Wavy horizontal sections are typically encountered in horizontal gas wells, which will result in gas accumulation on top of the wavy horizontal sections. This gas accumulation can be a problem and may trigger gas kick or blowout accident while tripping and pulling this gas into the vertical section. In this paper, a numerical model for gas accumulation and gas migration in the wavy horizontal sections of the horizontal gas well is developed; meanwhile, the gas accumulation and gas migration process is numerically investigated. The results show that the gas exhausting time in the wavy horizontal section increases with the increase of the wellbore curvature and the critical drilling fluid flow velocity for gas exhausting increases with the increase of the wellbore curvature. When the drilling fluid flow velocity is higher than the critical drilling fluid flow velocity for gas exhausting, no gas accumulation will occur. With all other parameter values set constant, the number of the wavy horizontal sections has a great effect on the gas-liquid flow pattern while it has little effect on the efficiency of the gas exhausting. This work provides drilling engineers with a practical tool for designing the drilling fluid flow velocity to avoid gas kick or blowout accident in horizontal gas well drilling.
Horizontal wells are widely used in petroleum and natural gas development, and they have many advantages over traditional vertical wells, such as increased drainage area and high production [
Gas accumulation in the wavy horizontal sections of the horizontal gas well.
In the past decades, several key studies have been conducted on horizontal well control and gas migration. Vefring et al. established new models for the gas slip and rise velocities in near horizontal wells and various models for different gas removal mechanisms [
The diameter of the borehole in the wavy horizontal sections of the horizontal gas well is 0.2159 m. Gas is accumulated on the top of the wavy horizontal sections. The schematic diagram of the gas accumulation in the wavy horizontal sections of the horizontal gas well is shown in Figure
Schematic diagram of gas accumulation in the wavy horizontal sections of the horizontal gas well.
Establishing a reasonable two-dimensional physical model of the wavy horizontal sections of the horizontal gas well according to the actual situation is the key to the simulation of gas accumulation and gas migration. In order to reduce the calculation amount and improve the simulation efficiency, only the fluid domain is established. The borehole diameter is 0.2159 m, and the horizontal distance on both sides of the model is 1 m. A horizontal wellbore physical model with different wavy horizontal sections is established, as shown in Figure
ICEM CFD is selected as the meshing software, and quadrilateral meshing is adopted [
Calculation model of wavy horizontal section of the horizontal gas well. (a) 1 time wavy horizontal section borehole model. (b) 1.5 times wavy horizontal sections borehole model. (c) 2 times wavy horizontal sections borehole model.
Meshing model.
The gas-liquid flow process in the wavy horizontal sections of a horizontal gas well is unstable. Even if the boundary conditions remain the same, the physical quantity of the fluid during the flow still has strong pulsation, so the simulation model should be run in a turbulent state [ Continuity equation: Momentum equation: Turbulence equation: Turbulent energy equation Dissipation rate equation
In the expression,
Gas-liquid density and dynamic viscosity are shown in Table
Gas-liquid density and dynamic viscosity.
Parameter name | Density (kg·m−3) | Dynamic viscosity (Pa·s) |
---|---|---|
Drilling fluid | 997.0 | 9.028 × 10−4 |
Gas | 1.185 | 1.86 × 10−5 |
An unsteady, implicit separation, and solving algorithm is used. The governing equations to be solved are continuity equations that satisfy mass conservation, momentum conservation, energy conservation, momentum equations, energy equations, and turbulence equations that take turbulence properties into account [
Definite solution conditions consist of a combination of boundary conditions and initial conditions. Inlet conditions: velocity inlet is used, and the inlet speed is set to 1.2 m/s, 1.6 m/s, and 2 m/s. The inlet boundary is set to a gas inlet volume fraction of 0. The turbulence definition method selects the hydraulic diameter and turbulence intensity. Outlet conditions: the pressure outlet and turbulence definition method are used to select the hydraulic diameter and turbulence intensity. Initial conditions: considering the influence of gravity, set
Fluent software uses residuals to reflect the convergence of the calculations and judges whether the iterative process converges through the final iterative residual output of each equation for each iteration step [
There are various combinations of gas accumulation simulations in the wavy horizontal sections of horizontal gas wells. Here, simulation studies of gas accumulation in single wavy horizontal section and complex wavy horizontal sections are performed.
According to the actual working conditions, wavy horizontal sections with different curvatures are selected, and models of gas accumulation of wavy horizontal sections with different curvatures are established to simulate gas-liquid two-phase flow. The diameter of the wavy horizontal section is 0.2159 m, the total horizontal length is 5 m, the drilling fluid flow velocity is 1.6 m/s, the initial gas accumulation at the top is 0.3 m3, and the curvature of the wavy horizontal section is 0.2, 0.3, 0.4, and 0.5. As displayed, red is drilling fluid and blue is gas.
It can be seen from Figure
Flow states under different curvatures. (a) Curvature is 0.2. (b) Curvature is 0.3. (c) Curvature is 0.4. (d) Curvature is 0.5.
The borehole diameter is 0.2159 m, the total horizontal length is 5 m, the initial gas accumulation at the top is 0.3 m3, the curvature is 0.3, and the drilling fluid flow velocity is 1.2 m/s, 1.6 m/s, and 2 m/s, as shown in Figure
Observing Figure
Flow states at different inlet velocities. (a) Drilling fluid flow velocity is 2 m/s. (b) Drilling fluid flow velocity is 1.6 m/s. (c) Drilling fluid flow velocity is 1.2 m/s.
From the above phenomenon, it can be seen that there is a critical flow rate to make the gas just exit the wavy horizontal section of the horizontal gas well. When the drilling fluid flow velocity is higher than the critical velocity, gas accumulation will not occur; when the drilling fluid flow velocity is lower than the critical velocity, gas retention will cause gas accumulation. At this time, the gas can only be exhausted by dissolving in the drilling fluid and increasing the speed of the drill pipe. In order to prevent the accumulation of gas in the wavy horizontal section, the critical flow rate needs to be determined. Therefore, this paper performed a series of gas-liquid simulation of the wavy horizontal section of the horizontal gas well with different curvatures. The simulation results are shown in Table
Exhaust conditions under different curvatures.
Curvature | Drilling fluid flow velocity/(m·s−1) | ||||||||
---|---|---|---|---|---|---|---|---|---|
0.8 | 0.9 | 1.0 | 1.1 | 1.2 | 1.3 | 1.4 | 1.6 | 2 | |
0.2 | F | F | Y | Y | Y | Y | Y | Y | Y |
0.3 | F | F | F | Y | Y | Y | Y | Y | Y |
0.4 | F | F | F | F | F | Y | Y | Y | Y |
0.5 | F | F | F | F | F | F | Y | Y | Y |
Y = exhaust; F = not exhausted.
As known from Table
Gao et al. [
The simulation results are exactly the same as the experimental results, verifying the accuracy of the simulation results.
Horizontal gas wells may have multiple wavy horizontal sections, so the total horizontal length is 20 m, the borehole diameter is 0.2159 m, the curvature is 0.2, the initial top gas accumulation is 0.98 m3, and the critical drilling fluid flow velocity is 1 m/s, 1.2 m/s, 1.6 m/s, 2 m/s, and 1.5 times wavy horizontal sections and 2 times wavy horizontal sections are simulated. The simulation results are shown in Figures
Flow states at different velocities during 1.5 wavy horizontal sections. (a) Drilling fluid flow velocity is 2 m/s. (b) Drilling fluid flow velocity is 1.6 m/s. (c) Drilling fluid flow velocity is 1.2 m/s. (d) Drilling fluid flow velocity is 1 m/s.
Observing Figure
Observing Figure
Flow conditions at different velocities when there are two wavy horizontal sections. (a) Drilling fluid flow velocity is 2 m/s. (b) Drilling fluid flow velocity is 1.6 m/s. (c) Drilling fluid flow velocity is 1.2 m/s. (d) Drilling fluid flow velocity is 1 m/s.
Observing Figures
Curves of gas volume at different flow velocity when there are 1.5 wavy horizontal sections.
Curves of gas volume at different flow velocity when there are two wavy horizontal sections.
In the case of the same length and the same flow velocity, the gas exhaust time increases with the increase of the curvature. Therefore, the lower the curvature of the wavy horizontal sections, the less likely it is to generate gas. When the drilling fluid flow velocity is extremely large, the drilling fluid directly carries the entire gas pocket out of the wavy horizontal section without shear fracture; as the inlet flow velocity decreases, the gas pocket formed by the accumulated gas is carried forward by the drilling fluid. When the gas pocket enters the downward section, the front end of the gas pocket is broken into small bubbles by the shear force. The gas is exhausted in the form of small bubbles. When the drilling fluid flow velocity is reduced to a certain speed, the drilling fluid pushes the gas pocket downward section. Then, the gas pocket is stationary, and the front section of the gas pocket is broken into small bubbles by the shear force. Over time, some of the gas cannot be broken into small bubbles and stays in the wavy horizontal section, causing gas accumulation. There is a critical flow velocity so that the gas is just completely exhausted from the wavy horizontal section. When the drilling fluid flow velocity is higher than the critical velocity, gas accumulation will not occur; when the drilling fluid flow velocity is lower than the critical velocity, gas retention will cause gas accumulation. And the critical drilling fluid flow velocity increases with the curvature. When the total length and curvature of the wavy horizontal sections are the same, the number of wavy horizontal sections has a great effect on the gas-liquid flow pattern but has little effect on the efficiency of the exhaust.
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that they have no conflicts of interest.
The authors gratefully acknowledge the financial support by “Thirteenth Five-Year Plan” China National Offshore Oil Corporation (CNOOC-KJ135ZDXM24LTDZJ01) and the National Science and Technology Major Project (2017ZX05009-003).