Hole Cleaning in Complex Wellbores

Hole Cleaning in Complex Wellbores such as Extended-Reach Wells, 3d Designer Wells and Wells with Wellbore Stability Problems present unique challenges that requires a completed system approach to the overall drilling process. The hole cleaning challenge begins at the planning stage of a well. Making sure that the well-designed focuses on the critical hole cleaning problems - is a key element to the success of these wells. When these elements are overlooked, the inherent risk of these wells increases. Once the plan is in place, the next step is to ensure that the entire rig team understands the downhole environment and the detailed response planned for tight hole. Understanding that tight hole in these wells is almost always cuttings related - is the first step towards a successful well. The Hole Cleaning System can be divided into three distinct environments. The first is the vertical hole section which generally ranges from zero to about 30 degrees. The next section ranges from 30 degrees up to about 65 degrees. And the final section is above 65 degrees of inclination. Each of these environments requires different set of rules for effective hole cleaning. Vertical hole certainly has its challenges. However, from a hole cleaning perspective, it is the easiest to clean.

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      Normally vertical hole cleaning is accomplished by plug flow of the drilling fluid that is designed to suspend the cuttings when the pumps are shut off. The cutting in this case has thousands of feet to fall before it reaches bottom. Fluid rheology is generally the key factor for affective hole cleaning in this section. A close-up of this is clearly shows the slower hindered settling typical of vertical intervals. In this environment, as cuttings begin to move downwards they run into other cuttings. They also display fluid as they move downwards which creates an upward flow that further slows the cutting. In this clip, some cuttings are actually moving up the hole, others seem stationary and still others are moving downward with the pull of gravity. As the angle increases over 30 degrees new challenges begin to come into play. Watch how the settling environment changes as this is tube moved from vertical to about 45 degrees. Those cuttings that once had thousands of feet to fall, now reach bottom in a matter of inches.  Pipe that was once concentric in the wellbore is now lane on the low side of the hole and fluid that was flowing all around the pipe now primarily flows on the top of a hole. We get a better picture of the pipe and flow situations later on this video. One phenomenon of cuttings behavior entangled wellbores call Boycott settling shown here at 45 degrees shows the clarified layer along the upper side and the slump along the lower side of the tube. This boycott settling causes some particles to move upward with the flow stream. Others are momentarily suspended while still others form a bed along the bottom of the hole and slumped opposite to the direction of flow. Here a highly dispersed mud is flowing at a 35 degree angle. Notice the rapid cuttings avalanche even when the pumps are shut off. These cuttings will slide downhole until they pile up on top of the P.A.J. or until they reach a high enough angle in the well and stop creating a large cuttings bed. As the well inclination reaches about 65 degrees, the cuttings will stop sliding down hole. Now, instead of a large cuttings bed forming in the well, along more evenly distributed cuttings bed will develop.

       Very large volume of cuttings can exist in these hole sections. For example, a 2 in cuttings bed in a 10,000 ft section of 12 ¼ in hole is approximately 85 barrels of cuttings in the well. Here during transport, a form of saltation flow is a typical transport mechanism. All this provides for a movement of a large volume of cuttings up the hole. It does not provide efficient hole cleaning. As we replayed this clip again notice how the movement of cuttings is very sudden with almost no cuttings movement on either side of the dawn. Movement of cuttings on a continuous basis only occurs at the top of these beds. Water in turbulent flow at 200 ft/min in a fully eccentric horizontal annulus can efficiently clean the hole. The same water at 45 degrees also in a fully eccentric annulus does not clean this interval at the same annular velocity. The net movement of the cuttings is downward clearly showing that effective hole cleaning in one interval does not necessarily translate to affective hole cleaning in the next interval of the same well. An essential element of high angle hole cleaning that needs to be understood is the fact that if cuttings are flowing over the shale shakers, the hole is being cleaned. The question now becomes how faster we cleaned the hole. Are we generating cuttings into the wellbore faster than we're getting them out? and is there a way that we can measure this? The first step towards answering these questions is to define the elements that affect efficient hole cleaning. There's no doubt that all of these elements play a role in how fast we're able to get cuttings out of the hole. Elements such as hole size, washout, drill pipe size, and wellbore instability all affect flow rates while rotary speed and mud rheology will play a role in how efficient we're able to get cuttings into the fluid flow. How the drilling fluid interacts with the rocks being drilled is also an essential element of the hole cleaning system.

       If we are joined with the dispersive system where the drilling fluid can penetrate the cutting and dissolve it into solution than most of the hole cleaning is accomplished through this mechanism. However if the drilling fluid is fully inhibitive such as this clear base soil then the entire cutting must be removed from the hole mechanically. Each of these systems have their place in high angle drilling. However, a very good rule of thumb is not to get caught in the middle between a highly dispersive and a highly inhibitive system. This drawing represents a section through the high angle portion of the wellbore. Here the drill pipe lays on the low side of the hole and the mud following the path of least resistance flows primarily on the top of the hole.in inhibited mud environment the cutting will lay on the low side of the hole away from the fluid flow that's preventing their efficient removal from a well. This computer simulation demonstrates the flow patterns as the center pipe becomes progressively more eccentric. Very high flow rates are seen along the top of the hole with little-to-know flow around the drill pipe at the bottom of the hole. Sweeps in high angle holes have proven largely ineffective. This clip demonstrates how the sweet deform zone along gates along the top of the hole. If high-speed rotation is introduced to stir the cuttings, the drilling fluid around the cuttings contaminates the sweep destroying its original properties. This clip demonstrates how fluid flow in a concentric annulus on the left differs from that of an 80% eccentric annulus on the right. Note the flow filling the hole on the left and the flow primarily along the top of the hole in the right.

       In order to effectively remove the cuttings from the high angle portion of the well, we must mechanically disturb the cuttings either with turbulent flow which is impractical in hole sizes over 8 ½ in or three pipe movement. here the addition of pipe rotation at 150 rpm effectively stirs the cuttings into the flow regime noticed the hopping motion of the cuttings as gravity  pulls them back out of the flow and the pipe movement pushes them back up. It's not just rotation that pushes the cuttings into the floor regime, but a combination of high rotary speed pipe eccentricity and mud rheology. Most of the hole cleaning takes place across the drill pipe tube. The high rotary speed and the viscous coupling between the drilling fluid and the drill pipe cause the fluid to span around the pipe. This fluid movement picks the cuttings up and carries them into the floor regime on the top of the hole. Without this viscous coupling hole cleaning in a laminar flow environment is reduced dramatically. In order to effectively maintain this viscous coupling, we recommend a 6 rpm reading of "(1.1 - 1.5)* Hole Size (inches)". This insures effective transfer of energy from the pipe to the fluid and then to the cuttings. The better the transfer of energy to the cuttings, the better the hole cleaning. Notice the poor transfer of energy to the cuttings on the left and the excellent energy transfer that's generating the helical flow pattern of the cuttings on the right. The importance of pipe rotary speed cannot be overlooked.

       The primary means of moving cuttings into the flow regime comes from pipe rotation. For all sizes over 8½ in, rotary speed is the single most important hole cleaning factor in the system. Field experience has shown that cutting flow over the shakers drastically improves that high rotary speeds and that it dramatically drops off as the rotary is slowed. Hurdle speeds of one 120 rpm and one 180 rpm have been repeatable regardless of hole size, drill pipe size, drawing fluid type. At these speeds significant increases in cuttings flow over the shakers is generally observed. The hole cleaning environment in a high angle well can be viewed like this conveyor belt. If the rotary speed and fluid rheology are not appropriate to get the cuttings onto the belt then hole cleaning efficiency is going to suffer. Likewise fluid flow rate represents the speed of the belt and that's the rate at which cuttings are being removed from the hole.

      In summary, hole cleaning efficiency is primarily affected by three things. Flow rate: flow rate moves along the top side of the hole and access the conveyor belt moving cutting out of the well bore. Rotary speed: rotary speed acts to get the fluid moving around the body of the drill pipe to throw cuttings up into the flow regime. Rotary hurdle speeds must be met in order to maintain effective hole cleaning.  In flow rheology, fluid rheology acts to create a viscous coupling with the drill pipe. It further acts to help to suspend the cuttings momentarily in the flow regime. It also has to provide hole cleaning in a lower angled portions of the wellbore. Getting the right combination of these critical parameters and then keeping them in the desired range throughout the drilling process requires full-time attention to detail. Good hole cleaning doesn't just happen, it requires a commitment from both the office and rig teams and a clear understanding of the downhole environment. Complex wells must be treated as an entire system with hole cleaning as the center of that system.

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