沿抽采井筒的煤储层渗透率模型研究

Research on permeability model of coal reservoir along producing wellbores

渗透率是表征瓦斯流动的重要参数,为保证煤矿瓦斯安全高效抽采,有必要探究距抽采井筒不同位置处煤层瓦斯渗流演化特征。然而,瓦斯抽采过程中伴随有效应力、煤基质对瓦斯的吸附/解吸能力以及煤储层温度的不断变化,甚至出现抽采损伤,使得煤层瓦斯运移行为异常复杂。为探究抽采过程的煤层瓦斯渗流特性,在圆柱坐标系下,考虑压力场与温度场变化对煤储层渗透率的影响,构建温度影响的孔隙压力时空演化函数,据此建立应力与温度作用下的煤储层渗透率模型。结果表明:建立的模型能合理描述沿抽采井筒孔隙压力的演化规律以及瓦斯的运移特性,即在恒定外应力的条件下,随抽采时间增加,不同位置处孔隙压力先降低后变化平缓,煤储层渗透率先降低后升高;此外,同一煤储层位置处,考虑温度比不考虑温度的渗透率计算值更低;通过讨论发现,随抽采时间增加,根据裂隙压缩与基质收缩对渗透率演化的不同效应,设置合理的负压抽采方式可提高瓦斯抽采量。

Permeability is an important parameter to characterize gas flow. In order to ensure the safe and efficient gas drainage, it is necessary to explore the evolution characteristics of coal reservoir gas seepage at different positions from the producing wellbores. However, the effective stress, the gas adsorption/desorption capacity of coal matrix and the temperature of coal reservoir are constantly changing with the gas drainage, and even the drainage damage may occur,which makes the gas migration behavior in coal seam extremely complicated. In order to explore the seepage characteristics of coal reservoir gas during gas drainage, the space-time evolution function of pore pressure affected by temperature was constructed in the cylindrical coordinate system with consideration to the influence of pressure and temperature fields on the permeability of coal reservoir, and the permeability model of coal reservoir under the action of stress and temperature was established accordingly. The results show that the established model could reasonably describe the evolution law of pore pressure along the producing wellbores and the migration characteristics of gas, that is, the pore pressure at different positions decreases firstly and then changes gently with the increase of drainage time under the condition of constant external stress, while the permeability of coal reservoir decreases at first and then increases. In addition,the calculated permeability of coal reservoir considering the temperature is lower than that at the same position without considering temperature. It is found through discussion that the gas drainage could be enhanced with the increasing of drainage time by setting a reasonable negative pressure drainage mode according to the different effects of fracture compression and matrix contraction on permeability evolution.