Permeability Dynamic Effect And Productivity Response Of Undersaturated Coalbed Methane Reservoirs

In order to study permeability dynamic change and production response of the undersaturated CBM reservoirs,this thesis discusses the dynamic change of porosity and permeability of the main target coals in lab,establishes permeability change models for undersaturated CBM reservoirs and carries out the models' sensitivity analysis and calculates the permeability changes of exploration wells,analyses the productivity characteristics for different cases of combination production,quantitative evaluates the factors affecting well production performance,and in end analyses the different factors influencing production performance in recovery of the multilayers undersaturated CBM reservoirs by numerical simulation technology based on the analysis of lab test data,production data and the usage of permeability models and production simulation technology.A number of lab tests were carried out to systemically study the stress effect on coal permeability during production.The test analyses show that the stress sensibility of poro-perm incease from 3#to 11#coalbed reservoirs,and slipage coefficient is closely related to permeability;macro porosity/fractures are more stress-depended than medium-micro porosity in nuclear magnetic resonance test;the increase rate of Young's modulus versus stress is undifferentiated in different coalbeds,and so is the decrease rate of Poisson ratio;the stress sensibility of gas relative permeability performs much stronger than that of water.Combinated the existed permeability change models and the exploitation mechanism of undersaturated CBM reservoirs,new models precisely describing the permeability change in production of undersaturated CBM reservoirs were established and the sensitivity of new models and example analysis were then studied.The essence of these new models is to piecewise describe the permeability dynamic change of undersaturated reservoirs in ciritical desorption pressure.The permeability was controlled only by effective stress effect above ciritical desorption pressure,while controlled not only by effective stress effect but also matrix shrinkage effect and Klinkenberg effect below ciritical desorption pressure.The sensitivity analyses show that gas content,Langmuir volume and Langmuir curve-match for shrinkage coefficient are the most important parameters affecting the permeability dynamic change.Taking the example of exploration wells,it concluded that the repound effect of permeability in 3#coal is the best and the strongest heterogeneity,while 5#coal is the worst and the weakest heterogeneity.Correlation analysis demonstrated that the repound effect of permeability in the study areas is mainly controlled by adsorption gas saturation.Meanwhile,production performance of CBM wells with different layer combinations was investigated by tracking production data,and the affecting factors were quantitatively evaluated.gas production performed best and water production generally low in those wells with combinated production of 3#and 5#.However,wells with production of 11?3#+11#and 3#+5#+11#performed unsatisfactory due to great difference of production and large water production as well as high water level in some wells,showing the existence of large water production in 11#coal.Grey correlation analyses found out that the most important effect on maximum gas production is pseudo-saturation,average gas production total thickness.And water production in the time of initially producing gas is the most vital factor affecting maximum gas production per meter and average gas production per meter.Besides,CBM production response of undersaturated CBM reservoirs with strong heterogeneity and differences of permeability dynamic change was discussed by numerical stimulation.The stimulation result shows the effect of gas content/saturation,permeability dynamic change and aquosity in surrounding rocks on CBM multilayer production in undersaturated reservoirs is very obvious.According to poro-perm dynamic change and multilayers production performance of undersaturated CBM reservoir in Hancheng CBM field,permeability model analysis,production dynamic analysis and numerical simulation study show together that gas saturation is the critical factor affecting permeability dynamic change and maximum gas production,and the impact of total coal thickness,cumulative water production in emerge of gas,permeability dynamic change characteristics and aquosity of surrounding rocks on average gas production are obvious.Thus,the favorable development area of those undersaturated CBM reservoirs is located on the CBM bearing zone which has high gas saturation,thich coal seam,homogeneous permeability and poor aquosity of surrounding rocks.