Characteristics Of Gas Production Pressure And Its Influence On Productivity In Fanzhuang Block Of Southern Qinshui Basin

CBM wells of Fanzhuang block was regarded as the research object in this paper. Based on geological exploration information of coal resource and drainage data of CBM wells and experimental results, chracteristics of NO.3 coal reservoirs were analyzed, the improved calculation method of the gas production pressure was proposed, and regional distribution of the gas production pressure was clarified. By comparing relationship between the gas production pressure and the critical desorption pressure and combining with gray correlation analysis, the main geological control factors and engineering control factors of the gas production pressure were analyzed. Finally, based on the relationship between decline rate of dynamic liquid level, reservoir pressure, migration resistance and the gas production pressure, the influences of the gas production pressure on drainage was investigated and corresponding drainage system suggestions were proposed.According to the prophase characteristic of production curve of CBM wells, the desorption point in time of adsorbed gas was determined, and the calculation formula of bottom hole flowing pressure in the state of gas-water two-phase was improved. By comparing and verification, the precision of improved calculation formula was high. The errors are generally smaller and less than 8%. So it can accurately determine the value of gas production pressure. The results show that the gas production pressure of NO.3 coal seam has the characteristic that it is higher in northern and central of study area, lower in southern and eastern regions and has greater difference in local area.Through comprehensive analysis, it was believed that the gas production pressure is the result of many factors coupling. It should be equal to the difference between the theory values of critical desorption pressure and the CBM migration resistance. It is mainly influenced by geological factors including CBM content, langmuir parameters, the depth of the coal seam, fault structure, reservoir pressure, coal petrographic pharacteristics. Meanwhile, in the development process of CBM wells, engineering factors such as fracturing, and reservoir sensitivity also have an important impact on the gas production pressure by affecting CBM migration resistance. The influence degree order of various factors on the gas production pressure is that CBM content> reservoir pressure> vitrinite content> depth> specific pore surface area> average pore diameter> vitrinite reflectance> langmuir parameter> faults and folds > effective stress effect > microfractures width> water sensitivity and speed sensitivity> critical dimension of fracturing fluid into the micropores> drainage system> gas components.The influences of gas production pressure on CBM productivity mainly performance in the influences of early drainage strength and the relationship between gas production pressure and coal reservoir pressure on productivity, and the effect of CBM migration resistance on CBM recoverable rate. For these three aspects, corresponding drainage system suggestions were proposed. Early drainage rate should not be too large, especially when bottom hole flowing pressure is close to the gas production pressure, the drainage rate should be controlled within 5m/d. During the development process of CBM wells, reservoir damage and CBM migration resistance should be reduced, and the gas production pressure should be improved.