Study On Dynamic Changes Of Coal Reservoir Permeability In Baode Block During Coalbed Methane Recovery

The research intended to present the dynamic variation characteristics of the permeability of the coal reservoir in Baode block. The research approaches consist of experimental research and theoretical analysis. Taking the coal reservoir in Baode as the research object, experiments are taken to study the separating effect of effective stress and matrix shrinkage on the permeability of coal reservoir, and the slippage effect on the strain, then the comprehensive influence and strain variation characteristics are observed as well. By analyzing existing dynamic prediction models of coal reservoir permeability and their assumptive conditions, the assumptive conditions fitting the structure and strain characteristic of the coal reservoir in Baode was presented, then a corresponding mathematical model of permeability was constructed. Based on the model, the author analyzed the model sensitivity, adapting it to the situations concerning multicomponent gas seepage and the well controlling block, which was used to analyze the dynamic variation characteristics in the range of well bore.1. The effect characteristics presented by the effective stress, the matrix shrinkage and the slippage effect on the permeability and strain of the coal reservoir in Baode.In the process of the drainage of the coal reservoir, affected by the effective stress, matrix shrinkage, slippage effect, the coal permeability changes along with the pore pressure. In order to find out the effect characteristics of the three effects on the permeability of the coal reservoir, as well as to provide the theoretical and data support for the modeling, the separating effect of the three effects on the variation of permeability and strain of the reservoir were studied respectively.To study the single effect of the effective stress on the permeability of the coal reservoir, the other two effects should be eliminated, which the author made it through the manipulation of the experiment. That was to use the non-absorptive gas as the gas medium, and control the variation range of the pore pressure in the experiment. Under these conditions, the following are analyzed: the variation law for the permeability of the coal reservoir, the stress sensitivity of the coal, variation characteristics of the coal cleat compressibility. The results show that the variation law between the effective stress and the permeability is constrained by the negative exponential function. The stress sensitivity decreases as the effective stress increases, so in the last stage of the drainage, the effect of the effective stress is getting less. The coal cleat compressibility is negatively correlated to the effective stress, and only in the stage with high effective stress can the coal cleat compressibility be taken as a constant.To study the single effect of the matrix shrinkage on the strain of the coal reservoir, the other two effects should be eliminated, which the author made it by using CO2 as the gas medium and adjusting the confining pressure in the experiment. The strain characteristic was analyzed, Langmuir volume strain and Langmuir pressure was determined, which was to provide the parameters for the subsequent modeling work. The study finds out that the drop of the pore pressure leads to the increase of the strain of the coal, and the meliorating action to the coal permeability by the matrix shrinkage effect is increasingly enhanced. The average of Langmuir volume strain is 0.010631, and the average of Langmuir pressure is 3.92 MPa.To study the single effect of the slippage effect on the permeability of the coal reservoir, the other two effects should be eliminated, which the author made it by setting the pore pressure range, adjusting the confining pressure, using the non-absorptive gas medium in the experiment. Through experiments, the slippage effect was proved to be existed in the reservoir, its effect on the permeability of the coal was analyzed, the correlation between confining pressure, pore pressure and slippage effect was analyzed, and the slip factor of the Baode was determined. The results are that as the confining pressure getting higher and the pore pressure getting lower, the slippage effect on the coal is getting more apparent. Under the experimental condition, the slippage effect contributes much to the permeability of the coal. The slip factors, namely helium, methane and carbon dioxide, are decreasing orderly under the same condition.2. The mathematical model of permeability of the reservoir in Baode blockThrough the analysis on the theory and experimental data, the research demonstrates the rationality of the assumptive conditions for existed mathematical models, and raises an idea that to construct a mathematical model of permeability suitable for the coal reservoir, the horizontal or voluminal strains should be considered objectively existed, but rather being assumed zero. Meanwhile, the following should be assumed: a) the principal strain in the horizontal are equal in coal reservoir, the coal matrix particles are incompressible, and the rock mechanics properties of the same coal bed are isotropic, b) coal beds are horizontal distributed and free from tectonic stress, the temperature is constant, and the permeability depends on the cleat net, without regard to the influence of the desorption and diffusion rate of gas. Under the above assumptions, based on the stress-strain constitutive equations of nonisothermal poroelastic medium and the horizontal strain model, the author constructed mathematical models of permeability for the coal reservoir with regard to different factors and reservoir conditions, including the models for saturated and undersaturated coal reservoirs with or without regard to the slippage effect.Based on model analysis, the recovery pressure and the rebounding pressure of the coal reservoir is defined, thereby the coal reservoirs can be divided into three types according to the variation characteristics for coal reservoir permeability. The types are permeability rebounding coal reservoir which can be divided into subtypes named sort I and sort II, permeability decaying coal reservoir, and permeability meliorating coal reservoir.3. Analysis and Adaption of the mathematical models of permeability for the coal reservoir.The author analyzed the parameter sensitivity of the models. Reservoir pressure?cleat compressibility?Langmuir pressure are negatively correlated to the permeability of the coal reservoir. Langmuir volume, Young modulus, Poisson ratio and slip factor show positive correlation.The CBM is a mixture of multi-component, so its adsorption property is different to the monocomponent gas. The author adapted the model which derived from the coal reservoir of monocomponent to analyze the permeability of the multi-component.Based on the models, the author studied the distribution characteristics of the permeability of the coal reservoir in well controlling range. In the drainage of CBM, there is the permeability meliorating zone and the decaying zone in the well controlling range of the low-rank coal. Furthermore, as the well bore pressure decreases, the radius of the permeability meliorating zone is going larger, and the increasing range is getting bigger. When well controlling range enlarges as the drainage proceeding, the decaying width is increasing in the permeability decaying zone of the coal reservoir.4. The coupling relationship between the stress-strain and the permeability.To confirm the rationality of the newly constructed models, as well as to study the coupling relationship between the stress-strain and the permeability while co-affected by effective stress, matrix shrinkage, slippage effect, the author simulated the co-effect circumstances and observed the variation of the permeability and strain in the coal reservoir. The influencing characteristics exerted by effective stress, matrix shrinkage on the coal deformation are also analyzed. The result illustrates that through the co-effect of the three effects, the permeability shows a trend of decreasing initially and increasing afterwards as the pore pressure getting lower. Besides, the larger the confining pressure is, the less the variation range will be. The previous research reported that a variation phase of permeability in which the slippage effect plays a leading role would emerge at the last stage of the drainage of the coal reservoir. But experiments in this article shows that in the last stage of the drainage of the coal reservoir, only the variation stages largely affected by the effective stress and slippage effect respectively are existed, and the slippage effect is a meliorating factor to the permeability. The confining pressure plays a decisive role in the permeability of the reservoir, based on which the three effects' action on the permeability would perform a limited adjustment. And the large the confining pressure is, the weaker the impact of the three effects will be. The strain characteristic of the coal indicates that under a low confining pressure, there are radial and voluminal expansion strains, which will transform to compressive strain as the effective stress increases. Under a high confining pressure, all the strains are compressive. For the coal reservoir, the horizontal and vertical strains are of large difference because of the shallow burial depth, and the difference is getting larger as the drainage goes on.The author used the parameters of the coal reservoir in Baode to confirm the predicting effectiveness of the permeability model for saturated coal reservoirs with regard to the slippage effect. Compared the predictive results with the experimental data, it is confirmed that the model constructed in this research is of high accuracy, and more suitable for the prediction of the coal permeability in the range of high pore pressure.