Logging Identification And Quantitative Evaluation Of Coal Macrolithotypes

Coal macrolithotypes control the reservoir heterogeneity,and the understanding of its distribution law is of great importance to the exploration and development of coalbed methane(CBM).Traditional methods to evaluate the coal macrolithotypes rely on coring and petrological observation in underground working face.However,these two techniques are non-economical and incapable of obtaining the comprehensive reservoir physical parameters.In recent years,well logging technology is widely used for CBM exploration and development due to its powerful ability and low cost.In this thesis,the logging method to identify coal macrolithotypes is established to evaluate coal macrolithotypes and to optimize the favorable exploration region of CBM,which is meaningful for exploration and development of CBM in the study area.In this study,the No.3 coal seam of Zhengzhuang field in the southern of Qinshui basin was taken as the target seam.Firstly,the L-index model to quantitatively evaluate the brightness degree of coal macrolithotypes was established by choosing the multiple geophysical logging data(AC,DEN,GR and RT)with the principal component analysis(PCA).And this model was adopted to quantitatively evaluate the vertical and regional distributions of coal macrolithotypes in Zhengzhuang field.Then the S-Index model which is based on the above L-index model was established to evaluate the coal seam brightness degree of a single well,which can be used to evaluate the brightness degree of the No.3 coal seam in this study area.Subsequently,the relationship between macrolithotypes and coal property,cleat density,adsorption capacity and gas content were discussed.Finally,the coal seam brightness of a single well was calculated by the S-Index model to investigate the relationships between the S-Index values and the length and height of the hydro-fracture.By combing the key parameters which effect the reservoir reconstruction effects with the gas content characteristics in this region,the favorable zone for CBM development in the study area is optimized.(1)The L-Index model by combing the multiple geophysical logging data with PCA method was established.The identification interval of coal macrolithotypes based on L-Index values(L)were divided into: the bright coal(L≤-32),the semi-bright coal(-32< L≤ 5),the semi-dull coal(5< L≤ 38),and the dull coal(38 < L ≤ 82).Moreover,the S-Index model for quantitatively evaluating the coal seam brightness degree in a single well was established,and the S-index values in Zhengzhuang field range from 1 to 3.7.The greater the value,the lower the coal seam brightness degree in a single well.(2)Based on the analysis of coal maceral composition,the coal facies were divided into three categories by the V/I ratio: the dry-shallow water covered environment(V/I<1),the moist-water covered environment(1.2< V/I< 2.7)and the moist-deep water covered environment(2.7< V/I< 4).Based on the analysis of coal property,cleat density,isothermal adsorption and gas content,it was found that the ash content,volatile content,water content and cleat density of coal show a downward trend with the increasing of the L-Index values,while the fixed carbon content have an opposite trend.When it turns to the brightness degree of coal reservoir,the vitrinite content decreases with the decreasing of coal seam brightness,and the adsorption capacity and gas storage capacity of the coal seam have the same trend.(3)The relationship between the coal seam brightness degree of a single well and the fracture length as well as fracture height was investigated.It was found that the coal seam with lower S-Index values have lower rupture pressures and this kind of reservoir can easily form long fractures with complex networks during the hydraulic fracturing.Long fractures with complex networks are conducive to connecting the distal coal seams,which can increase the CBM production.Based on the analysis the complex relationships between the coal macrolithotypes,the coal structure,the tensile strength difference between the coal bed roof and the coal seam,the tensile strength difference between the coal bed floor and the coal seam and the fracture expansion,the coal reservoir reconstruction effects of the study region can be evaluated.The CBM development potential were calculated by combining the evaluation results with gascontent data.As a result,the No.3 coal seam were divided into three categories.The evaluation results showed that the southwest and the southeast of the Zhengzhuang field are the most favorable zones.