Permeability Change Law And Its Effect On Production In The Process Of CBM Development

Based on the research of massive materials, the characteristics of CBM reservoir,occurrence, desorption, diffusion and seepage mechanism were analyzed. Through triaxialstress test of coal rock, its deformation characteristics were studied, the rock mechanicalparameters were calculated. The changing laws of porosity and permeability wereanalyzed under stress and strain condition. In process of CBM development, with thefalling of formation pressure, effective stress increased, reservoir permeability decreased,and yield of well was cut down. At the same time, as the continuous desorption of CBM,the effective pore space of reservoir increased, and its permeability got bounced. Stresssensitivity called negative effect and permeability rebound called positive effect madereservoir permeability in dynamic change. In this paper, permeability stress sensitiveequation was established at the foundation of bending distortion theory. Permeabilityrebound equation in the process of gas desorption was built using Langmuir Theory. Basedon these studies, the comprehensive mathematical model of coal reservoir permeabilitydynamic change was established, and permeability changing law was analyzed. At thebeginning of exploitation, stress sensitivity played a leading role, reservoir permeabilitykept decreasing, and gas yield went down. Effective pore space rised with the increase ofcumulative desorption quantity. When accumulated to a certain extent, permeability beganto rebound, and gas yield picked up periodically.According to large amounts of actual production data of Jincheng and Hancheng,production characteristics of CBM well were analyzed, and five decline types weresummarized. Stress sensitivity and permeability rebound were both important controllingfactors. Stress sensitivity showed strong in production rising period and stable period.Permeability rebound which periodical occurred mainly in decline period. In order toextend stable period and get a higher yield, it was necessary to reduce stress sensitiveeffect and promote permeability rebound effect. So optimization equation of reasonableformation pressure was set up, and formation pressure in the production process wasoptimized. Reasonable production system which control CBM well output fluctuation was put forward. In drainage decompression period, it could increase displacementappropriately, and shorten drainage time aiming at dynamic liquid level declining steadily.In production rising period, it should reduce displacement to ensure gas yield go upsmoothly. In production stable period, with the goal to prolong stable time, it must tosteady displacement to guarantee pressure drop funnel extend steadily and continually. Inproduction decline period, it should control displacement to ensure that CBM couldadequately desorb and the declining rate slow down. Research achievement has obtaineddesirable results in field application.