Slow Circulating Rates (SCRs) in Well Control Operations

Slow Circulating Rates (SCRs) refer to the use of slow pump rates during circulation to manage wellbore pressure, especially during situations where there is a need to control or “kill” the well. SCRs are integral in maintaining stability and safety in drilling environments, as they help manage bottom hole pressure, mitigate friction in the annulus, and provide additional control over circulation pressures. Let’s delve deeper into why SCRs are essential, how they are applied, and the importance of accounting for friction pressures, particularly in subsea operations.

Purpose and Benefits of Slow Circulating Rates

When performing well control operations, the primary goal is to maintain constant bottom hole pressure to avoid losses or taking unintentional influx, which occurs when formation fluids enter the wellbore uncontrollably. Well kills are typically executed at SCRs, which are lower than regular drilling rates, due to the following advantages:

1. Negligible Friction Pressure in the Annulus: At reduced pump rates, the friction pressure within the annular space (the area between the drill pipe and the casing) is minimized. This is beneficial because it allows for more accurate control over bottom hole pressure, reducing the chance of unplanned pressure surges that could compromise well integrity.
2. Better Choke Control: Lower flow rates make it easier for the choke operator to manage choke pressures precisely. During well control operations, the choke, a device used to control fluid flow and pressure, plays a pivotal role. The choke operator can more easily maintain stable pressure readings, allowing for safer well control.
3. Reduced Wear and Erosion of Equipment: Operating at SCRs limits the erosion and wear of the choke manifold and its components. High flow rates lead to more significant frictional forces, which can quickly degrade equipment. Slower rates extend the lifespan of essential well control components, thereby lowering maintenance costs and downtime.
4. Barite and Mud Weight Management: SCRs facilitate the control of barite (a weighting agent used in drilling fluids) and mud weight (MW). Managing these parameters effectively is crucial to maintaining appropriate mud properties, which, in turn, helps control wellbore pressure during a well kill operation.
5. Reduced Pressure on the Wellbore: The overall pressure exerted on the wellbore is lower with SCRs, reducing the risk of formation fractures or breakdowns. This is particularly beneficial when dealing with formations that have low fracture gradients, as excessive pressures could otherwise damage the well structure.

Measuring Slow Circulating Rates

SCR pressures are measured and recorded under various conditions to ensure accurate bottom hole pressure calculations. Regular recording of SCR values is essential for adjusting well control parameters in real-time and ensuring optimal performance during circulation. SCR readings are typically recorded when:

• Mud Weight (MW) or Mud Properties Change: Changes in mud composition affect circulation pressures, so measuring SCR helps recalibrate bottom hole pressure accurately.
• Bit Nozzle or Bottom Hole Assembly (BHA) Changes: Modifications to the drilling assembly alter flow dynamics, requiring new SCR measurements to gauge the resulting pressure effects.
• After Each Trip: Drilling trips, where equipment is raised or lowered in the wellbore, can affect pressure, and measuring SCR after each trip helps maintain consistent pressure control.
• Every 500-1000 Feet of Depth: As drilling progresses and the well depth increases, recording SCR pressures every 500 feet helps maintain safe and accurate well control as pressure profiles change with depth.
• After Equipment Changes or Repairs: Pump or surface equipment repairs can impact circulation performance, making updated SCR measurements critical for stable well control operations.

Friction Pressures in Subsea Operations

In subsea drilling, friction pressures in choke and kill lines are essential considerations. These lines form part of the fluid circulation path and can impose additional pressure on the wellbore, especially in deepwater environments where water depth and narrower inner diameters contribute to greater frictional resistance. The friction pressure in these lines must be routinely measured, recorded, and accounted for to prevent underestimating the wellbore pressure, which could lead to operational hazards.

Subsea well control is particularly complex due to the high pressures at depth and the reduced diameter of subsea lines, which increases friction as fluids are pumped through. This friction pressure, if unaccounted for, can reduce the precision of pressure control, increasing the risk of uncontrolled flow. Therefore, friction pressure measurements in choke and kill lines help ensure that the pressures applied to the wellbore remain within safe operational limits.

References

Cormack, D. (2007). An introduction to well control calculations for drilling operations. 1st ed. Texas: Springer.

Crumpton, H. (2010). Well Control for Completions and Interventions. 1st ed. Texas: Gulf Publishing.

Grace, R. (2003). Blowout and well control handbook [recurso electrónico]. 1st ed. Paises Bajos: Gulf Professional Pub.