采用双梯度钻井优化深水井井身结构

OPTIMIZATION OF CASING PROGRAM OF DEEPWATER WELL BY DUAL-GRADIENT DRILLING

针对深水钻井中因地层压力和破裂压力间隙过小引起的井身结构设计问题，提出采用双梯度钻井技术。介绍了双梯度钻井技术的基本原理，给出了海底泥线下井眼钻井液当量密度公式。根据传统井身结构设计方法和双梯度的井眼钻井液当量密度的变化规律，推导出套管下入深度的计算公式。结合深水钻井的特点，考虑钻井施工的特殊要求，开发了深水双梯度，钻井井身结构设计及优化程序。实例分析表明，与常规钻井方法相比，双梯度钻井井眼钻井液压力很好地匹配了地层压力和破裂压力之间的间隙。将去除使用2～3层套管鞋深度点，减少井眼和井口尺寸以及降低套管费用。双梯度钻井为深水钻井井身结构设计提供了一种新的优化方法，具有显著的经济效益和工业实用价值。

During deepwater drilling, due to the small difference between the formation pore pressure and fracture pressure, casing program design is challenging, a new technique known as 'Dual Gradient Drilling (DGD)' has been thus developed. This paper presents the basic principle of DGD, and proposes equation for calculating the equivalent density of the drilling fluid in the wellbore below seabed mudline. Based on the conventional casing program design method and the variation rule of the EMD of DGD well, the casing setting depth calculation formula is derived. And considering the features and particular requirements of deepwater drilling, a program for deepwater DGD well casing program design and optimization is developed. The case study herein shows that two or three casing strings can be eliminated by using DGD when compared to conventional riser drilling, which will ultimately reduce the size of both hole and wellhead as well as casing cost. Therefore, DGD may provide an alternative optimization method for deepwater well casing program design, which can produce remarkable economic and industrial values.