﻿ 低渗透油藏CO<sub>2</sub>埋存井筒泄漏风险评价模型

低渗透油藏CO2埋存井筒泄漏风险评价模型Leakage Risk Evaluation Model for Wells in the Process of CO2 Storage in Low Permeability Reservoirs

Abstract: In the process of Carbon Capture, Utilization, and Storage (CCUS), CO2 may leak from wellbore, faults, cracks and fractures, and may also overflow from reservoirs and caprocks by molecular diffusion. The high concentration CO2 from the wellbore leakage may cause great danger to the people’s life on the wellsite. Therefore, it is necessary to develop an evaluation model for CO2 leakage. The risk of CO2 wellbore leakage can be evaluated by the model of toxic gas leakage and diffusion. According to the gas diffusion concentration, the critical damage and toxicity value of CO2 concentration, the influence region of CO2 diffusion can be determined. Based on the simulation model, the leakage source area is divided into fatal region, heavy injury region, untoward effect region and safety region. The leakage risk of the CCUS is evaluated in a low permeability reservoir, in Changqing Oilfield, Ordos Basin, China. The forecast of the CO2 leakage risk is then used to guide the installation of the near surface CO2 concentration monitor and to design the CO2 leakage disposal scheme.

1. 引言

2. 井筒泄漏规模与风险评价

Figure 1. Wellbore schematic diagram (cited from literature 10)

3. CO2的泄漏模型

1) CO2的泄漏速率

${Q}_{G}=Y{C}_{d}QAp\sqrt{\frac{MK}{R{T}_{G}}{\left(\frac{2}{K+1}\right)}^{\frac{K+1}{K-1}}}$ (1)

$Y={\left(\frac{{P}_{0}}{P}\right)}^{\frac{1}{K}}×{\left[1-{\left(\frac{{P}_{0}}{P}\right)}^{\frac{K-1}{K}}\right]}^{\frac{1}{2}}×{\left[\frac{2}{K-1}×{\left(\frac{K+1}{2}\right)}^{\frac{K+1}{K-1}}\right]}^{\frac{1}{2}}$ (2)

2) CO2的泄漏浓度的分布

$C=\frac{{Q}_{G}}{2\text{π}\mu {\sigma }_{y}{\sigma }_{z}}\mathrm{exp}\left(-\frac{{y}_{r}^{2}}{2{\sigma }_{y}^{2}}\right)\left\{\mathrm{exp}\left[-\frac{{\left({z}_{s}+\Delta h-{z}_{r}\right)}^{2}}{2{\sigma }_{z}^{2}}\right]+\mathrm{exp}\left[-\frac{{\left({z}_{s}+\Delta h+{z}_{r}\right)}^{2}}{2{\sigma }_{z}^{2}}\right]\right\}$ (3)

${\sigma }_{j,k}^{2}={\sigma }_{j,k}^{2}\left({t}_{k}\right)-{\sigma }_{j,k}^{2}\left({t}_{k-1}\right)$ (4)

$\sigma =a+b{x}^{g}+dx$ (5)

Table 1. Diffusion coefficient in horizontal direction

Table 2. Diffusion coefficient in vertical direction

*A级x > 3.10 km，扩散系数恒等于5.00 km；B级x > 32.00 km，扩散系数恒等于5.00 km。

3) 烟羽抬升高度

$\Delta H={Q}_{n}^{\frac{1}{2}}{\left(\frac{\text{d}{T}_{a}}{\text{d}Z}+0.0098\right)}^{-\frac{1}{3}}{U}^{-\frac{1}{3}}$ (6)

${Q}_{h}=0.35{p}_{a}{Q}_{v}\frac{\Delta T}{{T}_{S}}=0.35{p}_{a}{Q}_{v}\frac{{T}_{S}-{T}_{a}}{{T}_{S}}$ (7)

4. 实例井筒泄漏分析

Figure 2. The relationship between the different distances of leak points and CO2 concentration

Figure 3. CO2 concentration at 100 meters from the leak point under different leak velocities

Figure 4. CO2 concentration at 100 meters from the leak point under different surface wind speeds

Figure 5. CO2 concentration at 100 meters from the leak point under different reservoir permeability

5. CO2泄漏风险评估

1) 影响范围预测

Table 3. Environmental risk values of CO2 leakage at different wind speeds

2) 泄漏风险评价

$r=\left\{\begin{array}{l}{r}_{0}\text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}0<{x}_{0}\le {l}_{0}\\ \frac{{r}_{0}}{l-{l}_{0}}\left(l-{x}_{0}\right)\text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}{l}_{0}<{x}_{0}\le l\\ 0\text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}{x}_{0}>l\end{array}$ (7)

${r}_{0}={R}_{1}{R}_{2}{R}_{3}{R}_{4}$ (8)

Table 4. Risk values for different areas near leakage sources

3) 风险分级

Table 5. The risk classification

6. 结论

H3区域CCUS实施过程中井筒泄漏风险评价表明：风速1 m/s的泄漏源点及距泄漏源点100 m处的环境风险值数量级为10−5，与中等危险等级相对应，CO2泄漏引起的事故危险性在井场附近，建议在距离井场中心50米范围内的四个方向安装四个用于监测空气中CO2浓度的实时在线监测装置，实时在线监测CO2浓度。

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