Permeability Change Of Different Coal Ranks Reservoirs During Drainage And Production Of Coalbed Methane Vertical Wells

The characteristics of pore and fracture,lithotype and coal property and coal quality during different rank coal reservoirs are various,initial reservoir permeability and dynamic evolution characteristics of permeability during drainage and production of coalbed methane maybe exist significant difference,explore permeability characteristics of different rank coal reservoirs and dynamic geological effects during drainage and production systematically has important theoretical and engineering significance for reveal the evolution mechanism of permeability deeply,customize drainage control plan scientifically and realize effective exploration and development of different rank coal reservoirs.Thesis selected subbituminous coal and high-volatile bituminous coal in the eastern margin of Ordos Basin in North China,volatile bituminous coal in Pingdingshan coalfield,low-volatile bituminous coal,semi-anthracite and anthracite in Qinshui Basin as research object,adopted“permeability testing device for CO₂ displacement and unsteady state method”independently developed by our research group and considered the conditions of formation temperature,pressure,ground stress and coalbed methane vertical well’s development,carried out the permeability dynamic evolution simulation experiments under different effective stress conditions and matrix shrinkage conditions of coal samples of different ranks,established fluid-solid coupling mathematical model of coalbed methane vertical well and solved by COMSOL Multiphysics,explored the dynamic evolution of permeability of different rank coal reservoirs under the constraint of vertical well drainage engineering,put forward some targeted suggestions for coalbed methane vertical wells during drainage and production.The main achievements and understandings of this thesis are as follows:（1）Under the condition of varying effective stress,the permeability of coal samples decreases with the increase of effective stress in the form of negative exponential(y=Ae^-bx),when the effective stress less than 6 MPa,the negative effect of the effective stress on the permeability of coal samples is more pronounced.By comparing different evaluation parameters,the results show that the stress sensitivity of coal samples is more reasonable.Data analysis shows that the stress sensitivity of coal samples decreases with t32he increase of effective stress and the relationship between stress sensitivity and effective stress conforms to exponential form.In addition,the stress sensitivity coefficient decreases with the increase of coal rank under the same effective stress conditions,that is,low-rank coal>middle-rank coal>high-rank coal.When the effective stress less than 6 MPa,the stress sensitivity coefficient of coal samples of low-rank significantly larger than that of middle-rank,high-rank,and decreases rapidly when the effective stress exceeds 6 MPa,which id lower than that of coal samples of middle and high rank.It is concluded that the mechanical properties of coal samples can reflect the sensitivity of coal samples to some extent,the mechanical properties and degree of fracture development of coal samples are positively correlated with the stress sensitivity of coal samples’permeability,and the compressive strength of coal samples is linearly correlated with the irreversible damage degree of coal samples’permeability under stress.（2）The response of the permeability of coal samples of different coal ranks to the adsorption equilibrium pressure is basically the same,confirm to exponential form(y=Ae^bx).The permeability of coal samples increases greatly under low adsorption equilibrium pressure and the inflection point of adsorption equilibrium pressure for coal samples of middle and low rank is larger than that of high rank coal samples,after which coal samples’permeability increases rapidly,with 4 MPa as the critical point basically.In addition,the improvement effect of coal matrix shrinkage on the permeability is obviously limited by effective stress,the improvement effect of coal matrix shrinkage on coal samples’permeability becomes worse and has phased characteristics.Because the stress sensitivity of high rank coal is low,the permeability increase caused by matrix shrinkage effect of high rank coal is higher than that of middle and low rank under the same effective stress condition,and the cumulative permeability increase of most coal samples first decreases and then increases with the increase of effective stress,while there is no obvious regularity for middle and low rank coal samples.The pressure inflection point at which matrix shrinkage effect begins to play a role in the middle and low rank coal samples is higher than that in the high rank coal samples.It is concluded that there are differences in the strength and micropore development characteristics during different coal samples,which affect the gas diffusion and seepage channels,and then affect the improvement of the permeability of coal samples by matrix shrinkage.（3）The results of simulation experiment and numerical simulation of coal reservoir’s permeability under different depressurization rates show that the negative effect of effective stress on coal reservoir’s permeability should be mainly considered in the depressurization stage of drainage,pressure holding phase and pressure control stage,and the depressurization rate should be determined based on the stress sensitivity of coal reservoir,mechanical strength of coal and elastic modulus.After the stage of high and stable production,more consideration should be given to the positive improvement of coal reservoir permeability caused by the coal matrix shrinkage effect,the depressurization rate should be determined by the mechanical properties of coal,degree of pore and fracture development and CH₄ adsorption capacity.In addition,the numerical simulation results show that the matrix shrinkage effect has different improvement effects on the permeability of coal reservoirs of different ranks,especially in the gas production attenuation stage,the matrix shrinkage effect of middle and low rank coal reservoir is significant,the permeability of the reservoir will be significantly improved.However,although the stress sensitivity of high rank coal reservoir is weak,the permeability improvement caused by matrix shrinkage effect is limited due to relatively undeveloped seepage channel.（4）The negative effect of effective stress and positive effect of coal matrix shrinkage on reservoir’s permeability should be fully comprehensive considered during drainage and production of coalbed methane vertical wells and based on this to formulate reasonable drainage control measures according to different coal reservoirs.Low rank coal reservoir should mainly consider stress sensitivity to avoid reservoir’s damage caused by rapid increase of effective stress during the whole process of drainage.Middle rank coal reservoir should pay more attention to the coal matrix shrinkage effect to ensure that the depressurization rate does not inhibit the improvement of reservoir’s permeability caused by matrix shrinkage before the pressure holding phase.As for high rank coal reservoir,the improvement effect of matrix shrinkage effect is not significant,more consideration should be given to using the negative effect of effective stress to promote fully play of coal matrix shrinkage effect.There exists a critical point about well bore’s depressurization rate in coalbed methane wells,which is between 0.06MPa/d and 0.075MPa/d.When the well bore’s depressurization rate is lower than the critical point,appropriately increasing the depressurization rate has positive effect on the expansion of the influence range of coal reservoir’s pressure drop,when it is higher than the critical point,excessively depressurization rate may cause serious permeability stress damage,which will affect the production of coalbed methane well.At the depressurization stage of drainage,well bore’s depressurization rate of coalbed methane wells of low rank,middle rank coal and high rank coal should be controlled at<0.03MPa/d,0.045MP/d～0.06MPa/d and0.06MPa/d～0.075MPa/d respectively.At the pressure holding phase and pressure control stage,well bore’s depressurization rate of coalbed methane wells of low rank,middle rank coal and high rank coal should be controlled at 0.045MPa/d～0.06MPa/d,0.03MP/d～0.045MPa/d and 0.045MPa/d～0.06MPa/d respectively.At the stage of high and stable production and gas production decline,we should maintain stable production of coalbed methane wells on the basis of full depressurization of coal reservoir in early stage to avoid reservoir’s damage caused by large fluctuations of reservoir pressure.This thesis has 70 figures,28 tables and 145 references.