﻿ 超临界CO<sub>2</sub>钻井井筒水合物形成区域预测

The Prediction of Hydrate Formation Regions in the Wellbore during Supercritical Carbon Dioxide Drilling
Sun Xiaohui, Sun Baojiang , Wang Zhiyuan, Wang Jintang
School of Petroleum Engineering, China University of petroleum(Huadong), Qingdao, Shandong, 266580, China
Abstract: In order to improve the safety and efficiency of SC-CO2 drilling (supercritical carbon dioxide drilling), it is necessary to deal with the formation of CO2 hydrate in the wellbores. Based on the hydrate formation mechanism analysis, a model for predicting the formation region of CO2 hydrate in the wellbore was built up, and the definite conditions and numerical resolution methods were proposed. Computational analysis was carried out by designing examples. It was shown that hydrate formation in the annulus decreased non-linearly in quadratic polynomial relations with the increasing of injection temperature or inhibitor dosage. And with the increasing of wellhead back pressure, hydrate formation increased with increasing rate reducing gradually (first in logarithmic relations and then in quadratic polynomial relations). The research achievements could provide a theoretical reference for hydrate prevention during SC-CO2 drilling.
Key words: supercritical carbon dioxide drilling    hydrate    inhibitor    mathematical model    temperature field    pressure field

1 水合物形成区域预测模型的建立

1.1 SC-CO2钻井温度场方程

CO2钻井流体循环过程中，主要通过热传导和热对流2种方式与周围地层进行热量传递。在考虑钻杆和环空中CO2流动特性和热物性参数变化的基础上，利用能量守恒原理，得到SC-CO2钻井的流体温度场方程[1, 14]

1.2 SC-CO2钻井环空压力场方程

1.3 水合物的热力学方程

 图 1 不同体系的CO2水合物相平衡曲线 Fig.1 Phase equilibrium curves of CO2 hydrate in different systems
1.4 其他辅助方程

2 模型求解 2.1 方程组的定解条件

2.2 数值求解流程

1) 已知井口注入温度和地层温度梯度，假设钻杆内温度分布；

2) 已知井口回压和钻杆内温度分布，计算环空内的压力分布、物性参数和温度分布；

3) 计算喷嘴压降，由井底向地面计算钻杆内的压力分布和物性参数；

4) 计算喷嘴温降，利用“追赶法”求解钻杆温度场差分方程组成的三对角方程组，得到钻杆内的温度分布；

5) 重复进行2)、3)、4)步，直至循环前后钻杆和环空内各点温度和压力的计算误差满足要求为止;

6) 计算不同条件下CO2水合物的相平衡条件，结合井筒内流体温度压力分布，判断井筒内水合物的形成区域。

3 计算结果分析

 图 2 SC-CO2连续油管侧钻井身结构 Fig.2 Casing program of SC-CO2 coiled tubing sidetracking
3.1 SC-CO2钻井井筒温度压力场分析

 图 3 SC-CO2钻井井筒温度压力分布曲线 Fig.3 Wellbore temperature and pressure distribution curves of SC-CO2 drilling

3.2 SC-CO2钻井环空水合物形成区域预测 3.2.1 不同温度和压力条件下的水合物形成区域

 图 4 不同注入温度下环空水合物形成临界井深曲线 Fig.4 Critical well depth curves of hydrate formation region in annulus at different injection temperatures

 图 5 不同井口回压下环空水合物形成临界井深曲线 Fig.5 Critical well depth curves of hydrate formation region in annulus at different wellhead back pressures
3.2.2 加入热力学抑制剂的水合物形成区域

 图 6 不同抑制剂加量下环空水合物形成临界井深曲线 Fig.6 Critical well depth curves of hydrate formation region in annulus at different NaCl mass fractions
3.3 钻头喷嘴处的水合物预测

SC-CO2流体通过钻头喷嘴时，由于存在节流效应，会引起较大的温降和压降，可能会在钻头喷嘴下游形成水合物，影响正常钻进。

 图 7 不同质量流速下钻头喷嘴处水合物形成预测曲线 Fig.7 CO2 hydrate predicting curves in bit nozzles at different mass velocities

 图 8 不同喷嘴直径下的钻头喷嘴处水合物预测结果 Fig.8 CO2 hydrate predicting results in bit nozzles at different nozzle diameters
4 结论及建议

1) 结合CO2水合物相态曲线及SC-CO2钻井井筒温度压力分布规律，建立了SC-CO2钻井井筒水合物形成区域预测方法，实现了对SC-CO2钻井井筒CO2水合物形成规律的数值模拟。

2) 随着井深增加，钻杆和环空内的温度逐渐升高，升高幅度逐渐减小，沿井筒呈二次多项式形式分布；随井深增加，钻杆和环空内压力逐渐增大，沿井筒近似为线性分布。

3) 环空内水合物形成区域的临界井深，随注入温度升高逐渐减小，减小幅度逐渐增大；随着抑制剂加量增大逐渐减小，减小幅度逐渐变小；随着井口回压增大，先呈对数函数关系增大，后呈二次多项式函数关系增大，增大幅度逐渐变小。

4) SC-CO2钻井过程中，适当增大注入温度、降低井口回压、添加水合物抑制剂以及优化钻头喷嘴尺寸，可以有效抑制井筒内水合物的形成。

5) SC-CO2钻井水合物防治问题的研究尚处于探索阶段，为进一步提高预测精度，建议考虑水合物形成和分解过程对井筒气液组分变化以及温度压力的影响。

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#### 文章信息

Sun Xiaohui, Sun Baojiang, Wang Zhiyuan, Wang Jintang

The Prediction of Hydrate Formation Regions in the Wellbore during Supercritical Carbon Dioxide Drilling

Petroleum Drilling Techniques, 2015, 43(06): 13-19.
http://dx.doi.org/10.11911/syztjs.201506003