SECCIÓN III- MARCO APLICATIVO
CAPÍTULO 7: PROYECTO DE INVESTIGACIÓN Y PLAN DE MEJORA
7.3 Plan de mejora organizacional (PMO)
7.3.3 Fase 3: Características de los dispositivos de seguimiento
A complete description of the extra pressure to which the open hole is exposed combines the effect of friction (ECD) and the effect of cuttings loading (effective mud density).
( )
Where: ECD = equivalent circulating density, ppg
∑ Pa = sum of friction pressure losses in the annulus, psi TVD = true vertical depth of hole, ft
SGc = specific gravity of the cuttings Ca = cuttings concentration, vol %
W = mud density, ppg
7.0 Hole Cleaning Guidelines
Additional information can be found in the Transocean SedcoForex Driller’s Stuck Pipe Handbook Section 2.
7.1 Guidelines for Vertical Holes
• Rheology is very important in transporting cuttings in vertical or deviated holes. Large diameter holes, in particular, cannot be cleaned by velocity alone. Providing that the mud rheology is correct, hole cleaning is not generally a problem in such wells. Annular velocity is generally greater than the cuttings slip velocity and so cuttings are effectively removed from the hole. To ensure that a low slip velocity is achieved, these wells are usually drilled with viscous, high yield point mud systems.
• Circulate at least 1.3 x bottoms-up for vertical wells. Monitor returns at the shakers ensuring the rate of return has decreased to acceptable levels before tripping.
• Limit the use of high viscosity pills. Instead, adjust the properties of the active system to provide optimum hole cleaning. High weight pills should not be used in vertical holes.
• Reciprocate rather than rotate the pipe prior to tripping. This helps remove cuttings from stagnant zones near the wellbore wall.
• Pull through tight spots only if the pipe is free going down. Agree a maximum allowable overpull in advance and work up progressively towards it, ensuring that the pipe is free to move downwards on each occasion. Stop and circulate if overpulls become excessive.
• Only backream if absolutely necessary. Backreaming may result in hole pack-off and stuck pipe.
It can also mask the onset of potentially serious hole problems which may have been detected at a much earlier stage had backreaming not taken place.
Mud Velocity Particle Velocity
Vertical Well
In vertical wells, annular velocity is generally much higher than the cuttings slip velocity and drilled cuttings are effectively removed from the annulus.
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7.2 Guidelines for Deviated and Extended-Reach Wells
The measures noted above are also valid for the effective cleaning of deviated wells. However, the geometry of deviated and extended-reach holes makes adequate hole cleaning rather more difficult than for vertical or near-vertical wells. Many of the problems encountered are associated with the nature of the cuttings beds which form on the low side of the hole.
7.2.1 Low-Angle Sections (10° - 40°)
Cuttings Bed Characteristics:
Cuttings beds form on the low side of the hole and are subject to 'particle recycling' which has a detrimental effect upon hole cleaning.
1. Because of increasing inclination, the cutting is forced toward the low side of the annulus, where it travels downward due to a lack of lifting force in the flow (low velocity near the wall).
2. At some point, due to a higher shear stress, the cutting is lifted and re-enters the high-velocity region at the middle of the annulus.
3. Then, it is swept upward and continues to travel until
4. its tendency to drop overcomes the lifting force in the flow and it is forced toward the low side of the annulus again.
This process can be repeated many times resulting in the cutting’s shape being altered through grinding. Measures used to minimise this problem include viscous sweeps.
Mud Velocity Particle Velocity
Directional Well
In deviated holes, cuttings will 'slip' to the lower side of the wellbore. In this situation, the velocity of the drilling fluid has to be higher in order to keep the cutting moving up towards the surface.
1
2 3 4
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Compared with vertical sections, low-angle sections require higher annular velocities for efficient hole cleaning. For this type of well section, laminar flow dominates cuttings transport. Therefore, viscosity, yield point and initial gel strengths have a significant effect on annular cuttings concentration and hole cleaning efficiency.
Field Guidelines:
• General use of laminar flow.
• Maintain a high yield point and gel strength to reduce the settling of cuttings when pumps are off.
• Always ensuring that the hole is clean before turning the pumps off.
• Maximise YP/PV ratio.
• Use viscous sweeps to reduce the effects of particle recycling.
7.2.2 Critical-Angle Sections (40° - 60°)
Cuttings Bed Characteristics:
In well sections with inclinations greater than 40 degrees, cuttings do not recycle as readily as in lower-angle sections. This is because gravity tends to hold them down on the low side of the hole.
Well sections with inclinations between 40° and 60° are considered critical, not only because a cuttings bed develops, but also because it is unstable and prone to sliding downward (avalanching).
The consequence of avalanching is an instantaneous build-up of cuttings around the drillpipe and/or BHA which, if not treated properly, can result in stuck pipe. Also,
• Turbulent flow exhibits a desirable, eroding effect on cuttings beds.
• Pipe movement (rotation/reciprocation) mechanically disturbs cuttings beds.
Field Guidelines:
In the well sections over 40 degrees, attention must focus on minimising cuttings beds.
• For the range of intermediate inclinations (40° - 60°), turbulent flow is recommended. Since cuttings transport in turbulent flow is not affected by rheological properties, lower mud parameters (i.e. YP, PV) may be used. However, static mud parameters such as gel strength are usually desirable even if turbulent flow is preferable. If turbulent flow cannot be used because of other adverse factors, like wellbore instability, annular velocity should be kept as high as possible.
• Rotating and/or reciprocating drillpipe has a mechanical, destructive influence on the cuttings bed (this influence is the main factor that provides a higher cleaning rate for higher RPM). As pipe rotation is typically governed by directional drilling needs, periodic wiper trips should be considered.
• Combination sweeps (low viscosity/high density) are effective at eroding the cuttings bed, and carrying the cuttings to surface.
7.2.3 High-Angle Sections (> 60°)
Cuttings Bed Characteristics:
At high angles of inclination the formation of a cuttings bed is almost instantaneous, and its thickness is governed primarily by annular velocity. The cuttings bed that forms at angles greater than 60 degrees is stable, this means it will not avalanche. The waving, vortex-like, character of turbulent flow has a destructive influence on the bed being formed. There is a tendency for cuttings to be withdrawn (lifted) from the bed and displaced upwards in the annulus, where such a process may occur again. This kind of interaction, together with the flat velocity-profile typical of turbulent flow, leads to better hole cleaning.
Pipe movement (rotation/reciprocation) mechanically disturbs cuttings beds.
Field Guidelines:
• Turbulent flow is preferable in high-angle wells. Generally, the same recommendations as those described for intermediate-angle wells are applicable in this region for turbulent flow. However,
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the requirements to ensure a mud gel strength are less important. If turbulent flow cannot be achieved, the YP/PV ratio should be maintained as high as possible.
• Again, rotating and/or reciprocating drillpipe has a mechanical, destructive influence on the cuttings bed. It is this influence that provides a higher cleaning rate for higher RPM, particularly at high inclinations where a considerable cuttings bed is formed. Pipe rotation > 120 RPM is highly recommended. Periodic wiper trips should be used if drillpipe rotation is restricted. The use of rotary steerable directional tools may prove helpful.
• Combination sweeps (low viscosity/high density) are effective at eroding the cuttings bed, and carrying the cuttings to surface. The effectiveness of a cuttings-bed-maintenance program can be determined through several indicators.
7.3 Poor Hole Cleaning Indicators
A number of rig-site indicators can be used as a guide to how effectively the hole is being cleaned.
These include:
• Shape and size of cuttings at the shakers - small, well rounded cuttings may indicate extended periods of regrinding down hole - an indication of the presence of cuttings beds.
• Rate of return of cuttings versus expected volume.
• Increased torque and drag.
• High pick-up weight.
• Poor weight transfer. A higher than normal surface weight is required to get a pressure-drop response from the mud motor.
• Difficulty orienting the motor, due to excessive friction between the cuttings and the drillstring.
• Excessively ground cuttings, due to extended particle recycling and drillpipe interaction with the cuttings bed.
7.4 Effects of Mud Type on Hole Cleaning Efficiency
• In highly deviated wells, and for lower values of yield point and plastic viscosity, cleaning performance for both mud types is roughly the same. However, at higher values of yield point and plastic viscosity water-base muds provide better cleaning. The general observation is that an increase in mud yield point and plastic viscosity results in increased cuttings concentration for both muds.
• As a result from this higher cuttings concentration, torque requirements for both muds increase with increasing yield point and plastic viscosity, at higher hole inclinations.
• Hole-cleaning performance of oil-base muds at critical angles (40° to 60°), is reduced by severe cuttings bed avalanching (due to reduced friction).
7.5 Hole Cleaning 'Aids'
Mud rheology, density, annular velocity and pipe movement are seen as being key to the success of efficiently cleaning the wellbore. The importance of sound operational practices and vigilant monitoring of the hole condition can not be underestimated. There are, however, tools available which, when used appropriately, can help to predict, and possibly alleviate, the onset of hole cleaning problems.
• Hole Cleaning Software Packages
Can be used to predict the likelihood and potential location of the build up of any cuttings beds.
Should be used with caution, making sure that results are combined with past experience and offset data.
• Cuttings Bed Impellers / 'Enhanced Performance' Drillpipe / Agitators
Lift the cuttings away from the low side of the hole into the area of higher annular velocity but, unless mud rheology and other conditions are sufficient, they are likely to drop out of suspension and form beds higher up the hole.
• 'Pressure While Drilling' Tools
In recent years, tools have been developed which enable downhole pressure measurements to be taken in real-time. The 'Pressure While Drilling' (PWD) tools give an indication of any
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changes in equivalent circulating density. PWD tools have a number of potential applications including:
- LOT
- Lost circulation detection
- Flow / kick detection and monitoring - Swab / surge information
- Mud property monitoring
- Measurements of differential pressure for overall drilling performance optimisation - Underbalanced drilling
- Hole cleaning and stability monitoring
By accurately measuring ECD whilst drilling, these tools allow engineers to assess the condition of the hole before serious hole cleaning problems occur.