Circulation Sub Series—2: Circulation Sub Uses in the Industry

How Do Circulation Subs Work?

A circulation sub is useful in many applications such as spotting remediation fluids, drilling, wellbore cleanup, subsea blow out preventer (BOP) jetting and surge pressure reduction.

  1. Spot Remediation Fluids

Loss of circulation occurs when drilling fluids flow into formations instead of returning up the annulus. It is one of the most time-consuming and cost inflating events in drilling operations. The effective solution is to deploy, or spot, lost-circulation material (LCM) into the formation. Due to LCM’s nature to plug holes in the formation, it is difficult to pump LCM through the bottom hole assembly (BHA) components with restricted flowpaths, such as bits, downhole motors and measurement while drilling (MWD) tools. To spot aggressive LCM, circulating subs are typically placed above the BHA and divert LCM to the annulus without causing damage to the motor or other tools below.

  1. Drilling

Cuttings removal and managing downhole pressure are two critical elements while drilling, particularly in deepwater and extended-reach conditions. In directional wells, rock cuttings fall to the low side of the wellbore. As cutting beds build up and annular cuttings concentration increases, the frictional pressure loss between the drillpipe and wellbore increases. This could lead to more torque and drag related problems such as buckling, stick slip, vibration, and lockup events.

Adequate annular velocity is required to transport cuttings to surface. However, because of the presence of mud motors, MWD tools, and other flow restricting components, it is often difficult to achieve annular velocities high enough to effectively transport cuttings without over spinning the motor. The narrow passage inside the BHA creates higher pressure losses, which could result in a high pump pressure requirement.

With pump rates hampered either by BHA restrictions or by equivalent circulating density (ECD) window considerations, a circulation sub, typically placed above the BHA, is often a convenient and simple solution. By bypassing the BHA and preventing motor overrun, a circulation sub can reduce wear on the motor and increase its reliability and operating hours. ‘Bottoms-up’ circulating time is greatly reduced and hole cleaning is improved. A percentage of flow to the drill bit is retained, which can be adjusted, keeping BHA components lubricated.

In summary, a circulating sub enables the rig to maintain a higher annular velocity, reduces pump pressure requirements, and reduces ECD at the bottom of the hole.

  1. Wellbore Cleanup

A clean well is essential prior to running expensive and sensitive completion strings or other debris sensitive equipment. The first step to ensure an optimum completion is to remove leftover drilling fluid residue and casing debris. This requires that the drilling mud be changed out with solids-free completion fluids. Completion fluid displacement involves multiple fluids sequenced in circulation.

Multiple fluids are used in wellbore cleanup operations, including drilling mud, water, spacers, pills, and flushes. Spacers are viscous fluids used to aid in the displacement or removal of other fluids. Pills are small volumes of specially prepared fluid designed to accomplish a specific task, such as lifting debris from a wellbore or removing scale on the internal diameter (ID) of casing. Flushes are used to prepare for or assist in production from the producing zone.

In wellbore cleanup operations, similar to a drilling scenario, a circulation sub permits an increased flow rate by opening flow paths to the annulus above the flow-restricting annular sections with smaller hole ID or large outer diameter (OD) string components. Bypassing the smaller annular sections allows the maximum amount of fluid to be directed to the annulus, thus boosting annular velocity for more effective wellbore cleanup above the circulation sub location and lowering the pump pressure. These operations can be optimized by adjusting the port sizes of circulation sub so that the desired downward flow rate is achieved to clean up the hole sections below with restricted annular clearances.

  1. Blowout preventer (BOP) stack jetting

Circulation subs are also used to hydroblast the subsea wellhead or BOP cavities. The nozzles, or ports, on the circulation sub direct fluid out to the BOP stack and create jet impact forces that thoroughly dislodge junk and debris.

  1. Surge pressure reduction

During casing or liner running, a circulation sub can be used in conjunction with auto-fill float equipment. Normally located on drillpipe immediately above the liner, the ports of the circulation sub allow the fluid trapped in the liner access to the larger annulus between drillpipe and previous casing, in addition to the flowpath through the restrictive drill pipe ID. The auto-fill float equipment and circulation sub establish 2 places of fluid communication between pipe interior and annulus. Fluid displaced by the string seeks the least resistant flowpath. A circulation sub opens a less restrictive flowpath, which helps to reduce the surge pressure.

These circulation sub applications are illustrated in Figure 1.

Applications of Circulation Sub

Figure1: Applications of Circulation Sub

Future Blogs

The third and fourth articles will discuss the numerical analysis used and the results on changes in the critical variables that affect the flow split created by a circulating sub. These variables are:

  • Total Flow Area (circulating sub ports)
  • Depth of the Circulating Sub
  • Flow Rate
  • Fluid Viscosity
  • Fluid Density

The Significance of Cleanups: What a difference it makes!

Picture this: a filthy driveway that has not been washed in months, covered in dead leaves, mud, and dirt. All those lifeless bits of wilted foliage and dirt have mixed resulting in gunk being stuck to the concrete driveway. So what do you do? Dig through your garage, pull out the pressure washer and get to work! Whether it is a dirty driveway or a wellbore, in order for all things to serve their purpose, we must make sure we are able to clean out the mess and residue, to get us through to our next step.

Successful well completions rely on a lot of factors. As mentioned before, one main aspect is maintaining a clean wellbore, free of debris or any other fluid residue that has been left behind due to the nature of drilling fluids. Whether it is a dirty driveway or a wellbore, the process of cleaning highly increases the chances for us to foresee what is to come next. Enter CleanMax, the next generation of wellbore cleanup.

Avoiding mishaps is quintessential for any project we take upon ourselves. When it comes to operations, failure to conduct wellbore cleanups could lead to potential failed completions, not to mention the high costs associated with it. It is essential that we not take risks when it comes to this and use the tools that we have at hand to accurately conduct successful wellbore cleanups and safer operations. One of our most recent software, CleanMax, does just that and more. We have created the go-to software that meets the needs for both service companies and operators, helping minimize spacer interfacing and reducing rig time, pill volumes, and filtration costs.

CleanMax - Wellbore Cleanup Software

We are all cognizant that drilling comes with its complexities. During this challenging time in our industry, we have had to make crucial decisions when it comes to getting the job done efficiently while keeping costs in mind.  At PVI, we know this all too well. “How?” you may ask, and the answer is pretty straightforward: because we are the ones who create the tools to turn this into a sophisticated, yet simple process (that’s our slogan!). We are your eyes when it comes to successful drilling completions!

For more information about CleanMax and CleanMax+, please visit:

The Best is Yet to Come

The following is told by an American lady:

“My grandmother always used to tell us, “keep your forks.” when the main dishes were being cleared from the table. It was my favorite part of dinner, because I knew that something wonderful was coming… like a velvety chocolate cake or a deep-dish apple pie.”

A similar expression would be “You have not seen anything yet.” or as my 8-year old daughter told me, “I am not done yet!” when I gave her an applause after she sang the song from the movie “Frozen”.

Life is a simple and normal routine. Everyone has the same number of hours in a day and the same number of days in a year. It is up to us to make our daily routines more interesting, to fill our time doing things that will make us grow in every aspect of our lives, to make the most of our time. We cherish the hope that our present situation is not our final destination. The best is yet to come.

For us software developers, we are continuously enhancing the drilling software that is being developed. We certainly can’t add more hours to our day, but we can make our development more efficient. We will probably spend the same amount of time as we did last year, but we will have better products in 2014.

The latest release of our cementing software CEMPRO+ is a milestone. Why? Because this is the first time we address the displacement efficiency during the multi-fluid displacement operations such as cementing or wellbore cleanup. Before CEMPRO+ we used to assume that the drilling mud was completely displaced by the cement slurry with the use of a piston. This convenient assumption makes the hydraulics relatively easy, but it fails to predict the mud channeling which occurs due to many factors such as, differences in the cement properties and mud flow rates, and the casing centralization. A typical illustration of mud channeling is shown here:

Mud channel left on the narrow side of the annulus

Mud channel left on the narrow side of the annulus (Macondo incident-Chief Counsel’s report, 2011)

CEMPRO+ can help predict the mud fractions in the annulus during a cementing job. The following picture is a snap shot from the program. It looks neat. Graphics are more significant, because they show what really happens in the wide and narrow sides of the annulus.

Displacement Efficiency Illustration in CEMPRO+

Displacement Efficiency Illustration in CEMPRO+

If our CEMPRO has been on your software menu, keep your forks, because the best is yet to come.