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K. Zhang et al.
also known as the underbalanced drilling (UBD), is a technique to generate a pathway
connecting the reservoir to the surface equipment, in which commonly compressed
gases are applied to remove the cuttings and cool the bit instead of the conventionally
used fluids. Over the past 70 years, several types of gas drilling have emerged with the
advancement of the industry, such as mist drilling [1, 2], foam drilling [3, 4], and aerated
drilling [5, 6]. In these methods, the compressed gases are injected into the well combined
with the incompressible liquids, generally water, surfactants, and drilling muds. Due to
the significantly increased viscosity of the mixture compared with the single gas phase,
the gas injection rate or method becomes a less important parameter in cuttings transport.
Therefore, the term of gas drilling in this work denotes only the drilling method using
the compressed gas (air, nitrogen, or natural gas) as the sole circulating medium.
Gas drilling has many advantages, such as increasing the rate of penetration, reducing
the formation damage (especially the water-sensitive formation), reducing the risk of lost
circulation, and improving the drill bit life, etc. [7]. However, a major disadvantage of
the gas drilling is the severe deposition of the drilled cuttings, especially in the horizontal
gas drilling, which mainly caused by the poor transport capacity of the gas phase due to
its low density and viscosity. Generally, the solution is increasing the gas injection rate.
But if the gas injection rate is too high, the cost due to the higher gas injection volume
and the investment of the surface equipment will increase accordingly. Besides, the high
gas injection rate may cause ice-balling of the drill bit [8, 9]. On the contrary, a too low
gas injection rate means a low cuttings transport efficiency, which may lead to a series
of potential downhole problems, such as the dill pipe sticking and bore-hole instability.
To the best of our knowledge, most of the papers focus on the effects of the rate of
penetration (ROP), drill pipe rotation speed, fluid flow rate, fluid viscosity, and cuttings
sphericity, etc. on the cuttings transport efficiency [10–13]. The effects of different gas
injection methods on the cuttings transport efficiency are seldom reported. In this paper,
a pulsed gas injection method is proposed to overcome the shortcoming of the low
cuttings transport efficiency in horizontal gas drilling. The pulsed gas injection method
based on different pulse amplitudes and pulse repetition frequencies has the same gas
injection volume with the constant-rate gas injection method, but has a wider velocity
range due to the velocity fluctuation. The Eulerian-Eulerian two-fluid approach with the
kinetic theory of granular flow is employed to simulate the gas-solid two-phase flow
in a 3D eccentric horizontal annulus using CFD modelling. The RNG k-ε turbulence
model is adopted to describe the turbulence behavior of the gas phase. The effects of
various gas injection methods on the gas inlet velocity, pressure drop, cuttings volume
fraction, granular temperature, turbulence kinetic energy, and turbulence dissipation rate
are systematically investigated. The results can provide a reference for the petroleum
engineers in using this pulsed gas injection method for horizontal gas drilling.
K. Zhang et al.
also known as the underbalanced drilling (UBD), is a technique to generate a pathway
connecting the reservoir to the surface equipment, in which commonly compressed
gases are applied to remove the cuttings and cool the bit instead of the conventionally
used fluids. Over the past 70 years, several types of gas drilling have emerged with the
advancement of the industry, such as mist drilling [1, 2], foam drilling [3, 4], and aerated
drilling [5, 6]. In these methods, the compressed gases are injected into the well combined
with the incompressible liquids, generally water, surfactants, and drilling muds. Due to
the significantly increased viscosity of the mixture compared with the single gas phase,
the gas injection rate or method becomes a less important parameter in cuttings transport.
Therefore, the term of gas drilling in this work denotes only the drilling method using
the compressed gas (air, nitrogen, or natural gas) as the sole circulating medium.
Gas drilling has many advantages, such as increasing the rate of penetration, reducing
the formation damage (especially the water-sensitive formation), reducing the risk of lost
circulation, and improving the drill bit life, etc. [7]. However, a major disadvantage of
the gas drilling is the severe deposition of the drilled cuttings, especially in the horizontal
gas drilling, which mainly caused by the poor transport capacity of the gas phase due to
its low density and viscosity. Generally, the solution is increasing the gas injection rate.
But if the gas injection rate is too high, the cost due to the higher gas injection volume
and the investment of the surface equipment will increase accordingly. Besides, the high
gas injection rate may cause ice-balling of the drill bit [8, 9]. On the contrary, a too low
gas injection rate means a low cuttings transport efficiency, which may lead to a series
of potential downhole problems, such as the dill pipe sticking and bore-hole instability.
To the best of our knowledge, most of the papers focus on the effects of the rate of
penetration (ROP), drill pipe rotation speed, fluid flow rate, fluid viscosity, and cuttings
sphericity, etc. on the cuttings transport efficiency [10–13]. The effects of different gas
injection methods on the cuttings transport efficiency are seldom reported. In this paper,
a pulsed gas injection method is proposed to overcome the shortcoming of the low
cuttings transport efficiency in horizontal gas drilling. The pulsed gas injection method
based on different pulse amplitudes and pulse repetition frequencies has the same gas
injection volume with the constant-rate gas injection method, but has a wider velocity
range due to the velocity fluctuation. The Eulerian-Eulerian two-fluid approach with the
kinetic theory of granular flow is employed to simulate the gas-solid two-phase flow
in a 3D eccentric horizontal annulus using CFD modelling. The RNG k-ε turbulence
model is adopted to describe the turbulence behavior of the gas phase. The effects of
various gas injection methods on the gas inlet velocity, pressure drop, cuttings volume
fraction, granular temperature, turbulence kinetic energy, and turbulence dissipation rate
are systematically investigated. The results can provide a reference for the petroleum
engineers in using this pulsed gas injection method for horizontal gas drilling.