Well-logging Identification And Quantitative Evaluation Of Free Gas In Coal Measure Strata

Free gas in coal measure strata plays a key role for the high production of coal measure strata gas reservoirs,and it is still difficult to be identified qualitatively and calculated quantitatively.In this paper,firstly,the collected raw logging data are pre-processed,and then the basic reservoir parameters of coal measure strata are evaluated.In terms of fluid property recognition method of coal measure strata,the traditional neutron-density cross plot is insensitive to identify the free gas in coal measure strata,which mainly reflects the quality,rank and water feature of the coal seam.During the study,the paper finds that the acoustic logging value is more sensitive to gas,recognizes the free gas in coal measure strata through establishing the cross-well acoustic background value.As the advanced logging technologies,Poisson's ratio and fluid compressional coefficient overlap method and formation micro-resistivity imaging logging apparent formation water resistivity method are used to identify the free gas in coal measure strata and get effective results.In terms of free gas in coal measure strata quantitative evaluation research,the paper regards the free gas in coalbed similar to gas-water layer issue.Using the gas saturation model of acoustic data indicator factor which is recurred through the field test water production rate,the free gas in coalbed evaluated quantitatively.By electric logging and neutron-density intersection methods,the gas saturation of sandstone formation and limestone formation have been evaluated.Based on the single well evaluation,stratigraphic correlation analysis and plane distribution law of coal measure strata in the study area are studied.Vertically,it is generally considered that if the coal seam contains free gas,then its top and bottom sandstone formation and limestone formation that contain free gas.On the contrary,if the coal seam does not contain free gas,then its top and bottom sandstone formation and limestone formation that do not contain free gas.In the plane,free gas is mainly concentrated in the structure of the high position,and the free gas enrichment zone basically does not develop faults.The favorable area of the study area is mainly located in the central and northern zones.The technical innovation of the paper is mainly manifested in the following two aspects:(1)in terms of qualitative identification of free gas in coal measure strata,for the first time,the paper proposes the cross-well acoustic background value method,combined with the measured gas saturation and drainage data analysis to determine the reference wells which contains free gas and does not contain the free gas,and then using the no free gas reference well to identify the free gas in coal measure strata.This method has solved the identification problem of free gas in coal measure strata effectively;(2)in terms of quantitative calculation of free gas in coal seam,for the first time,the paper established empirical relationships between the gas production rate and cross-well acoustic value and reconstruction value differential ratio to achieve the estimation of free gas saturation.The method has settled the problem of quantization of free gas in coal seam successfully.