MPD boosted with hands-free connection of mud bypass

The increasing need to drill difficult reservoirs in a cost-effective way is why the technique of managed-pressure drilling (MPD) has become more popular. MPD has made it possible to drill reservoirs that 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.

One of the major problems for MPD was cycling the mud pumps off and on for connections, which affected the pressure. An earlier version, the continuous circulation system (CCS), was developed to make a connection while continuing to circulate the drilling mud to maintain constant pressure. The CCS has allowed continuous circulation during connections with traditional jointed drillpipe by using a chamber around the connection. Several companies have developed continuous circulation subs that are threaded between tool joints to achieve continuous circulation without a pressure chamber.

Many operators have begun using the continuous circulation chamber as a way to drill un-drillable wells, specifically those having “tight” pore- and fracture-pressure windows. The industry has realized that operators cannot accommodate the crew required to operate the continuous circulation chamber, so a new design was devised with a streamlined package reducing the footprint on rig operations compared to the CCS, which needs a manifold system and valves. The well-center connection for the new sub can be retracted, and the new system combines the machine with the mud valves.

The new CCS is a hands-free unit that is threaded into the drillpipe in the slips until the next stand of pipe is connected. (Images courtesy of National Oilwell Varco)

There have been many successful attempts to create a CCS that allows the mud flow into pipe sitting in-slips without flowing through the top drive, but these attempts have introduced a major safety concern. There is a replaceable barrier in the CCS – the middle blind rams, which can be changed out if these are found to be leaking. However, during change-out, the drillers would divert the flow and assume the barrier is good. This is the only barrier between high-pressure mud and the rig crew, which is critical.

Both the CCS and new circulation sub are the only equipment at this time to have a barrier that can be replaced and tested in the field.

The new sub was designed with a focus on safety and reliability and is becoming part of the drilling process. The sealing component between the high-pressure drilling mud and the atmosphere is easily removed and inspected between every connection, and each connection is checked for seal integrity before separation. 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.

The CCS 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 drillpipe was fed through several ram-type BOP style seals. This provided a sealed chamber around the connection that was positioned at well center and simply retracted when not in use. That posed a problem because the floor was not clear for access.

Many have tried different techniques to connect the mud pumps to the side entry sub, but all of them were potentially exposing the rig crew to increased danger. The new CCS is hands-free for this reason. The mud is transported from the standpipe using industry-accepted swivel joints and piping, to allow movement, and it is easy to visually inspect movements. 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.

Replaceable barrier

The novelty of the sub design is based on the barrier, which is a round ball with sealing characteristics. This presents a perfect and reusable sealing surface. The ball is inserted through the side of the sub and shifted “up” using differential pressure. When the ball in the sub is “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 remains sealed. The barrier is tested for leakage before the control system proceeds with breaking the connection to add/remove pipe. The replaceable barrier is removed from the sub, using the same pressure differential.

Testing of the barrier

All continuous circulation processes using a chamber have the advantage of being able to test the barrier for leakage prior to breaking the connection. The CCS has a similar ability that allows “safe” pressure differential by using two mud lines – one provides pressure and flow from the mud pumps, and the other is a vent line.

If a leak is detected, the process can be reversed and attempted again. In the new sub, the ball can be returned to the valve system and reinserted, or simply replaced. If difficulty is encountered when seating the ball, the dynamics of the pressure differential can be tuned to provide a more positive “hammer” when it seats. Additionally, the low cost of the ball encourages the precautionary measure of changing it for each connection. Many ball material choices are available that can cope with the specific mud conditions experienced.

Rig crew operation

The first continuous circulation chamber was an all-inclusive tool, and it was found that the rig crew had difficulty in changing their workflow to use the newly combined functionality. In the new CCS the crew uses all the same equipment they are trained to use providing an improvement in efficiency and connection time.

A benefit of the system is that the drill crew can see every part of the connection process where the continuous circulation chamber conceals the connection process. The slips are set, the system is extended to engage the sub, a visual indicator signals that the barrier has passed its pressure check, and the iron roughneck is used to break the connection directly above the CCS.

It is expected that the new sub will take roughly one minute to engage the pipe, insert the ball, and perform the barrier integrity check. With a predicted five minutes or less connection time, MPD is becoming much more efficient.

Initial testing

The replaceable barrier has been tested using a flow fixture with various arrangements of valves to simulate the rig’s standpipe 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 valve’s opening and closing plays a role in how efficiently the ball is shifted into the circulation sub. The ball can be shifted with little pressure differential, and velocity appears to be the largest influence on the operation.

Another development was the position and shape of the internal geometry, such that the ball is encouraged to travel “up” into the sub. A pressure gauge on the downstream side of the sub registered almost no pressure changes during shifting of the ball due to the flow area not being restricted throughout the sequence.

The ball material was also experimented with, and the initial testing was done with a rubber-coated steel core where the weight of the ball posed no issues. Another area of interest was the clearance between the internals of the CCS and the associated plumbing with respect to the ball. The clearance did not prove to be a concern, even when using low viscosity fluids and larger than practical clearances.

The new system is a solution for continuous circulation in today’s MPD market, and it addresses safety, reliability, and performance by using a replaceable barrier and testing the barrier at each connection.