低渗透地层气体钻井液体侵入数值分析

气体钻井过程中会遇到低渗透地层，地层液体的侵入遵循低速非达西渗流规律，用基于达西渗流建立的传统预测模型来计算气体钻井过程中低渗透地层液体的侵入量将会产生较大误差。为此，在分析井底压力与地层液体侵入量关系的基础上，考虑了实际钻井中逐渐打开储层的过程，提出了气体钻井过程中低渗透地层液体侵入量的预测方法，并建立预测模型计算钻至任意井深时地层液体侵入量。计算表明，启动压力梯度对低渗透地层液体的侵入量影响较大，用基于达西渗流建立的预测模型计算的低渗透地层液体侵入量较实际值偏高，不考虑钻井过程也不能准确预测气体钻井过程中地层液体的侵入量，且预测误差随储层渗透率、厚度的增加而增大。进一步说明不能用传统的达西公式计算气体钻井中低渗透地层液体侵入量。建议用低速非达西渗流理论来预测气体钻井过程中低渗透地层液体的侵入量具有重要的意义。

NUMERICAL ANALYSIS OF LIQUID INFLUX OCCURRED DURING GAS DRILLING IN LOW-PERMEABILITY RESERVOIRS

Low-permeability formations will be encountered during gas drilling,the formation liquid will flow into the well as low-speed non-Darcy seepage.If the conventional prediction model,established on basis of Darcy seepage,is used to calculate the liquid influx form the low-permeability formation during gas drilling,it will result in big errors.In this paper,on basis of analyzing the relationship between the bottom hole pressure and the liquid influx volume and by taking into consideration the actual process of gradual opening of the reservoir,a method is introduced to predict the formation liquid influx during gas drilling in low-permeability formations,and the corresponding prediction models are established to calculate the influx volume at any depth.The calculation indicates the liquid influx calculated by Darcy-seepage based model is higher than the actual,and the forecasting error will become greater and greater by the increment of permeability and thickness of the formation.It further proves that the traditional Darcy formulae cannot be used to calculate the liquid influx during gas drilling of low-permeability formations.It is recommended to use low-speed non-Darcy seepage theory to predict the liquid influx.