| The exploration and development of oil and gas shows that,the widely developed fracture reservoir has become one of the main areas of oil and gas production in the 21st century.D gas field is one of important natural gas production base for SINOPEC,due to the ceaseless great developments in upper Paleozoic sandstone reservoir.And,the effect of fracture in the lower Paleozoic Ordovician carbonate reservoir on development of gas reservoirs is gradually revealed,some problems such as fracture development characteristics,logging identification and effective evaluation,distribution and the relationship with gas enrichment are still not clear.Thus,this research takes carbonate fracture in Ordovician weathering crust in D gas field as research object,foucses on the problems mentioned above,and trys to establish a set of fractxUre evaluation methods suitable to research object and find out the the distributing disciplinarian of fracture and its effect on gas enrichment.Firstly,based on coring data,the macro characteristics of cracks(morphology and surface features,filling,crack width,length,etc)are obtained and analyzd.At the same time,the micro crack characteristics of fractures are also obtained by sampling and petrophysical analysis.On this basis,combined with the results of karst system and regional structure of this region,and tested by crack filling isotope determination,AE and inclusion measurement,a comprehensive analysis of the formation period and causes of the cracks in the study area were carried out to make clear fracture types and the difference of characteristics.Secondly,the fractures were qualitatively and quantitatively evluated by calibrating logging data with core datas:? The logging response to fractures and relationship between fracture and the logging curves were established by cores,drilling and testing data.? through analysis and selection of conventional logging characteristics which are sensitive to the fracture,the multi-steps variable optimization method ineluding the sensitivity analysis of the variables,the normalization of the variables and the correlation analysis of the characteristic variables,is proposed,and then taking discriminant analysis of fracture by Bayesian and stepwise discriminant methods.?The fracture parameters such as porosity and width are calculated,and verified by compared with the core observation results.The method and standard of fracture evaluation based on conventional logging in well profile are established.Finaly,based ont the research above,the main controlling factors of fracture development in vertical and plane were analyzed.And then,combined with the test data,the role of fractures in the migration and accumulation of natural gas was further analyzed.Through the research above,a set of suitable to Ordovician carbonate rock reservoir fracture identification methods was setteled,and obtain the following understanding:Fracture types in study area include Structure,weathering and dissolution fractures.Structural fracture developed in multiple geology times,especially in Yanshan-Himalayan period.Weathering and dissolution fracture development presents zonality vertically,Weathering fracture developed in the first phase of vertical seepage in karst belt(M51-2 sub-section),and dissolution fractures mainly developed in the second and the third phase of karst belt(M53-5 sub-section).Logging response of foll filled fractures is insensitive,but part of unfilled low angle extensional fractures are characterized with higher acoustic and low density value.Half-unfilled structural fractures are characterized with platform or spikes low resistivity value in Micro-spherical focused logging.The average fracture porosity and width are 0.26%and 0.012 mm respectively,in Msm dolostone stratum,Effective fracture porosity and width are greater than 0.35%and 0.019 mm respectively,in M5s limestone stratum,Effective fracture porosity and width are greater than 0.31%and 0.015 mm respectively.Structural deformation,karst paleomorphology and rock thickness are the three main controlling factors of fracture development in the study area.the fracture distribution of the main developmental stratum(M51?M52?M55)based on the evaluation method of the two main controlling factors indicate the fractures in M51-2 sub-section are mainly developed in the karst highland and karst slopes overlapping area.The sub-section of M55 is characterized by the development of Structural fractures and collapsible dissolution under the control of lithology,the extensive development and interconnection of the fracture system in M51-5 sub-sections is not only provides a channel for the Carboniferous Taiyuan group of natural gas down to the Majiagou group,but also forms some localized enrichment zones of natural gas in the plane of the karst highlands,karst slopes and other steep zone. |
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