A short story, The Adventure of Hole Cleaning
Picture this: you’re on a daring expedition, not in the sweltering jungle or the frozen Arctic, but instead you are drilling deep down into the darkness underground. Your objective? To find and extract black gold!
You’ve got a trusty drill bit, BHA, drill pipe, and the determination to locate a new potential discovery that could open up the field for a major development. However, there’s a catch. As you drill through the geological layers of rock, you’re not just creating a neat wellbore down to those sweet, sweet hydrocarbons. You’re also creating a whole lot of cuttings (that drilled rock has to go somewhere…).
Enter the hero of our story: hole cleaning. This unsung process isn’t about tidying up for rig visitors or ensuring your drill string is feng shui-compliant. Oh no, it’s about ensuring that your wellbore remains the model of cleanliness and that your drilling operation doesn’t come to a grinding halt with a pack off so tight you’ll be thinking about mobilising the fishing crews!
Why Hole Cleaning Is Important
Imagine your kitchen sink where every surface is covered with flour, eggshells, and mysterious goo from last night’s leftovers. Not imagine trying to wash all the mess down the sink hole. It’s not just unpleasant; it’s a recipe for disaster. Depending how good or bad you are at cooking, all that mess might end up plugging up the sink drain and causing a blockage. The same principle applies to drilling.
When you drill, you’re constantly generating cuttings – those little fragments of rock that don’t just politely disappear. They need to be transported up to the surface by the drilling fluid and out of the mud system, and that’s where hole cleaning comes in. Effective hole cleaning ensures that these cuttings are efficiently carried away from the drill bit, round the BHA, up past the drill string and out of the wellbore, allowing for smooth drilling operations. If you don’t clean up these cuttings, you’re setting yourself up for a range of problems such as increased Equivalent Circulating Density (ECD), fractured formations, poorer drilling performance or resulting in pack offs. All of which can be costly or catastrophic to the well bore and drilling progress.
Equivalent Circulating Density (ECD)
Now let’s talk about ECD, or Equivalent Circulating Density. ECD is the effective density of the drilling fluid circulating in the wellbore, and it can have a big impact on your drilling operation. Think of ECD as the Goldilocks principal of the drilling world: too high or too low and you’re asking for trouble, from losses to well control events. ECD is made up of the drilling fluid density (or ESD, Equivalent Static Density), plus the forces acting on the fluid (e.g. mud pumps). The drilling fluid density is carefully managed and continuously maintained by the mud provider based on pre well modelling and engineering calculations to stay in sweet spot between the fracture gradient and pore pressure window. However, if there are additional additives that are not properly managed and removed, e.g. solids and drill cuttings, this can have an adverse effect on the mud density and thus ECD. Effective hole cleaning helps keep ECD at a manageable level, reducing the risk of wellbore instability, fracturing, or an unplanned visit from the local formation fluids (also known as a kick).
Ensuring the Shakers are Clean and Green
For hole cleaning, if the mud pumps are the heart of the operation, then the shakers are its kidneys, filtering out the cuttings and rock debris to keep everything running smoothly. Ensuring that the shakers are doing their job effectively is crucial. If the shakers aren’t properly screened, maintained and cleaned, this can lead to poor solids separation efficiency and an ineffective mud system, which risks the mud density becoming out of spec. Cuttings or finer material can bypass the shakers and be either trapped in the surface pits or recirculated through the drilling system, back into the wellbore. This, in turn, can lead to increased ECD values, reduced drilling performance, and requiring time and effort to correct before further issues develop.
Avoiding the Dreaded Pack Offs
One of the primary nightmares in drilling is the infamous pack off. When drill cuttings aren’t effectively transported in the mud system up away from the bit and to surface, they can accumulate in the wellbore, either suspended in the mud, or along the bottom of the well bore as cuttings beds. If not identified or addressed, these increased cuttings can result in an increasing risk of experiencing a pack off. A pack off occurs when there is either a partial or full blockage of the annulus around drill string that can trap the bit, BHA or drill pipe in place. As an example, if cuttings beds build up and avalanche down the wellbore towards the bit, this can cause a pack off of the BHA. Pack offs typically lead to costly downtime while attempting to get free again, stuck pipes events where jarring or freeing and fishing operations take place, or in extreme cases, potentially the loss of the wellbore and an expensive sidetrack.
Better Wellbore Conditions
Let’s face it nobody likes a messy workspace, and the wellbore is no exception. Proper hole cleaning helps stick to planned for ECD’s. Engineered ECD’s help create safer wellbore conditions, which in turn translates to easier and generally faster drilling, smoother well bore quality, reduced risk of wellbore cavings or collapse and quicker casing running operations. Getting the ECD and hole cleaning right (or wrong) can have significant time, effort and cost implications.
The Role of Drilling Fluids and the Quest for Lower ECD’s
Drilling fluids, also known as mud, are like the Swiss Army knife of the drilling operation. They cool the drill bit, stabilize the wellbore, and most importantly for our discussion transport those pesky cuttings to the surface. But here’s the catch, if your drilling fluid isn’t optimized, it can cause your ECD to rise faster than the price of crude oil during a geopolitical crisis.
A well formulated drilling fluid can help lower ECD, improve hole cleaning, and minimize the risk of pack-offs. It’s all about striking the right balance, too thick, and you might end up with excessive ECD and poor hole cleaning. Too thin, and the cuttings won’t be carried out effectively, leading to, yep, you guessed it, those dreaded pack offs.
Technology To Help Manage Hole Cleaning
A technology which can bring significant benefits to hole cleaning as well as overall drilling performance is Continuous Circulation Units (CCUs). These specialized systems are designed to maintain circulation of drilling fluids in the wellbore continuously, even during activities where traditionally mud would stop circulating. The use of Continuous Circulation Units in drilling offers several significant benefits over conventional connection procedures where the pumps are typically stopped, these advantages stem primarily from the ability of CCUs to maintain the circulation of drilling fluid throughout the entire drilling process, even during connections when adding or removing sections of drill pipe
Continuous circulation is crucial for maintaining well control, optimizing drilling efficiency, and minimizing the risks associated with wellbore instability. In this section we will take a look at how one provider of this technology DHANS Drilling CCU is designed and operates.
The DHANS CCU is specially designed, API approved and tested successfully to provide continuous mud circulation throughout a drilling connection. Their CCU can be installed retroactively to rigs worldwide and is a cost-effective solution to counter traditional Managed Pressure Drilling (MPD) systems. With minimal rig floor footprint, the CCU is used in place of traditional rig tongs or iron rough neck systems to make up pipe connections, while also managing and maintaining the flow of drilling fluids down hole. The CCU is designed to seal against the hung off stand and provide continual circulation, rather than using traditional side entry subs which can be time consuming to instal and remove into the drill string, as well as the manual and rig floor handling associated with their use. A EX zone 1 rated remote control panel, typically mounted inside the drillers cabin for optimal communications, is hard wired to the CCU unit on the fig floor. At a connection, stands are made up and broken out inside the CCU itself. For example, when drilling at stand down, prior to making the connection, when the drill pipe joint stick up is above the table, the CCUs internal slips and sealing element clamps the pipe in place. Flow from the rig mud pumps is then diverted from travelling down through the top drive connection to the drill pipe, instead through the CCUs flow route, without breaking circulation or changing the down hole pressure. Once the connection is made and threads torqued up, the flow is then rediverted back to the top drive flow path before the CCU will unseal from the drill string and drilling recommences. Once drilling continues, the CCU will remain stationary on the rig floor, until the next stand down and connection.
Key Functions and Benefits of Continuous Circulation Units (CCUs):
The continuous circulation unit is a device designed to provide continuous circulation whilst making a drilling connection. Some key benefits of the technology include:
- Consistent Bottom Hole Pressure: Proper management of wellbore pressures, ECD and ESD, is essential to prevent wellbore collapse or fracturing. The CCU provides consistent bottom hole pressure through the elimination of -vs and +vs pressure surges by removing the need to stop and starting circulation over a connection.
- Improved Hole Cleaning & Cuttings Removal: Continuous circulation keeps the drilling fluid moving, which improves the removal of cuttings from the wellbore. This is particularly important in deviated, horizontal or ERD wells, where cuttings can easily accumulate due to the increased forces required to achieve effective hole cleaning.
- Lower Risk of Wellbore Instability: Continuous circulation helps to stabilize the wellbore by minimizing fluctuations in ECD, which is especially critical in wells with narrow pressure windows (the difference between formation pressure and fracture pressure). By maintaining constant pressure, CCUs reduce the risk of wellbore instability, which can lead to hazardous situations like wellbore collapse or loss of circulation.
- Well Control: By maintaining steady pressure, CCUs reduce the risk of well control issues such as kicks (unexpected influxes of formation fluids) or blowouts. Continuous circulation helps prevent the influx of fluids from the formation into the wellbore by keeping the hydrostatic pressure stable. Should a well control event occur, reduced or eliminated downtime in HPHT wells to circulate out connection gas.
- Reducing Stuck Pipe Risks: By preventing cuttings from dropping out of suspension of the mud system and settling on the wellbore walls, CCUs help reduce the risk of the drill pipe getting stuck while drilling and over connections, which can be a costly and time-consuming problem to resolve.
- Improved Drilling Performance: CCUs enable faster shoe to shoe drilling and reduced section times due to continual circulation of mud system cutting or eliminating dedicated circulation periods to clean the wellbore.
- Reducing Total Connection Time (TCT): By allowing circulation to continue during pipe connections, CCUs help reduce achieve unrealised efficiency gains Vs the amount of time required to drop the pumps, make a connection, establish circulation, wait for a MWD survey, go back to bottom, leading to more efficient drilling operations. More on bottom drilling time by Reduced Total Connection Time (TCT)
- Faster Data Updates: Reduce MWD survey time and improve LWD data transmission capabilities by providing MWD and LWD systems both a continual power supply (mud turbine) and uninterrupted mud flow for pulser systems. Surveys can be taken with pumps on during pipe make up.
- Alternative to Managed Pressure Drilling: CCU offers many of the benefits of MPD back pressure method of well control, with a reduced rig footprint and rig up time Vs conventional MPD systems. No need for additional MPD riser joints, extensive piping and telemetry as associated with back pressure based systems.
- Faster Casing Runs: Superior hole cleaning due to always on mud circulation lifting more cuttings to surface and out of your well Vs traditional connection systems. Reduce open hole and formation exposure time by minimising or eliminating altogether circulating hole clean at section or well TD
- Flexible System: The CCU is a bespoke and modular system, designed for retrofitting onto existing drilling assets with minimal critical path time rig up, available in 5000psi, 7500psi & 10000psi rated units
- HSE Focused: Reduce risk on the rig floor and improve safety with a remote controlled, fully automated operation connection method. Reduce manpower from red zone, reduced chance of manual handling related accidents and injuries.
- Well Control During Connections: The CCU has inverted slips which allow a pressurised seal around the drill pipe. This has allowed formation fluid influx events (aka kicks) to be controlled rather than activating the BOP shear rams.
- Precises Fluid Volume Control: Typically monitoring drilling volumes over connections has been problematic due monitoring the well volume on a trip tank which is affected by pumps on and off events as mud volumes stabilise. As the CCU removes the pumps on/off events, this allows improved drilling fluid management
How CCUs Work:
- Integration with Rig Systems: CCUs are integrated with the rig’s existing circulation system. During the drilling process, when a drill string connection is made, the CCU takes over the circulation of the drilling fluid, without interruption, and makes up the subsequent pipe joint (i.e. replacing the function of rig togs and/or iron roughneck) before the top drive takes over and feeds the new pipe into the wellbore to continue drilling.
- Pressure Management: The CCU maintains the desired pressure in the wellbore by controlling the flow of drilling fluid. It uses pumps and control valves to ensure that the fluid circulation remains continuous. Elimination of negative and positive pressure surges when stopping and starting circulation to make a connection.
- Adaptation to Drilling Operations: CCUs are designed to adapt to various drilling operations, with clear benefits in deviated, horizontal wells, or ERD wells making them versatile tools in complex drilling environments.
Applications of CCUs
Continuous Circulation Units are valuable systems in the modern drilling operator’s toolbox, allowing for continuous circulation of drilling fluids, which helps maintain wellbore pressure, improve hole cleaning, enhance safety, and increase overall drilling efficiency. Their ability to maintain circulation during critical operations like pipe connections makes them invaluable in managing complex and challenging well conditions:
- Narrow Pressure Margin Wells: In wells with a narrow margin between pore pressure and fracture gradient, CCUs help maintain the delicate balance needed to drill safely.
- High-Pressure, High-Temperature (HPHT) Wells: In HPHT wells, where the pressure and temperature conditions are extreme, CCUs help manage these challenges by maintaining continuous circulation and preventing pressure fluctuations.
- Horizontal and Extended Reach Drilling: In horizontal or ERD type wells, the well geometry, length of open hole and forces required for hole cleaning often pose a challenge during planning and operations. By ensuring continual, effective hole cleaning, CCUs can help manage these challenges as well as improve drilling performance.
- Deepwater Drilling: In deepwater environments, where pressure management is critical, CCUs play a vital role in ensuring wellbore stability and preventing costly and dangerous well control incidents.
Several companies in the oil and gas industry provide Continuous Circulation Units (CCUs) and related technology.
1. Weatherford International
- Continuous Flow System (CFS): Weatherford offers a Continuous Flow System that enables continuous circulation during connections. Their system is designed to enhance well control, improve hole cleaning, and reduce non-productive time.
2. Halliburton
- Continuous Circulation Sub (CCS): Halliburton provides continuous circulation technology through their Continuous Circulation Sub, which allows for uninterrupted circulation of drilling fluids during pipe connections. This system is part of their broader suite of well control and drilling efficiency tools.
Wrapping It All Up
In conclusion, hole cleaning might not be the most glamorous part of drilling, but it’s undoubtedly one of the most crucial. Get it right, with hole cleaning ‘in the box’, and it’s the unsung hero that ensures your wellbore remains free of debris, your drilling fluid stays in the Goldilocks zone of downhole pressures. Get it wrong, and your operation can grind to a halt with slow drilling, pack offs, stuck pipe or wellbore stability issues that could have been avoided.
There is a wealth of information, training and coaching available on effective hole cleaning and stuck pipe mitigations. Multiple complex considerations are factored in when designing a well, with the pressures, fluids and equipment required to safely and efficiently deliver each well all parts of the puzzle. Continuous Circulation Units offer multiple benefits and are worth considering, from improving the quality of the wellbore through effective ECD and hole cleaning, performance improvements with faster section times, more on bottom times and less time spent circulating, to helping manage HSE risks from both a well control and manual handling aspects while making connections on the rig floor.
So, let’s take a moment to appreciate the humble task of hole cleaning. It can be the difference between a smooth, efficient operation and a costly, frustrating and potentially dangerous situation. And remember, when in doubt, keep those shakers clean, drilling fluid in spec, pumps working and don’t let the cuttings win!
Now, get out there and keep those wellbores clean!
