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煤层井下低温取心过程瓦斯损失量推算模拟试验

Simulated test of gas loss calculation during the freezing coring process inunderground coal seam

  • 摘要: 低温取心技术既能在煤层中定点取样,又可有效减少取样过程的瓦斯逸散,在井下煤层瓦斯含量精准测定中应用前景广阔。为了准确推算低温取心过程的煤心瓦斯损失量,依托含瓦斯煤低温取心吸附解吸模拟平台,在不同管壁外热、不同平衡压力(1~4 MPa)条件下开展了低温取心型煤瓦斯解吸模拟试验,研究了低温环境下的瓦斯解吸特征;并基于图解法,采用3种不同扩散模型对低温取心的解吸曲线进行拟合分析,评价了3种损失量推算模型的优劣。结果表明:常规取心过程产生的摩擦热会大大增加煤心瓦斯损失量;瓦斯解吸量随取心管壁温度上升逐渐增大,当管壁外热分别为60、70、80、90 ℃时,30 min内解吸量分别为6.587、7.082、7.460和7.981 cm3/g,较之恒温30 ℃解吸量的增幅分别达到13.71%、22.25%、28.78%和37.77%。低温取心时,煤心解吸出现倒吸回流现象,这是降温导致煤样罐内压力小于大气压造成的。经不含瓦斯煤倒吸对比试验校正,低温取心时的真实解吸量随着管壁外热降低逐渐减小,管壁外热分别为60、70、80、90 ℃时,30 min内低温取心解吸量分别为3.578、3.842、4.215和4.76 cm3/g,较之常规取心的解吸抑制率在40%~46%间。低温环境下,瓦斯解吸量随吸附平衡压力升高逐渐增大,但增长幅度逐渐减小;低温取心时的扩散系数较之常规取心减少得多,且随着降温呈现线性降低。采用Logistic增长模型对低温取心解吸曲线的拟合精度明显优于 \sqrtt 模型和指数模型,损失量推算误差小于0.5%,能够满足低温取心时瓦斯损失量推算的需要。

     

    Abstract: The freezing coring technology can not only perform the fixed-point sampling in coal seams, but also effectively reduce gas leakage during the sampling process. It has broad prospects for application in the precise measurement of gas content in coal seams. To accurately evaluate the gas loss of coal sample during the freezing coring process, the simulated tests of gas desorption during coring were carried out under the different external heats and the equilibrium pressures (1−4 MPa) by using a simulation platform for gas adsorption/desorption on gas bearing coal. The gas desorption characteristics in low-temperature conditions were also studied. Based on the graphical method, the three diffusion models were adopted to analyze the gas desorption curves under low-temperature conditions, and the fitting performance of the gas loss estimation models were evaluated. The results show that the tube frictional heat generated during the conventional coring process greatly increases the gas loss; and the gas desorption amount gradually increases with the temperature rise in the core tube wall. When the friction heat of the tube wall is 60, 70, 80, and 90 ℃, the desorption amounts within 30 min are 6.587, 7.082, 7.460, and 7.981 cm3/g, respectively; and the increments in desorption amounts compared to that of 30 ℃ are 13.71%, 22.25%, 28.78%, and 37.77%, respectively. During the freezing coring, there is a back-flow phenomenon in the desorption, which is caused by the coal sample cooling leading to the pressure inside the sample tank lower than atmospheric pressure. Through the correction tests of the pure back-flow of coal without gas, the gas loss during the freezing coring gradually decreases as the external heat of the tube wall. When the external heat of the tube wall is 60, 70, 80 and 90 ℃, the desorption amounts within 30 min are 3.578, 3.842, 4.215, and 4.76 cm3/g, respectively; and the desorption inhibition rates reach 40%−46%, compared with the conventional coring. At low temperatures, the desorption amount gradually increases with the rise of adsorption pressure, but the growth rate gradually decreases. The diffusion coefficient during the freezing coring is much reduced compared to the conventional coring, and it shows a linear decrease with cooling. The fitting accuracy of the gas desorption curve at low temperatures via the logistic growth model is significantly better than that of the \sqrtt model and the exponential model, with a loss estimation error of less than 0.5%. Therefore, the logistic growth model can meet the needs of gas loss estimation during the freezing coring in coal seams.

     

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