平衡钻井井底压力自动控制技术研究

在欠平衡钻井期间，钻井液密度确定后，如何适时适量控制井口回压是个难题。其核心问题是如何在钻井过程中保持井底循环压力低于地层孔隙压力或储层压力在一个合适的范围内（即设计负压值） 。目前，现场普遍采用的调节井底压力的方法是通过手动调节节流阀改变套压使井底压力保持在合适的范围内，但该方法存在不少问题。为此，提出了对井底压力实施自动控制来保证欠平衡钻井过程中井底压力的精确、有效控制的方法，采用理论计算和现场试验相结合的技术，总结出了一套节流阀压降随开度的变化规律，从而很好地解决了欠平衡钻井过程中井底压力自动控制的技术难题，适合现场推广应用。

AUTOMATIC CONTROL OF BOTTOMHOLE PRESSURE DURING BALANCED DRILLING

During the underbalanced drilling, once the drilling fluid density is determined, the difficult work left will be how to control the wellhead back pressure at the proper value at the proper time. The core of this difficulty is how to keep the circulating pressure at the well bottom lower than the pore pressure or reservoir pressure within a proper margin (i.e., to design the underbalance degree). Currently choke valve adjusted manually is commonly used to change the casing pressure so as to keep the well bottom pressure within the proper range. This paper introduced an automatic control method which could maintain the well bottom pressure more accurately and effectively. Theoretic calculation and field tests were combined to study the variation relationship between pressure drop and the opening size of the choke valve, and discovered how to consequently solve the troublesome problem by automatically controlling the well bottom pressure during the underbalanced drilling. This paper recommends this to be used in the field production.