Development Dynamics And Productivity Responses Of Coalbed Methane Reservoirs In Hancheng Area

In order to investigate the development dynamics and production responses of the Hancheng coalbed methane(CBM)reservoirs,lots of the main target coals were measured in lab with the conventional methods and new lossless methods and the differences of reservoirs among No.3,No.5 and No.11 coals were analyzed systematically.Then,the production stages were divided with the method of dimensionless gas rate and a comprehensive evaluation system for CBM wells productivity which is on the basis of the correlation analysis and regards grey correlation analysis as the focus,were established after considering the characteristics under the different reservoir combinations.Finally,the CBM well productivity in the future was forecasted with the methods of autoregressive integrated moving average model(ARIMA)and logging well data.These experimental results indicate that the metamorphism degree of the coal is high and the vitrinite is the most important part of the coal composition which results that the coal lithotype is made up of bright and semi-bright,while the mineral content is low.The representation for the porosity and permeability by five methods shows that the coal is a typical low porosity and low permeability reservoir and their distribution is anisotropic.What's more,the change and distribution for the lump coals,lump-pulverized coals as well as pulverized coals could be identified qualitatively through the resistivity,caliper and natural Gamma ray logging curves,which demonstrates the pulverized coals and the lump-pulverized coals could be found along with the tectonic zone.The different production stages including water drainage stage,unstable gas production stage,stable gas production stage and gas decay stage could be divided by the way of dimensionless gas rate in the Hancheng area.The boundary of these stages is 0.3,0.6 and 0.8,respectively.At present,the most obvious characteristics of the production wells is high water rate and low gas rate,indicating the effects of gas production need be improved.Under the condition of different reservoir combinations,the gas production characteristics are various.If the No.5 coal is added into production,the production effects would become poorer whereas when the No.3 coal is added into production,the production effects would be better.In addition,it is worth noticing that if the well is located near the faults,the No.11 coal should not be fractured or not even been drilled.The wells which have kept producing for more than two years and their distributions are reasonable were used to analyze the well productivity.The results indicate that the Hancheng area is in the early stage of development.The most important three factors affecting gas production of the CBM wells are,in decreasing order,the ratio of the critical desorption pressure to the initial reservoir pressure,the volume of the fracturing fluid per meter,the gas content.Among these factors,the ratio of the critical desorption pressure to the initial reservoir pressure determines frequently the degree of difficulty of the water drainage and decompression;the gas production potential(the thickness x the gas content)and the gas production deliverability(the gas production potential x the permeability)could characterize the comprehensive effect of the thickness of the coal seam,the gas content and the permeability on gas well productivity.Higher ratio of the critical desorption pressure to the initial reservoir pressure,higher gas production potential and deliverability are more favorable for gas productivity.In combination with gas well production performance,the parameter indexes of high production CBM wells are presented as follows:the buried depth range should be 400?700 m;the thickness of the coal seam should not be lower than 5 m;the ratio of the critical desorption pressure to the initial reservoir pressure should be greater than 0.3;the gas content should be greater than 6 m3/t;the volume of the fracturing fluid per meter should not be less than 40 m3/m;the volume of fracturing sand per meter should not be less than 3 m3/m;and 1%KCl active water fracturing fluid should be used as the fracturing fluid.The gas rate history matching and forecast for the wells in the Hancheng area with the method of ARIMA revealed that the average relative error is less than 5%.At the same time,through the relationship between compensation density logging,compensation neutron logging and productivity,the intersection template of the well productivity was established which is consist of high gas productivity index area,middle gas productivity index area and low gas productivity index area.With the well logging data of a new well,the productivity could be forecasted qualitatively.