井筒内气体扩散侵入定量计算模型

油基钻井液中更易发生气体扩散侵入,伴随油基钻井液使用的增多,有必要研究气体扩散侵入的机理并建立气体扩散侵入定量计算模型。将气体扩散侵入通道划分为气体与滤液界面、滤液滞留区、内滤饼区、外滤饼区、外滤饼区内壁与钻井液界面5个部分,在分析气体扩散侵入机理的基础上,推导了井筒内气体扩散侵入控制方程及解析解。基于解析解,给出了气体扩散侵入参量(循环工况气体侵入速率、循环工况井筒内气体总量、静止工况气体最大浓度、静止工况井筒内气体总量)的定量计算公式。结合算例分析,得到了储层长度、储层孔隙度、钻井液滤失对气体扩散侵入的影响规律。除了静止工况最大气体浓度外,气体扩散侵入参量与储层长度成线性关系。储层孔隙度的增加会促进气体的扩散侵入,但这种促进作用非常弱,低孔隙度储层和高孔隙度储层的扩散侵入参量没有数量级的差别。钻井液滤失对气体的扩散侵入具有非常明显的抑制作用,无滤失时的气体扩散侵入参量显著大于滤失时的情况。

Quantitative calculation model of diffusion gas influx in wellbore

Diffusion gas influx is more prone to happen in oil-based mud. Along with the increased use of oil-based mud, it is necessary to study the influx mechanism of diffusion gas influx and establish its quantitative calculation model. The diffusion gas influx channels are divided into five parts, including gas-filtrate interface, filtrate stagnant area, inner filter cake, outer filter cake, inner wall of outer filter cake and mud interface. On the basis of analyzing diffusion gas influx mechanism, the governing equation and analytical solution of diffusion gas influx in wellbore were derived. Based on analytical solution, the quantitative calculation formula of diffusion gas influx parameters (gas influx rate and gas gross in wellbore under circulated conditions as well as maximum gas concentration and gas gross in wellbore under static conditions)was established. The influence laws of reservoir length, reservoir porosity and mud leak-off on diffusion gas influx were obtained by analyzing calculation examples. In addition to maximum gas concentration under static conditions, the parameter of diffusion gas influx shows a linear relationship with reservoir length. The increase of reservoir porosity can promote diffusion gas influx, while such promotion is very weak, and there is no difference in order of magnitude between the diffusion-influx parameters of low-porosity and high-porosity reservoirs. The mud leak-off has an obvious inhibiting effect on diffusion gas influx, and the diffusion gas influx parameter under no leak-off is significantly greater than that under the opposite condition.