Influence of depressurization rate on gas production capacity of high-rank coal in the south of Qinshui Basin, China
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摘要
A desorption simulation experiment with the condition of simulated strata was designed. The experiment, under different depressurizing rates and the same fluid saturation, was conducted on the sample from 3# coal of Daning coal mine in Jincheng, Shanxi Province. The gas production rate and pressure change at both ends of the sample were studied systematically, and the mechanisms of some phenomena in the experiment were discussed. The experimental results show that, whether at fast or slow depressurizing rate, the methane adsorbed to high-rank coal can effectively desorb and the desorption efficiency can reach above 90%. There is an obvious inflection point on the gas yield curve during the desorption process and it appears after the pressure on the lump of coal reduces below the desorption pressure. The desorption of methane from high-rank coal is mainly driven by differential pressure, and high pressure difference is conducive to fast desorption. In the scenario of fast depressurization, the desorption inflection appears earlier and the gas production rate in the stage of rapid desorption is higher. It is experimentally concluded that the originally recognized strategy of long-term slow CBM production is doubtful and the economic benefit of CBM exploitation from high-rank coal can be effectively improved by rapid drainage and pressure reduction. The field experiment results in pilot blocks of Fanzhuang and Zhengzhuang show that by increasing the drainage depressurization rate, the peak production of gas well would increase greatly, the time of gas well to reach the economic production shortened, the average time for a gas well to reach expected production reduced by half, and the peak gas production is higher.
A desorption simulation experiment with the condition of simulated strata was designed. The experiment, under different depressurizing rates and the same fluid saturation, was conducted on the sample from 3# coal of Daning coal mine in Jincheng, Shanxi Province. The gas production rate and pressure change at both ends of the sample were studied systematically, and the mechanisms of some phenomena in the experiment were discussed. The experimental results show that, whether at fast or slow depressurizing rate, the methane adsorbed to high-rank coal can effectively desorb and the desorption efficiency can reach above 90%. There is an obvious inflection point on the gas yield curve during the desorption process and it appears after the pressure on the lump of coal reduces below the desorption pressure. The desorption of methane from high-rank coal is mainly driven by differential pressure, and high pressure difference is conducive to fast desorption. In the scenario of fast depressurization, the desorption inflection appears earlier and the gas production rate in the stage of rapid desorption is higher. It is experimentally concluded that the originally recognized strategy of long-term slow CBM production is doubtful and the economic benefit of CBM exploitation from high-rank coal can be effectively improved by rapid drainage and pressure reduction. The field experiment results in pilot blocks of Fanzhuang and Zhengzhuang show that by increasing the drainage depressurization rate, the peak production of gas well would increase greatly, the time of gas well to reach the economic production shortened, the average time for a gas well to reach expected production reduced by half, and the peak gas production is higher.
A desorption simulation experiment with the condition of simulated strata was designed. The experiment, under different depressurizing rates and the same fluid saturation, was conducted on the sample from 3# coal of Daning coal mine in Jincheng, Shanxi Province. The gas production rate and pressure change at both ends of the sample were studied systematically, and the mechanisms of some phenomena in the experiment were discussed. The experimental results show that, whether at fast or slow depressurizing rate, the methane adsorbed to high-rank coal can effectively desorb and the desorption efficiency can reach above 90%. There is an obvious inflection point on the gas yield curve during the desorption process and it appears after the pressure on the lump of coal reduces below the desorption pressure. The desorption of methane from high-rank coal is mainly driven by differential pressure, and high pressure difference is conducive to fast desorption. In the scenario of fast depressurization, the desorption inflection appears earlier and the gas production rate in the stage of rapid desorption is higher. It is experimentally concluded that the originally recognized strategy of long-term slow CBM production is doubtful and the economic benefit of CBM exploitation from high-rank coal can be effectively improved by rapid drainage and pressure reduction. The field experiment results in pilot blocks of Fanzhuang and Zhengzhuang show that by increasing the drainage depressurization rate, the peak production of gas well would increase greatly, the time of gas well to reach the economic production shortened, the average time for a gas well to reach expected production reduced by half, and the peak gas production is higher.
引文
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[2]WU Shiyue.Preliminary approach on the low of gas diffussion and permeation on coal seams.Journal of Shanxi Mining Institute,1994,12(3):259-263.
[3]LI Xiangfang,SHI Juntai,DU Xiyao,et al.Transport mechanism of desorbed gas in coalbed methane reservoirs.Petroleum Exploration and Development,2012,39(2):203-213.
[4]NIE Baisheng,ZHANG Li.Diffusion micro-mechanism of coal bed methane in coal pores.Coal Geology&Exploration,2000,28(6):20-22.
[5]ANNA L O.Ground water flow associated with coalbed gas production,Ferron sandstone,east central Utah.International Journal of Coal Geology,2003,56(3):69-95.
[6]ANDREW R S.Hydrogeologic factors affecting gas content distribution in coal beds.International Journal of Coal Geology,2002,50(2):363-387.
[7]KOHLER T E.Development and application of a compositional coalbed methane production model.Pennsylvania:The Pennsylvania State University,1999.
[8]ZHAO Qingbo,SUN Fenjin,LI Wuzhong.New theory for coalbed gas exploration in China.Natural Gas Industry,1996,16(5):25-28.
[9]ZHAO Qun,WANG Hongyan,LI Jingming,et al.Study on mechanism of harm to CBM well capability in low permeability seam with quick drainage method.Journal of Shandong University of Science and Technology(Natural Science),2008,27(3):27-31.
[10]LI Jinhai,SU Xianbo,LIN Xiaoying,et al.Relationship between discharge rate and productivity of coalbed methane wells.Journal of China Coal Society,2009,34(3):376-380.
[11]DUAN Pinjia,WANG Zhiyin,ZHAI Yuyang,et al.Research on reasonable depressurization rate in initial stage of exploitation to coal bed methane.Journal of China Coal Society,2011,36(10):1690-1692.
[12]YANG Zhaobiao,ZHANG Zhengguang,QIN Yong,et al.Optimization methods of production layer combination for coalbed methane development in multi-coal seams.Petroleum Exploration and Development,2018,45(2):297-304.
[13]LI Mengxi,WANG Lilong,CUI Xinrui,et al.Output characteristics of vertical wells and dewatering control method used in Fanzhuang Block of Qinshui CBM Field.China Coalbed Methane,2011,8(1):11-13.
[14]LI Yangmin,WANG Lilong,LIU Guowei,et al.Study on coal reservoir damage mechanism in dewatering and extraction process of CBM wells.China Coalbed Methane,2010,7(6):39-43.
[15]LIU Huihu,SANG Shuxun,FENG Qingling,et al.Study on stress sensitivity of coal reservoir during drainage of coal-bed methane well in Southern Qinshui Basin.Journal of China Coal Society,2014,39(9):1873-1878.
[16]ZHANG Suian,CAO Lihu,DU Caixia.Study on CBM production mechanism and control theory of bottom-hole pressure and coal fines during CBM well production.Journal of China Coal Society,2014,39(9):1927-1931.
[17]LI Laicheng,FU Xuehai,LUO Bin.Experimental study on large coal sample desorption under successive depressurization.Coal Geology of China,2015,27(9):18-21.
[18]KANG Yongshang,SUN Liangzhong,ZHANG Bing,et al.Discussion on classification of coalbed reservoir permeability in China.Journal of China Coal Society,2017,42(S1):186-194.
[19]KANG Yongshang,DENG Ze,LIU Honglin.Discussion about the CBM well draining technology.Natural Gas Geoscience,2008,19(3):423-426.
[20]XU Xiaokai.Stress sensitivity and pressure drop propagation law of coal reservoir during drainage process of CBM vertical well.Beijing:China University of Mining and Technology,2016.
[21]GAO Caixia,SU Xuefeng,LIU Junying,et al.Study on characteristics of stress sensitivity and permeability variations of high-rank coal in Qinshui Basin.China Coalbed Methane,2017,14(5):11-13.
[22]LI Mengxi,WANG Lilong,HU Qiujia,et al.Coalbed methane development characteristics and management in Fanzhuang Block of Qinshui Basin.China Coalbed Methane,2013,10(3):3-8.