Existing pressure drilling technologies are based on different principles and display distinct characteristics in terms of controlling the pressure and degree of formation adaptability. In the present study, the constant bottom hole pressure (CBHP) and controlled mud level (CML) dual gradient drilling methods are considered. Models for the equivalent circulating density (ECD) are introduced for both drilling methods, taking into account the control pressure parameters (wellhead back pressure, displacement, mud level, etc.) and the relationship between the equivalent circulating density curve in the wellbore and two different types of pressure profiles in deep-water areas. The findings suggest that the main pressure control parameter for CBHP drilling is the wellhead back pressure, while for CML dual gradient drilling, it is the mud level. Two examples are considered (wells S1 and B2). For S1, CML dual gradient drilling only needs to adjust the ECD curve once to drill down to the target layer without risk. By comparison, CBHP drilling requires multiple adjustments to reach the target well depth avoiding a kick risk. In well B2, the CBHP method can drill down to the desired zone or even deeper after a single adjustment of the ECD curve. In contrast, CML dual-gradient drilling requires multiple adjustments to reach the target well depth (otherwise there is a risk of lost circulation). Therefore, CML dual-gradient drilling should be a better choice for well S1, while CBHP drilling is more suitable for well B2.

Due to the continuous emergence of applications of managed pressure drilling technology, a systematic process theory has been gradually formed, and different types of managed pressure drilling technology have been developed, such as CBHP managed pressure drilling, pressured mud cap drilling technology, dual gradient drilling technology, etc. [

In this paper, the most mature and widely used CBHP managed pressure drilling and CML dual gradient drilling are used as the research objects [

The main influencing factors of the ECD value are the frictional pressure drop, PVT characteristics of the drilling fluid and cuttings mixing [

^{3}; ^{3}; ^{−1}; ^{−1};

The coupling relationship between the temperature, pressure and density of the drilling fluid, can be effectively analyzed by a numerical method. The pressure node is arranged at the lower boundary of the control unit. When the drilling fluid is circulated naturally or the wellhead is circulated with back pressure, the wellhead node pressure is known, which is the upper boundary condition. Therefore, the annular node calculates the lower boundary pressure of the node control unit in a top-down order. Assuming that the pressure _{i −1} is known, the calculation steps for _{i} are as follows:

Take the surface drilling fluid density as the reference density:

Calculate the pressure of node

Calculate the drilling fluid density based on the grid average pressure and average temperature:

Calculate the node pressure based on density:

Calculate the relative error of the two node pressures:

If the relative error

^{3}; ^{3};

Under the natural circulation conditions of drilling fluids, the ECD is composed of the annulus friction pressure drop, drilling fluid density considering the influence of temperature and pressure, and additional density of cuttings. The ECD expression corresponding to node

^{3}; and

A lift pump is connected to a riser at considerable depths. The drilling fluid level in the riser is managed by controlling the power of the drilling pump, thereby adjusting the wellbore ECD. There is a certain height of static drilling fluid above the connection between the riser and the lift pump, and the height depends on the power of the drilling pump. Therefore, there is a section of air above the riser, a section of static drilling fluid below, and a normal circulating drilling fluid below the interface of the lift pump. The calculation expressions of the friction pressure drop and hydrostatic column pressure under this working condition are as follows, and substituting them into

Under this working condition, the ECD consists of the equivalent density of the wellhead back pressure, equivalent density of the frictional pressure drop, drilling fluid density and additional density of the cuttings.

According to the established ECD calculation model in the wellbore, the distribution characteristics of the ECD curve with the depth of the well under different drilling methods are calculated. The figure shows the change in ECD with well depth in the four drilling modes.

The control pressure parameters of CBHP managed pressure drilling are mainly the wellhead back pressure and displacement.

The CML dual gradient drilling pressure management parameters are mainly the mud level depth and displacement.

According to the ECD distribution law of different pressure control parameters, the two MPD methods have better formation adaptability than conventional drilling. Both kinds of managed pressure drilling can adapt to deep-water narrow pressure formations, but different narrow pressure profiles play a screening and decisive role in the adaptability of managed pressure drilling. Therefore, combined with two typical narrow pressure formation profiles in a deep water area, the adaptability of the two managed pressure drilling methods to different types of narrow pressure formation profiles was analyzed.

The characteristic of the formation pressure profile in

MPD type | ECD curve adjustment times | Control pressure parameters value | Well depth of drilling/m | Downhole risk |
---|---|---|---|---|

CBHP managed pressure drilling | The first time to adjust the ECD curve | ^{−3}, P = 0.5 MPa, Q = 30 L·s^{−1} |
3249.15 | overflow |

The second time to adjust the ECD curve | ^{−3}, P = 3 MPa, Q = 30 L·s^{−1} |
4187.9 | no risk | |

CML dual gradient drilling | The first time to adjust the ECD curve | ^{−3}, Q = 30 L·s^{−1}, riser pump depth = 800 m, mud level depth = 350 m |
4187.9 | no risk |

The formation pressure profile of deep-water well B2 is characterized by a relatively uniform pressure increase in the upper formation as the well depth increases. In the deep formation, with increasing well depth, the formation fracture decreases, and the formation pore pressure increases, forming a narrow pressure window.

MPD type | ECD curve adjustment times | Control pressure parameters value | Well depth of drilling/m | Downhole risk |
---|---|---|---|---|

CML dual gradient drilling | The first time to adjust the ECD curve | ^{−3}, Q = 30 L·s^{−1}, riser pump depth = 800 m, mud level depth = 500 m |
1688.44 | overflow |

The second time to adjust the ECD curve | ^{−3}, Q = 30 L·s^{−1}, riser pump depth = 800 m, mud level depth = 350 m |
3205.81 | overflow | |

The third time to adjust the ECD curve | ^{−3}, Q = 30 L·s^{−1}, riser pump depth = 800 m, mud level depth = 200 m |
3991.05 | no risk | |

CBHP managed pressure drilling | The first time to adjust the ECD curve | ^{−3}, P = 2 MPa, Q = 30 L·s^{−1} |
3991.05 | no risk |

CBHP managed pressure drilling and CML dual gradient drilling ECD calculation models are established to study the influence of pressure control parameters on the two managed pressure drilling methods. The results show that the wellhead back pressure and displacement are proportional to the ECD in MPD with CBHP. In CML dual gradient drilling, the mud level depth and riser pump depth are inversely proportional to the ECD value and proportional to the displacement. The wellhead back pressure is the main pressure control parameter for CBHP managed pressure drilling, and the mud level depth is the main pressure control parameter for CML dual gradient drilling. Displacement can be used as an auxiliary means for the two pressure control methods. Based on the ECD curve trend of multiple drilling methods and the ability to adjust the ECD curve, managed pressure drilling is much more satisfactory for deep water and narrow pressure formations.

The applicability of the two pressure control methods is analyzed based on two typical pressure profiles common in deep water areas and combined with the ECD curve. Taking well S1 as an example, the formation pressure in this type of deep water increases or decreases synchronously with the depth of the well, and there is a pressure reversal formation. CBHP managed pressure drilling needs to adjust the ECD curve twice; if not, there is a kick risk. CML dual gradient drilling only needs to adjust the ECD curve once to go down to the target well depth without risk. Hence, this type of formation pressure profile is more suitable for CML dual gradient drilling. Taking well B2 as an example, the characteristics of this type of deep-water formation pressure are that the upper formation pressure increases with increasing well depth, the lower formation fracture pressure has a significant decreasing trend, and the lower formation pressure profile shifts to the left as a whole. This kind of formation pressure has good applicability to CBHP managed pressure drilling, which can drill into the target well or even deeper at one time. In contrast, CML dual gradient drilling needs to adjust the ECD curve several times to meet the bottom depth; otherwise, there will be a risk of lost circulation. Therefore, this type of formation pressure profile is more suitable for CBHP managed pressure drilling.

The wellhead back pressure is in the range of 0–3 MPa, and the mud level depth is in the range of 200–500 m. Judgments of the formation pressure of each well in advance pave the way for research on the adaptability of managed pressure drilling technology. The two typical pressure profiles account for more than 95% of the pressure profile types in deep-water areas and contribute to the selection and analysis of subsequent pressure management methods and well structure optimization.