Leakage Risk Evaluation Model for Single Wells of CO2: A case study

ABSTRACT

From the experience of CO₂ storage in the past, the leakage part of CO₂ may be the wellbore, fractures, faults, and cap rocks in the formation. Once CO₂ leaks from the wellbore, it will pose a serious threat to the life safety of workers near the well pad in a short time. Therefore, from the perspective of single well leakage in the process of CO₂ storage, this paper studies the risk of single well CO₂ injection leakage in Changqing Oilfield, Ordos Basin, simulates CO₂ leakage from the wellbore, and simulates it with the diffusion model after toxic gas leakage, so as to obtain the CO₂ concentration at different distances from the leakage point. The range of affected area after CO₂ leakage is determined by the critical value of CO₂ leakage diffusion concentration. According to the calculation in this paper, the area near the leakage source is divided into (taking summer as an example: the wind speed is about 3.3 m/s): fatal area (2.2 m), serious injury area (4.3 m), inhalation reaction region (19.5 m) and safety area (> 50 m). The leakage risk of CO₂ injection buried wellbore in low-permeability reservoir in Ordos Basin is evaluated.

KEYWORDS:

• CO₂
• CCUS
• wellbore leakage
• risk evaluation
• H₃ low permeability reservoir

Acknowledgments

This work was supported by the China Postdoctoral Science Foundation (No. M2019650965), SKL of Oil & Gas Reservoir Geology and Exploitation Engineering (No. PLN201920), Major R & D Plan of Sichuan Province (No.2020YFQ0034).

Nomenclature

Abbreviation	=
CCUS	=	carbon capture, utilization and storage
CCS	=	carbon capture and storage
NRAP-IAM-CS	=	National Risk Assessment Partnership Integrated Assessment Model for Carbon Storage
PLUME	=	Predicting Leakage Using Multi-phase Equations
N-S equations	=	Navier-Stokes equations
C6	=	Chang 6
C8	=	Chang 8
C9	=	Chang 9
Parameters	=
QG	=	gas leakage rate, mg/s
Cd	=	gas leakage coefficient
A	=	crack area which is Tubing area in this paper, m2
M	=	relative molecular mass
R	=	gas constant, J/(mol·K)
TG	=	gas temperature, K
Y	=	outflow coefficient, constant
P	=	medium pressure in the container, Pa
P0	=	pressure of surface condition, Pa
K	=	adiabatic index of gas, K value used in this study is 1.30
C	=	CO2 concentration of the reception point r (xr, yr, zr) with the leakage point source locates at s(0, 0, zs)
Δh	=	lift height of the smoke plume, m
μ	=	local wind speed, m/s
σy	=	horizontal diffusion parameters of the horizontal wind direction at the leeward distance xr
σz	=	vertical diffusion parameters of the horizontal wind direction at the leeward distance xr
xr	=	Position in X direction at receiving point, m
yr	=	Position in Y direction at receiving point, m
zr	=	Position in Z direction at receiving point, m
zs	=	Position in Z direction at leakage point, m
${\sigma }_{j,k}$	=	A time step of vertical diffusion parameters or horizontal diffusion parameters of the horizontal wind direction at the leeward distance xr
$t_k$	=	Current time step
$t_{k-1}$	=	Previous time step
a	=	Diffusion coefficient parameter
b	=	Diffusion coefficient parameter
c	=	Diffusion coefficient parameter
d	=	Diffusion coefficient parameter
e	=	Diffusion coefficient parameter
$d{T}_a/dZ$	=	atmospheric temperature gradient above the geometric height of the leakage point, K/m
$U$	=	average speed of the leakage point, m/s
$Q_h$	=	heat release rate of leakage gas, kJ/s
$Q_v$	=	actual gas leakage rate, m3/s
$T_S$	=	outlet temperature of the leakage gas, K
$T_a$	=	ambient atmospheric temperature, K
$P_a$	=	atmospheric pressure, Pa
r	=	environmental risk value of the calculation point
r0	=	environmental risk value of the source of the leakage risk
l0	=	maximum impact radius of the severe injury region, m
l	=	maximum influence radius, m
x0	=	distance between the calculation point and the leakage source point, m
R1	=	number of people in the research region
R2	=	the percentage of death
R3	=	occurring probability of accident
R4	=	probability of adverse weather condition

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data Availability statement

All the Data about this research are available in this manuscript. Readers can access the data supporting the conclusions of the study. The basic field data is shown in following.

“In the research region, the atmospheric pressure is 88,000 Pa, the ambient temperature is 20°C, and the temperature gradient in the height direction is 0.006 K/m. In the process of CO₂ leakage, wellbore pressure decreases sharply. The pressure at the wellhead (Tubing pressure) is 0.2 MPa, and the wellbore temperature at the surface is 35°C. The shape of the oil pipe leakage vent is circular. The diameter of the vent pipe is 76 mm (3^1/2 in), and the leakage coefficient C_d is 1. The average wind speed at the test point surface is 3 m/s. When the leaked gas is CO₂, the relative molecular mass M is 44 g/mol (0.044 kg/mol), the gas constant R is 8.314 J/(mol·K). When calculating the plume height, the average wind speed at the outlet of vent is 20 m/s, and the actual exhaust rate is 0.01 m³/s.”

Based on this data, using theoretical Equations (1)(1) $Q_G=Y{C}_{d}QAp\sqrt{\frac{MK}{R{T}_G}{\left(\frac{2}{K+1}\right)}^{\frac{K+1}{K-1}}}$(1) to (7), the result can be obtained as shown in Figure 4 to 7. According to Equations (8)(8) $r=\left\{\begin{array}{c}{r}_{0}0<x_0\le l_0\\ \frac{r_0}{l-l_0}\left(l-x_0\right)l_0<x_0\le l\\ 0x_0>l\end{array}\right.\hspace{0.17em}$(8) , (9(9) $r_0=R_1{R}_2{R}_3{R}_4$(9) ) and Table 5, the risk value of CO₂ leakage environment by different wind speeds and the environmental risk values in different regions near the leakage source can be obtained as shown in Tables 6, 7, respectively.

Additional information

Funding

This work was supported by the China Postdoctoral Science Foundation [No. M2019650965]; Major R & D Plan of Sichuan Province [No.2020YFQ0034]; SKL of Oil & Gas Reservoir Geology and Exploitation Engineering [No. PLN201920].