirs and the establishment of accurate reservoir geological model.To tackle the problem,we calibrated the conventional logging data with cores and image logs and selected five vug-sensitive parameters and three fracture-sensitive parameters.These parameters were used to build a norma-lized and weighted function for identification of small fracture-vugs in Tahe oilfield.We used DEN-V_sh,R_LLD-V_sh cross plot method to recognize the types of fillings,including clastics,breccias and chemical fillings.The results of recognition are fairly good compared with that from core observation and image log interpretation.The vug recognition rate is above 81%,fracture recognition rate is larger than 82%,and cave fills recognition rate is over 84%.The method can meet the basic requirements for identification of small vugs and fractures.The application of the method to 117 wells in Tahe oilfield proves to be quite encouraging and the results also show the potential of the method for identification of caverns and fractures with a height over 0.5 meters and their fillings.It provides a new approach for fine study of reservoir space of paleokarstic reservoirs.

Application of seismic data to the design of horizontal well trajectory in tight sandstone gas reservoirs

Xu Jie, Dong Ning, Zhu Chenghong, Liu Junzhou, Chen Tiansheng

2012, 33(6):  909-913.  doi:10.11743/ogg20120612

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The 1^st member of Permian Xiashihezi Formation (He-1) in Daniudi gas field,Ordos Basin,is braided channel deposits with large thickness.As heterogeneity of the gas layer distribution is strong,development of the gas reservoirs with horizontal wells is difficult.In order to increase the success rate of drilling"sweet spot"in this tight sandstone layer,we studied in detail the sedimentary facies and characteristics of the braided channel deposits in He-1 based on analyses of high resolution sequence stratigraphy and sedimentary system,and presented methods to integrate various seismic data into design of horizontal well trajectory,such as 3D seismic interpretation,waveform analysis,AVO gas detection and geo-statistical inversion.Among these seismic data,negative AVO gradient and geo-statistics inversion are the most important and can provide strong technical support to the development of the tight sand gas reservoirs with horizontal wells in Dajiudi gas field.The integration of sequence stratigraphy theory with geophysical prediction methods is an effective way to improve the success rate of "sweet spot"drilling in tight gas sandstone reservoirs.

Sedimentary characteristics and evolution of the Banqiao reservoir in Banqiao oilfield,Huanghua Depression

Song Fan, Su Nina, Hou Jiagen, Yang Shaochun, Zhu Chunyan, Zhang Ruixiang

2012, 33(6):  914-922,943.  doi:10.11743/ogg20120613

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The sedimentation of the Banqiao reservoir in the Banqiao oilfield occurred during the most active tectonism of the Huanghua Depression in the Bohai Bay Basin.The basin expanded so quickly that the accommodation space it provided was far more spacious for sedimentation.The typical deep-water gravity flow deposits were formed in the northern and central parts of the oilfield,while the shallow-water beach-bar deposits were formed in the southern part under the control of palaeo-geomorphology.Eight sedimentary micro-facies were identified in the study area through core observation and study of facies markers and logging facies.The gravity flow channel facies can be divided into 4 microfacies including main channel,distributary channel,channel flank and overflow.The beach-bar facies also can be divided into 4 microfacies,including main bar,bar flank,inner-margin beach and outer-margin beach.The sedimentary scale shrunk vertically due to many factors such as lake basin expansion,sediment input,palaeo-geomorphology and palaeo-climate.At the end of deposition,the gravity flow channel deposits were only developed in the northern part and the winding and forking phenomenon can be easily observed,while the beach-bar disappeared.The fine study of the sedimentary characteristics and evolutions in the area lays a sound foundation for adjustment of development well pattern.

Coupling relations between the petrologic characteristics and stress-sensitiveness in low-permeability sandstone reservoirs:an example from the lower 8^th member of the Lower Shihezi Formation in Sulige-Jier area,Ordos Basin

He Jiang, Fu Yonglei, Shen Guichuan, Zhu Xiaoyan, Wu Qilin

2012, 33(6):  923-931.  doi:10.11743/ogg20120614

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Low-permeability reservoirs are usually developed through natural depletion and the loss of formation pressure is not restored,causing stress sensitivity of reservoirs.We chose the lower 8^th member of the Lower Shihezi Formation in Sulige-Jier area,the Ordos Basin as a case study.Through downhole geological survey,core observation,systematic sampling and testing,and lab correlation and analysis,we studied the coupling relation between the petrologic characteristics and stress-sensitivity and found out that the rock types of the layers of interest are mainly lithic quartz sandstone,followed by siliceous quartz sandstone and small amount of lithic sandstone.The samples were tested with model CMS-400 high-pressure porosity and permeability analyzer.The initial permeability was low and stress-sensitivity was medium to strong and the two presented an obvious positive correlation.Among the rock components,the soft debris and matrix that have low hardness and stability are easily deformed,crushed and displaced under exogenic action.Their contents have positive correlations with the stress-sensitivity.Detrital quartz,rigid debris and cements such asquartz and daphnite have higher hardness and stability and remain stable under exogenic action.And their contents have negative correlations with stress-sensitivity.Different rocks have various stress-sensitivities because of their different petrologic characteristics.During the development of oilfields,we suggest to optimize deliverability by using production pressure differentials at different levels.

Experiments on permeability change of carbonate gas reservoirs in the northeastern Sichuan Basin

Lai Fengpeng, Li Zhiping, Guo Yandong, Jiang Shuangze

2012, 33(6):  932-937.  doi:10.11743/ogg20120615

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The terrestrial stress variation is a comprehensive triaxial change in nature.The traditional stress sensitivity experiment results do not match well with actual situation.In order to establish the relations between permeability and effective stress of carbonate gas reservoirs with ultra-high pressure,we designed two triaxial deformation experiments by using cores,one with and another without pore fluid pressure being con-sidered.Through experiments we established mathematical model for calculating permeability change based on experiment data and analyzed the relations between permeability and effective stress of carbonate gas reservoirs with ultra-high pressure in northeastern Sichuan Basin.The results show that the permeability declines more quickly in the experiment with pore fluid pressure being considered than that without pore fluid pressure being consi-dered under the same effective stress.The dimensionless permeability reduces by 57.48% for the No.3 core sample when the effective stress reaches 50MPa.For cores with well developed fractures,the experiment with pore fluid pressure being considered can better reveal the changing patterns of the permeability.In contrast,for those with poorly developed fractures,the experiment without pore fluid being considered works better for understanding the permeability variation of the matrix.Compared with other ultra-high pressure gas reservoirs both domestic and abroad,the reservoirs in the study area have higher stress-sensitivity.The experimental study provides a solid basis for highly efficient development of the gas reservoirs in the study area.

Research and application of gravity drainage in steam flooding:an example from the high-dip angle area of Qi-40 block in Liaohe oilfield

Jiang Yanyan, Yang Jun, Chen Difang

2012, 33(6):  938-943.  doi:10.11743/ogg20120616

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Steam channeling is one of the main factors influencing production.Taking the high-dip angle area in the Qi-40 block of the well group 65 in Liaohe oilfield as an example,the steam injected into the reservoirs tends to channel upwards by density difference and causes production to decline,especially in reservoirs with high-dip angle.The numerical simulation analysis of oil saturation for the well group 17-028 shows that the remaining oil distributes mainly in down-dip bottom of the reservoirs.Based on the study we put forward three adjusting schemes:drilling new wells,reorganizing the well pattern and performing gravity drainage.Comparison of the operation index of these three schemes shows that gravity drainage through simply adjusting steam injection parameters is the most effective and the simplest scheme of all.By performing the technology,we could produce a uniform downward pressure gradient and push the steam to flow down.Through optimization of the technology,we could determine the reasonable parameters for steam injector.The prediction shows that the recovery-reserve ratio of the high-dip angle area in the Lotus Ⅱ reservoir can reach 7.4% when the dryness degree of the upper steam injector array is 30%,and the gas injection volume of the middle and lower steam injector arrays declines by 20% and 50% respectively.Field testing indicates that the technology is feasible in oilfield development since it can effectively slow down steam channeling and increase gas-oil ratio.

Application of probabilistic method in reserve calculation for the CLFS oilfield project

Li Jiemei, Tan Xuequn, Xu Huaming, Zhang Peng

2012, 33(6):  944-950.  doi:10.11743/ogg20120617

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The Monte Carlo simulation was applied to reserve calculations to honor the uncertainties of reserve parameters.As geologic parameters themselves are random in nature,they are treated as stochastic variables that randomly choose values in real number domain with different probabilities.Taking reserve calculation of the CLFS project as an example,suitable valuing methods were selected for parameters with uncertainties in accordance with the SPEC standards.These parameters include oil-bearing areas,porosity,effective thickness,saturation and crude volume coefficient.The value ranges of each parameter were determined and their values at P10,P50 and P90 were calculated.The final calculation result is just a range of reserves,covering the full value ranges from conservative to optimistic.By doing so,we could master the overall situation concerning the reserve of the oilfield so as to provide more reliable bases for exploration and development strategies and reduce decision-making risks.