Managed Pressure Drilling – EPISODE 1 (Introduction to the Technology)

Managed Pressure Drilling – EPISODE 1 (Introduction to the Technology)

Managed Pressure Drilling – EPISODE 1 (Introduction to the Technology)

“A new way of looking at drilling hydraulics…Overcoming conventional drilling challenges” Don M. Hannegan, Weatherford International Ltd.

As current reserves deplete, it is necessary to drill to reservoirs that are deeper and more complex. Some industry professionals would say that 70% of the current hydrocarbon offshore resources are economically undrillable using conventional drilling methods. Managed Pressure Drilling (MPD) is a new technology that uses tools similar to those of underbalanced drilling to better control pressure variations while drilling a well. The aim of MPD is to improve the drillability of a well by alleviating drilling issues that can arise.

MPD can improve economics for any well being drilled by reducing a rig’s nonproductive time (NPT). NPT is the time that a rig is not drilling. Many of the drilling problems in any well can be reduced by using MPD.

Pressure-Gradient Windows: 
As a well is drilled, drilling fluid is circulated in the hole to obtain a specific bottomhole pressure. The density of the fluid is determined by the formation and pore pressure gradients and the wellbore stability.

 

1

Fig. 1 shows a pressure gradient profile of a well. This profile shows the change in pressure as the depth increases. The pressure window is the area between the pore pressure and the fracture pressure. The goal when drilling a well is to keep the pressure inside this pressure window. In a static well, the pressure is determined by the hydrostatic pressure of the mud. In conventional drilling, the only way to adjust the pressure during static conditions is to vary mud weight in the well.

2
Where:

  • BHP= Bottom Hole Pressure , HHMW= Hydrostatic Head of mud , AFP= Annular Friction Pressure

Fig. 2 shows the problem that can occur when dealing with tight pressure-gradient windows. When the well is static, the pressure in the well is less than the pore pressure and the well takes a kick; that is, hydrocarbons flow into the well. After a connection, the pumps restart, the BHP (Bottom Hole Pressure) increases, and the pressure goes above the fracture-pressure, resulting in lost circulation, or fluid flowing into the formation. The goal of managed pressure drilling is to manage the pressure and remain inside this pressure gradient window to avoid many drilling problems.
How Managed Pressure Drilling Works:

The basic technique in MPD is to be able to manipulate the BHP and the pressure profile as needed. In conventional drilling, the BHP can be calculated by summing the mud weight hydrostatic head and the Annular Friction Pressure (AFP). The AFP is the friction pressure that results from the circulation of the mud while drilling. ECD is defined as the Equivalent Circulating Density of the BHP. It is basically the BHP while circulating converted into the units of mud weight. During a connection, the pumps turn off and the fluid stops circulating, thus eliminating the annular friction pressure and reducing the BHP to the mud hydrostatic pressure.

MPD uses a closed and pressurizable mud system. With a closed system the equation for the BHP can be varied to include backpressure. Adjusting backpressure while drilling can quickly change the BHP. The basic configuration for MPD is to have a Rotating Control Device (RCD) and a choke. The RCD diverts the pressurized mud returns from the annulus to the choke manifold. A seal assembly with the RCD enables the mud returns system to remain closed and pressurized. The choke with the pressurized mud return system allows the driller to apply backpressure to the
wellbore. If the pressure starts to climb above the fracture pressure of the formation, the driller can open the choke to reduce backpressure and bring the pressure down. If the driller needs to increase the pressure throughout the well, closing the choke will increase back pressure.

The Need for Managed Pressure Drilling

The need for MPD is clearly illustrated by current drilling statistics and problems that currently exist. Fig.3 shows the results of a database search of NPT (Non-Productive Time) while drilling offshore gas wells.

3

MPD can solve a large percentage of the problems the database lists, especially those that are caused by wellbore pressure deviating out of the pressure gradient window during drilling operations. Table 1 shows the NPT from Fig. 3 that could be reduced by using MPD.

4

Numerous problems can occur if the wellbore pressure goes below the pore pressure gradient. At shallow depths, water or gas can flow into the wellbore. As noted above, a kick can occur. With a lower pressure in the wellbore, the hole can also become unstable and start to fall in on the drillpipe. This can lead to the pipe becoming stuck and could cause a twist off, which is breaking the pipe. The main problem when the pressure exceeds the fracture pressure-gradient is lost circulation, losing mud into the formation. Reservoir damage can also occur and the wellbore can become unstable. These problems account for more than 40% of drilling problems in the 10 years this study covers. Also according to this study ,If we can eliminate the problems with MPD, we could reduce hole costs by about $39 per foot drilled, which means that in wells drilled to 15,000 ft, that can equate to an average savings of $585,000 per well.

Conclusion
Drilling programs using MPD technology are in progress enabling more precise wellbore pressure management, MPD is now being widely recognized as a step-change technology more readily acceptable to offshore drilling decision-makers than true-state UBD.

 

Reference:

  • Matthew D. Martin, “Managed Pressure Drilling techniques and tools”,2006, Texas A&M University.
  • Charles R. Stone, “Sometimes Neglected Hydraulic Parameters of Underbalanced and Managed Pressure Drilling”, 2008 SPE/IADC Managed Pressure Drilling and Underbalanced Operations Conference and Exhibition.
  • E&P Magazine , “Managed Pressure Drilling adds value”, September 2004
  • Schlumberger’s Oil Field Glossary.

Ahmed Radwan

Related Posts
Leave a reply