有效应力恢复条件下水力压裂后煤层气分区渗流特征OA北大核心CSTPCDEI

Hydraulic fracturing is dominantly used to improve the production of coalbed methane(CBM)well,but it may induce fracture zones in the coal seams.Due to the effective stress,the flow behaviors in fracture zones have direct impacts on the effectiveness of CBM extraction.To determine such impacts,taking the coal samples from the Qinshui Basin as examples,this paper designs a serial/parallel multi-cavity test using intact coal samples,microfractured coal samples and fracture-penetrated coal samples to investigate the transverse CBM flow along zones and longitudinal CBM flow crossing zones after hydraulic fracturing.A permeability model of different serial-parallel damaged coal rocks under the influence of effective stress is established to analyze the flow behaviors in fracture zones induced by hydraulic fracturing under effective stress recovery conditions and predict the overall permeability of coal seams at different fracturing depths and in different disturbance ranges.The findings demonstrate that the total flow exhibits a negative exponential pattern of drop with the restoration of effective stress in both serial and parallel tests.In the parallel tests,the percentage of flow through the intact coal samples is very low,the percentage of flow through the microfractured coal samples gradually rises when the effective stress is restored,and the percentage of flow through the fracture-penetrated coal samples progressively falls until it becomes equal.Longitudinal flow shows a smaller total permeability than transverse flow.The total permeability of longitudinal flow is greatly affected by the permeability of intact coal samples,while the total permeability of transverse flow is dependent on the permeability of fracture-penetrated coal samples.Based on the size of transverse/longitudinal permeability,the CBM production profiles can be divided into monomodal and bimodal curves.It is concluded that increasing the hydraulic fracture depth and fracture width around the borehole is conducive to improving the gas extraction.This offers a reference for designing the parameters of hydraulic fracturing in CBM reservoirs to promote the beneficial extraction of CBM.