原位保压取心气体组分对煤层瓦斯压力测算的影响

Influence of gas components on the determination of gas pressure in coal seams under in-situ pressure-preserved coring

基于保压取心工艺测定煤层瓦斯压力和含量,是实现瓦斯参数快速精准测定的有效方法。瓦斯是一种以甲烷为主的多组分混合气体,煤对于不同气体的吸附能力不同,混合气体中各组分气体分压也存在差异,不同气体组分占比将影响煤层瓦斯含量和压力的测算结果。为实现保压取心煤层原位瓦斯压力的精准测算,采用数值模拟和理论计算相结合,研究气体组分对瓦斯压力测算的影响。通过分析取心器内煤心瓦斯压力演化过程,将多组分气体吸附模型引入瓦斯运移理论,建立考虑多组分气体的双重介质瓦斯压力演化方程,并应用于数值仿真。模拟结果显示,煤心进入岩心筒后裂隙瓦斯压力初始先迅速下降而后逐渐上升,基质瓦斯压力一直缓慢下降,岩心筒内自由空间的瓦斯压力由初始0.1 MPa缓慢上升,数小时后3个压力平衡,平衡压力远小于煤心原始瓦斯压力;CO_(2)组分对平衡瓦斯压力值的影响最大,当气体为纯CH_(4)时平衡压力最小。同时,推导了考虑多组分气体的保压取心煤层原位瓦斯压力计算公式,根据设定的平衡压力反算煤层瓦斯压力,理论计算煤层瓦斯压力值与数值模拟数据的对比结果吻合较好,Pearson相关系数为99.89%;利用取心器平衡压力反算煤层瓦斯压力时,计算结果随CO_(2)、CH_(4)组分增加和N_(2)组分减少而减小,气体为纯CH_(4)时反算的煤层瓦斯压力值最小。利用保压取心测算煤层原位瓦斯压力时,若不考虑气体组分而以纯CH_(4)计算,可能会低估煤层瓦斯压力值,尤其是煤层瓦斯压力较大且CO_(2)气体组分占比高时,误差更大,为准确测算煤层瓦斯压力,应考虑瓦斯气体组分的影响。

Determining the pressure and content of coal seam gas based on pressure-preserved coring is an effective method for quickly and accurately calculating gas parameters.Coal seam gas is a multi-component gas mixture dominated by methane.Given that coals show different adsorption capacities for different gases and that the components in the mixed gas exhibit different partial pressures,the proportions of gas components can influence the calculation results of the content and pressure of coal seam gas.To accurately determine the in-situ gas pressure in coal seams based on the pressure-preserved coring,this study investigated the influence of gas components on the determination of gas pressure by combining numerical simulation with theoretical calculation.Based on the analysis of the evolutionary process of coal-core gas pressure in the coring device,this study introduced an adsorption model for multi-component gas into the gas migration theory.Then,we developed an equation for the evolution of dual-medium gas pressure considering the effects of multicomponent gas and applied this equation to numerical simulation.The numerical simulation results are as follows:(1)After coal cores entered the core barrel,the gas pressure in fissures decreased rapidly and then increases gradually,the gas pressure in the matrix decreased slowly,and the free-gas pressure in the core barrel rose gradually from the initial 0.1 MPa.The three pressures reached equilibrium after a few hours,with the gas pressure after equilibrium being much less than the initial pressure of coal cores.(2)The CO_(2) component had the greatest impact on the gas pressure after equilibrium,which,however,was the lowest for gas consisting only of pure CH_(4).Furthermore,this study deduced an equation for calculating in-situ gas pressure in coal seams based on the pressure-preserved coring that considers the effects of multi-component gas.For the gas pressure in coal seams obtained through the reverse calculation based on the set gas equilibrium pressure,it agreed well with the data from numerical simulation,with a Pearson correlation coefficient of 99.89%.For the gas pressure in coal seams determined through the reverse calculation based on the equilibrium pressure in the coring device,it decreased with increased CO_(2) or CH_(4) component and decreased N_(2) component and reached the minimum for coal seam gas consisting only of pure CH_(4).Therefore,when determining the in situ gas pressure in coal seams based on pressure-preserved coring assuming that coal seam gas consists only of pure CH_(4),the determined in-situ gas pressure in coal seams may be underestimated,especially in the case of a high proportion of the CO_(2) component.Therefore,to accurately determine the gas pressure in coal seams,it is necessary to consider the influence of the components of coal seam gas.