# Circulation Sub Series—4: Case Study Part II of II

3. Effect of Flow Rate

To study the sensitivity of flow rate on a circulation sub’s performance, 3 cases were chosen: one is without a circulation sub, and the other two have 1 (in2) and 2 (in2) of TFA, respectively. As we increase the flow rate from 1 to 10 (bpm), bypass ratios for both cases decrease. One might wonder why the bypass ratio decreases as flow rate increases. Does not the circulation sub play a bigger role in tougher conditions such as a high flow rate situation? Here is the reason behind the reverse change: the pressure drop across circulation sub nozzles (Path B) is proportional to the square of the flow rate, regardless of the rheological model. The frictional pressure loss along Path A is proportional to the flow rate to the power of 1.75 for Newtonian fluids in turbulent flow conditions. When the flow rate increases, it is relatively easier for fluid to flow along Path A than Path B. Therefore, at higher flow rates, the bypass ratio is smaller.

Figure 10: Circulation Sub Bypass Ratio vs Flow Rate

However, even with the slightly decreased bypass ratio at higher flow rate, the presence of a circulation sub greatly reduces pump pressure and bottom hole ECDs, as illustrated in Figure 11 and Figure 12. The benefits become more pronounced at higher flow rates. As noted before, the inclusion of a circulating sub makes a dramatic impact up to a certain TFA, in this case 1 square inch.

Figure 11: Pump Pressure vs Flow Rate

Figure 12: Bottom Hole ECD vs Flow Rate

4. Effect of Viscosity

Since viscosity has a small impact on the analysis, the circulation sub’s nozzles have been changed to 2 x 10 (1/32in) for this portion of the analysis, which yields a TFA of 0.153 (in2) for our base case.

The flow split at a circulation sub is the result of flowing fluid seeking the path of least resistance and pressure balance. The frictional pressure loss along Path A is a function of fluid viscosity, density, flow rate and flow path geometry. If the flow is laminar, the pressure loss is proportional to the fluid viscosity for Newtonian fluid. The resistance of path B is dominated by the pressure drop across nozzles, where the viscous frictional effects are essentially negligible. As fluid viscosity increases, it is more difficult for fluid to flow through Path A. The bypass ratio will increase as illustrated by Figure 13. Both pump pressure and bottom hole ECD increase as fluid viscosity becomes higher. However, they would be much higher if no circulation sub is present.

Figure 13: Circulation Sub Bypass Ratio vs Fluid Viscosity

Figure 14: Pump Pressure vs Fluid Viscosity

Figure 15: Bottom Hole ECD vs Fluid Viscosity

5. Effect of Fluid Density

If the flow is turbulent, the pressure loss along Path A is proportional to the fluid density to the power of 0.75 for a Bingham plastic fluid. On the other hand, the pressure drop across Path B is proportional to the fluid density. As the fluid density increases, it is relatively more difficult for fluid to flow through Path B. The bypass ratio will decrease when fluid density increases as illustrated by Figure 16. The pump pressure increases as fluid density increases. The bottom hole ECD increases because both hydrostatic pressures and frictional pressure loses increase with greater fluid density.

Figure 16: Circulation Sub Bypass Ratio vs Fluid Density

Figure 17: Pump Pressure vs Fluid Density

Figure 18: Bottom Hole ECD vs Fluid Density

The above case study is performed for a particular wellbore cleanup scenario. In order to have a better understanding of your particular case, it is recommended to use engineering software to take into account of well configurations and fluid properties to optimize circulation sub performance.

# Circulation Sub Series—3: Case Study Part I of II

Case Study

Engineers may have some basic ideas on how to optimize the design parameters of a circulation sub to achieve their goals. For example, increasing the total flow area of a circulation sub will increase the bypass flow rate, reduce pump pressure, etc. This case study will quantify the impacts of various circulation sub parameters and fluid properties on pump pressure and ECD for a wellbore cleanup operation. We used a wellbore cleanup hydraulics software to perform this case study. Numerical methods are employed to obtain the correct flow split percentage at the location of the circulation sub. The flow split is obtained such that the summation of the frictional pressure losses inside the pipe below the circulation sub and in the annulus below the circulation sub should be equal to the pressure loss through the circulation sub nozzles.

Figure 2 shows the wellbore configuration used for the example calculation. This is the basic case, from which we will perform sensitivity studies on each of 5 parameters. Note that the flow rate is left blank because it is run at several values for all stages.

Figure2: Example Case

Figure3: Flow Paths

1. Effect of Total Flow Area (TFA)

Circulation sub’s adjustable nozzles enable you to define how the flow is split between the annulus and the pipe interiors. By adjusting the TFA of the circulation sub, you can control the amount of fluid that is diverted.

The flow split at a circulation sub is determined as the fluid chooses the path of least resistance. The rates of flow through the circulation sub and down the string are determined when these two flow paths reach a pressure balanced state. When fluid inside pipe travels to the circulation sub, it faces 2 choices. The first one is to flow downward through the pipe and up the annulus. Let us call this Flow Path A. The alternative path is sideways through the circulation sub’s nozzles. We will call this Flow Path B.

As illustrated by Figure 3, Flow Path A involves a long, but wide conduit, while Flow Path B is an array of short constrictions (nozzles).

The circulating fluid does not have a preference as which path to flow. When the fluid passes the circulation sub, it senses the resistances of both paths and chooses the split of fluid so that it yields an overall minimum resistance.

The frictional pressure loss, or flow resistance, along Path A is a function of fluid viscosity, density, flow rate, pipe ID, hole ID, pipe OD and flow path length. On the other hand, the resistance of Path B is dominated by the pressure drop across the nozzles, which is reversely proportional to the square of the TFA of those nozzles. As we increase the TFA of a circulation sub, it becomes much easier for fluid to flow through Path B. As a result, less fluid will flow through Path A and the frictional pressure losses in the lower pipe and annular sections will be reduced. Whatever the percentage of flow split, the pump pressure and ECD of the system are both reduced by the fluid bypass.

In our example, we increase the TFA from 0, representing a case of no circulation sub, to 2 (in2). Figure 4 shows increased fluid bypass ratios as the TFA increases for 3 flow rates, 2 (bpm), 4 (bpm) and 6 (bpm). The circulating sub bypass ratio is the percentage of flow exiting the string through the circulating sub nozzles, as opposed to the bit.

Figure4: Circulation Sub Bypass Ratio vs TFA

Accompanying these increased bypass ratios, both the pump pressure and bottom hole ECD reduce rapidly at beginning and more gradually later, as shown in Figure 5 and 6, respectively. The pump pressure is reduced by almost 80% when TFA is increased from 0 (in2) to 1 (in2) for a flow rate of 6 (bpm). Meanwhile, for the same flow rate, bottom hole ECD is reduced by 7.6%. Further increase of TFA from 1 (in2) to 2 (in2) yields only marginal reduction.

Figure5: Pump Pressure vs TFA

Figure6: Bottom Hole ECD vs TFA

2. Effect of Circulation Sub Depth

The location of the circulation sub affects the overall downhole hydraulics. A circulation sub establishes a communication path between fluid inside the pipe and fluid in the annulus. The closer a circulation sub is to surface, the greater the fluid bypass ratio is, because Flow Path A is getting longer and creates a higher frictional pressure drop. Figure 7 shows the bypass ratios at various circulation sub locations along the wellbore. As expected, if we place the circulation sub at the bottom of the pipe, it would have no effect on pump pressure or bottom hole ECD.

Figure7: Circulation Sub Bypass Ratio vs Circulation Sub Depth

To take advantage of its unique characteristic for wellbore cleanup operations, a circulation sub is often placed at the depth where the wellbore geometry changes, such as the previous casing shoe. By increasing the pump rate, the hole section below the circulation sub with a smaller annular clearance can maintain the required fluid velocity from the downward split flow. The velocity of the fluid in the larger OD annulus above the circulation sub will see both the flow rate traveling down the string and through the sub’s ports, increasing the annular velocity to closely match that in the narrow clearance hole below.

Greater reductions in both the pump pressure requirement and bottom hole ECD are achieved when a circulation sub is placed closer to surface, as seen in Figures 8 and 9. The pressure and ECD drops because less fluid is traveling through the narrower clearance section of the annulus.

Figure8: Pump Pressure vs Circulation Sub Depth

Figure9: Bottom Hole ECD vs Circulation Sub Depth

# 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.

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