IADC/SPE 151249

Continuous Circulation Drill String Sub J. Weir, R. Goodwin, R. Macmillan, SPE, Natl. Oilwell Varco

Copyright 2012, IADC/SPE Drilling Conference and Exhibition This paper was prepared for presentation at the 2012 IADC/SPE Drilling Conference and Exhibition held in San Diego, California, USA, 68 March 2012. This paper was selected for presentation by an IADC/SPE program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the International Association of Drilling Contractors or the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflect any position of the International Association of Drilling Contractors or the Society of Petroleum Engineers, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the International Association of Drilling Contractors or the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of IADC/SPE copyright.

Abstract There is an increasing need to drill difficult reservoirs in a cost effective way. Over the past few years Managed Pressure Drilling (MPD) has made it possible to drill reservoirs which have a narrow window between pore and fracture pressure gradients. Before the introduction of MPD techniques, safely drilling and completing these formations was very costly and not always successful. Cycling the mud pumps off and on for connections affects the pressure and is a major problem for MPD. Techniques and equipment have been developed to make a connection while continuing to circulate the drilling mud to maintain constant pressure. Since 2005, the Continuous Circulation System (CCS) has allowed continuous circulation during connections with traditional jointed drill pipe, by using a chamber around the connection. Several companies have developed continuous circulation subs which are threaded between tool joints to achieve continuous circulation without a pressure chamber. These have had varying degrees of success from both an operational and safety standpoint. This paper describes the current MPD market and describes the philosophy adopted for a new side-entry sub. Introduction History was made in July of 2003, when the first continuous circulation chamber was tested in prototype form on a land rig. Jointed drill pipe was separated inside a sealed chamber, which was filled with drilling fluids provided by the mud pumps. The drill string was separated, another joint added, and drilling continued without ever shutting down the mud pumps. After its introduction in 2005, many

operators have begun using the continuous circulation chamber as a way to drill un-drillable wells, specifically those having tight pore pressure and fracture pressure windows. This means that the bottom-hole pressure must be very closely controlled at all times (even during connections) to avoid either fracturing the formation, or allowing an influx. The system has worked reliably for the last six years. However, many operators cannot easily accommodate the dedicated crew required to operate the continuous circulation chamber. As operators weighed the benefits against the increase in drill crew, the obvious solution was to provide the same continuous circulation functionality, with a streamlined package that reduced the footprint on the rig operations. There have been many attempts to create a continuous circulation sub which allows the mud flow into pipe sitting in-slips without flowing through the top drive. Many of these attempts have been successful, but have introduced a major safety concern. When looking at continuous circulation chambers, the barrier to prevent fluid from erupting out of the separated connection is typically a gate valve, with replaceable rubber seals. This barrier is replaceable and inspectable before exposing the crew to a potential leak. The recently introduced continuous circulation subs have all introduced a non-replaceable, non-inspected barrier. The rig crew is also responsible for attaching high pressure connections to the sub during operation. With all these points in mind, existing continuous circulation subs have presented major safety concerns and reliability weaknesses. The new continuous circulation sub is focused on safety, reliability, and becoming part of the drilling process. The design does not place rig personnel near the high-pressure drilling mud. The sealing

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component between the high-pressure drilling mud and atmosphere is easily removed and inspected between every connection. Each connection is checked for seal integrity before separation. This ensures reliability of the barrier, regardless of how long the sub has been in operation. The entire system is designed with the idea that the driller and existing crew will operate the equipment just as easily as existing floor equipment (ie, iron roughnecks, pipe rackers, etc). The solution aims to become a standard part of the drilling process; some key contributing features will be described. With the initial testing phase completed, the results will also be presented and discussed. The Concept The new continuous circulation sub has been designed with the same attention to detail as the original continuous circulation chamber. It has hands-free connection of the mud bypass, a replaceable barrier, automated connection integrity checks, and integration into the drilling process with the rig crew. Hands free connection When using the continuous circulation chamber, the drill pipe was fed through several ram type Blow Out Preventer (BOP) style seals. This provided a sealed chamber around the connection. These seals were positioned at well center and were always in position, simply retracted when not in use. This posed a problem during operation, the floor was not clear for access. It is desired to improve the access to the well center when drilling ahead or otherwise not continually circulating. A method of connecting the mud pumps to the side entry sub is no easy task. Some have tried using industry accepted hammer-unions, even though blunt force and subsequent sparks should be avoided with high pressure containment and in potentially explosive environments. Others have used threaded plugs, or clamp-on housings. All of these are potentially exposing the rig crew to increased danger. The new continuous circulation sub is a totally hands-free unit for this reason. The mud is transported from the standpipe using industry accepted swivel joints and piping, to allow movement. The sub is converted from drilling mode to side entry mode using very simple, easy to visually inspect, movements. The exact layout of the movement will be customer specific. The current proof of concept is an extending arm arrangement. Potential alternate versions are track mounted or pedestal mounted with an arm system, or even adapted to the existing iron roughneck. The final articulation method will depend largely on the customer and their requirements to either be part of the rig floor, or as a temporary installation.

Replaceable barrier - The novelty of the sub design is based on the barrier, which is a round ball, with sealing characteristics (ie, soft compared to the seat). This presents a perfect sealing surface, which can be re-used multiple times. The ball is inserted through the side of the sub and shifted up, much like a shuttle valve, using differential pressure. Minimal amounts of pressure differential will be sufficient to shift the ball (a small test fixture was used to prove this theory). With the ball in the sub, and seated against the top-drive side of the connection, mud is pumped down the string, through the same hole from which the ball was introduced. While in this condition, the connection directly above the sub is still made up and sealed. The barrier is then tested for leakage before the control system gives the green light to proceed with breaking this connection and either adding or subtracting joints of pipe. The replaceable barrier is then removed from the sub, using the same pressure differential. In normal drilling mode the sub has an unrestricted central bore with no seals or internal components. Testing of the barrier - All continuous circulation processes utilizing a chamber have the advantage of being able to test the barrier for leakage prior to breaking the connection. The new circulation sub has a similar ability by utilizing two mud lines; one provides pressure and flow from the mud pumps, and the other is primarily a vent line. By having two high pressure access points to the connection, it is possible to have a safe pressure differential. If a leak is detected, the process can be reversed and attempted again. In the new continuous circulation sub, the ball can be returned to the valve system and re-inserted, or simply replaced. Because the ball is a self-aligning seal mechanism, it is very difficult to forsee a non-sealing situation. If difficulty is seen when seating the ball, the dynamics of the pressure differential can be tuned to provide a more positive hammer when it seats, or a stronger application of pressure differential. Additionally, the low cost of the ball encourages the precautionary measure of changing it for each connection. Many material choices are available which can cope with the specific mud conditions experienced; high pressure, gas, chemistry of the mud, and extreme temperatures. Rig crew operation The first continuous circulation chamber was an all-inclusive tool; having a pipe handling system, spinning wrench and torque wrench, pipe slips, and even functioning as a mud bucket. The rig has provisions for all of these operations, either manual or automated. It was found in operation that the rig crew had difficulty in changing their work flow to use the newly combined functionality. In the new continuous circulation sub,

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and existing circulation subs, the crew uses all the same equipment they are trained to use. This provides an improvement in efficiency and connection time.When using a continuous circulation chamber, the connection process was essentially concealed. With continuous circulation subs, however, the drill crew can see every part of the process. The slips are set, the system is extended to engage the sub, a green strobe light or similar visual indicator signals that the barrier has passed its pressure check, and then the iron roughneck is used to break the connection directly above the new continuous circulation sub. In this case the roughnecks are very much a part of the connection process, and have a direct impact on the speed of the connection. It is expected that the new continuous circulation sub will take roughly one minute to engage the pipe, insert the ball, and perform the barrier integrity check. The same or less time will be taken when removing the ball. With a predicted five minute or less connection time, MPD is becoming much more efficient. This is a very significant leap forward for MPD. Initial Testing Certain elements of the design have been developed based on the results of testing. The replaceable barrier has been tested using a flow fixture with various arrangments of valves to simulate the rigs stand pipe manifold. The flow fixture was transparent where the ball enters the sub and seals. Water is flowed through the setup to simulate very thin drilling mud. The timing of the valves opening and closing plays a role in how efficiently the ball is shifted into the circulation sub. It was a challenge to reduce the number of valves to a minimum, but also provide double-valve-protection at all times. Figure 1 shows the typical valve schematic for the new continuous circulation sub. During testing it was found that the ball can be shifted with very little pressure differential and that velocity appears to be the largest influence on the operation. Once seated the pressure differential required to generate a seal is also negligible. Another development was the position and shape of the internal geometry, such that the ball is encouraged to travel up into the sub, even at very low transition speeds. A pressure gage on the downstream side of the sub registered almost no pressure changes during shifting of the ball. This is primarily due to the flow area never being restricted throughout the shifting sequence. The ball material was also experiemented with, and the initial testing was done with a rubber coated steel core. The significant weight of the ball posed no issue with being transferred using almost no pressure differential. Another area of interest was the clearance between the internals of the continuous circulation sub and the associated plumbing with

respect to the ball. This ball must be transported efficiently within these pipes for a short distance, so the leakage cannot be excessive. The clearance didnt prove to be as much concern as expected, even when using low viscosity fluids and much larger than practical clearances. The system is also very intuitive; first impressions have been positive from people familiar with the drill floor and connection processes. The Market Managed Pressure Drilling has gained a lot of momentum in the last six years since the commercial introduction of a continuous circulation chamber in 2005. The demand for these chambers is growing, from its initial applications in the North Sea and the Mediterranean Sea, to offshore Gulf of Mexico and Brazil. A 2011 survey published in the Journal of Petroleum Technology reached an industry consensus that 40% of offshore wells would be using MPD within five years. There is a large demand onshore also that is not able to be accessed by the continuous circulation chambers. This is primarily due to restricted rig space and configuration. This potentially opens up a market for very compact continuous circulation subs which have minimal footprint on the rig floor with MPD-Ready land rigs also becoming more common. Conclusion The new continuous circulation sub is a solution for continuous circulation in todays Managed Pressure Drilling market. The new system addresses safety, reliability and performance by using a replaceable barrier and testing the barrier at each connection. Initial testing has verified both the simplicity of the system and its intuitive operation. References

1. Calderoni A, et al. The Continuous Circulation System from Prototype to Commercial Tool paper SPE 102851, presented at SPE Annual Technical Conference and Exhibition, San Antonio, Texas, Sept 2006.

2. Jacobs S and Donnelly J, Crossing the Technology Chasm: Managed Pressure Drilling, online article featured on Journal Of Petroleum Technologys website, www.JPTonline.org, 2011

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Figure 1 Typical Mud Valve schematic.

Figure 2 Flow simulation through sub using CFD software.

igure 4 Full scale test setup

Figure 5 Cross section showing internal ball seat.

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Figure 3 Turbulance prediction using CFD software.