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Hole cleaning is the ability of a drilling fluid to transport and suspend drilled cuttings.
Hole cleaning in directional-well drilling
Throughout the last decade, many studies have been conducted to gain understanding on hole cleaning in directional-well drilling. Laboratory work has demonstrated that drilling at an inclination angle greater than approximately 30° from vertical poses problems in cuttings removal that are not encountered in vertical wells. Fig. 1 illustrates that the formation of a moving or stationary cuttings bed becomes an apparent problem, if the flow rate for a given mud rheology is below a certain critical value.
Inadequate hole cleaning can lead to costly drilling problems, such as:
- Mechanical pipe sticking
- Premature bit wear
- Slow drilling
- Formation fracturing
- Excessive torque and drag on drillstring
- Difficulties in logging and cementing
- Difficulties in casings landing
The most prevalent problem is excessive torque and drag, which often leads to the inability of reaching the target in high-angle/extended-reach drilling.
Hole cleaning in extended reach drilling operations
Effective drilling-fluid selection and management is important to the successful outcome of a high-angle or horizontal extended-reach drilling (ERD) operation. In addition to formation protection, the most important ERD challenges include:
- The narrow margin between the pore pressure and the fracture gradient
- Equivalent circulating density (ECD) management
- Adequate hole cleaning
- Reduction of torque and drag
- Wellbore stability
- Barite sag
- Loss of circulation
Years of operational data indicate that hole angles of between 30 and 60° create the most difficult hole-cleaning conditions. Hole-cleaning problems can be minimized by good management of:
- Annular velocities
- Drilling-fluid viscosity
- Pipe-rotation speed
- Pipe eccentricity
Because of their reliable performance under adverse downhole conditions, invert-emulsion oil-based and synthetic-based drilling fluids (OBFs and SBFs, respectively) are usually the first choice for ERD operations. The use of invert-emulsion muds, however, is becoming more restricted, because of environmental considerations, and several inhibitive water-based fluid (WBF) systems have been developed for use as alternatives.
Tools for hole cleaning optimization
Using hydraulics-modeling software that is programmed specifically for oilfield applications, it is possible to accurately predict drilling-fluid properties under actual downhole conditions, including:
- Static and dynamic temperature profiles
- Hydraulic pressures
- Annular pressure loss
- Rheological properties
- Cuttings loading and transport efficiency
- Effects of pipe eccentricity
- Pressures required to break gels
The modeled properties are confirmed by real-time pressure-while-drilling (PWD) data. The immediate feedback from the modeling process can allow the operator to optimize hole cleaning by several means, including:
- Adjustment of surface mud properties to meet changing downhole conditions.
- Adjustment of mechanical parameters such as penetration rate, flow rate, pipe-rotation speed, and tripping speed.
- Design and implementation of an effective sweep program.
Even while still in the well-planning stages, the casing design, bit selection, and drilling-fluid properties can be optimized to achieve the best drilling conditions, given the rig’s pumps and fluid-handling capabilities. An accurate hydraulics modeling package should incorporate Bingham-plastic, power-law, and the Herschel-Bulkley rheological models.> Surface rheological properties are measured with a six-speed rheometer. Such input allows the hydraulics-modeling software to determine actual annular shear rates at any depth in the well, taking into account the temperature and pressure regime at that depth.
The basis for the rheological modeling is either a matrix database of rheometer data or real-time data obtained from a high-pressure/high-temperature (HP/HT) viscometer while drilling. A built-in routine can calculate the pressure required to break gels, allowing the operator to minimize surge pressures while tripping and running casing, and to reduce the risk of breaking down the formation.
A comprehensive modeling software package should accurately predict:
- The cuttings loading in the annulus
- The cuttings-bed height
- The effect of drillstring rotary speed and pipe eccentricity
- The maximum recommended rate of penetration (ROP) for the given conditions
These tools are useful not only for drilling extended-reach wells, but also for optimizing drilling performance in deepwater operations, HP/HT wells, and slimhole-drilling operations.
Hole cleaning in underbalanced drilling (UBD)
Fig. 2 shows annular liquid velocity vs. gas-injection rate and liquid-flow rate. Hole cleaning while UBD horizontally must be monitored closely. There is a reduced fluid rheology (a very thin, nonsolids-suspending mud), turbulent two-phase flow, and, normally, an increased rate of penetration (ROP). A result of two-phase flow is accelerating mud and cuttings transport velocities (because of gas expansion) as the fluid moves upward from the bit.
The main areas of concern for hole cleaning are the region where the hole angle is from 45 to 50°, and the region immediately behind the bit. The area immediately behind the bit can become the critical hole-cleaning area, because there is limited reservoir inflow. Liquid-phase velocity and hole cleaning in this area depend only on the fluid(s) and rate(s) being pumped or injected down the drillstring.
Two phase hole cleaning
Two-phase hole cleaning is largely dependent on the same criteria as single-phase. Hole-cleaning efficiency and solids transport are primarily controlled by liquid-phase velocities and solids concentration. Studies and field experience have shown that removal of cuttings is more efficient with two-phase fluid. The addition of a gas medium generates a turbulent flow regime, which minimizes solids bed formation. Liquid velocity is the critical parameter controlling the system’s ability to transport solids. From experience, it has been concluded that a minimum liquid-phase annular velocity of 180 to 200 ft/min is required in a wellbore with a deviation greater than 10°.
High-viscosity sweeps that provide effective hole-cleaning in vertical wellbores might not be the best option for high-angle and horizontal wells, because of the flow distribution around eccentric drillpipe. To induce flow, the stress applied to a fluid must exceed that fluid’s yield stress. In the narrow annular space created by eccentric drillpipe, it is possible that little or no flow will occur, and that the cuttings bed will remain in place. Pumping a high-viscosity sweep might exacerbate this problem in a deviated well.
Applying a weighted sweep program that targets the silt bed that accumulates on the low side of the hole can mitigate hole-cleaning problems that often occur in ERD wells. As early as 1986, hole-cleaning research indicated that turbulent flow produced by relatively thin drilling fluid is more effective at silt-bed removal than is flow produced under a high-viscosity flow profile. Consistent results in silt-bed removal have been achieved with fully-circulated, low-viscosity, weighted sweeps that exceed the drilling mud weight by 3 to 4 ppg and provide a 200- to 400-ft column in the annulus. The guidelines for an effective weighted sweep program are:
- The sweep is pumped at regular intervals at the normal circulating rate.
- The pipe-rotation speed is ≥ 60 rev/min once the sweep has reached the bit.
- The sweep is allowed to return to the surface with continuous circulation. 
The additional buoyancy that a weighted sweep provides helps to reduce cuttings-settling tendency while the sweep travels up the annulus. The efficiency of the weighted sweep in dislodging cuttings might cause an increase in ECD, however, while the annulus becomes loaded. If a PWD tool is used, effects on the ECD can be monitored and the pump rate reduced as needed to maintain an acceptable ECD without allowing cuttings to settle.
Factors in hole cleaning
Flow rate is the dominant factor in cuttings removal while drilling directional wells. An increase in flow rate will result in more efficient cuttings removal under all conditions. However, how high a flow rate can be increased may be limited by:
- The maximum allowed ECD
- The susceptibility of the openhole section to hydraulic erosion
- The availability of rig hydraulic power
Hole inclination angle
Laboratory work has demonstrated that, when hole angle increases from zero to approximately 67° from vertical, hole cleaning becomes more difficult, and flow-rate requirement increases. The flow-rate requirements reach a maximum at approximately 65 to 67°, and then slightly decrease toward the horizontal. Also, it has been shown that at 25 to approximately 45°, a sudden pump shutdown can cause cuttings sloughing to bottom, and may result in a mechanical pipe-sticking problem. Although hole inclination can lead to cleaning problems, it is mandated by the needs of drilling inaccessible reservoir, offshore drilling, avoiding troublesome formations, and side tracking and to drill horizontally into the reservoir. Objectives in total field development (primary and secondary production), environmental concerns, and economics are some of the factors that intervene in hole angle selection.
Laboratory studies have shown, and field cases have reported, that drillstring rotation has moderate to significant effects in enhancing hole cleaning. The level of enhancement is a combined effect of:
- Pipe rotation
- Mud rheology
- Cuttings size
- Flow rate
- String dynamic behavior
It has been proved that the whirling motion of the string around the wall of the borehole when it rotates is the major contributor to hole cleaning enhancement. Mechanical agitation of the cuttings bed on the low side of the hole, and exposing the cuttings to higher fluid velocities when the pipe moves to the high side of the hole are results of pipe whirling action.
Although there is a definite gain in hole cleaning caused by pipe rotation, there are certain limitations to its implementation. For example, during angle building with a downhole motor (sliding mode), rotation cannot be induced. With the new steering rotary systems, this is no longer a problem. However, pipe rotation can cause cyclic stresses that can accelerate pipe failures due to fatigue, casing wear, and, in some cases, mechanical destruction to openhole sections. In slimhole drilling, high pipe rotation can cause high ECDs due to the high annular-friction pressure losses.
In the inclined section of the hole, the pipe has the tendency to rest on the low side of the borehole, because of gravity. This creates a very narrow gap in the annulus section below the pipe, which causes fluid velocity to be extremely low and, therefore, the inability to transport cuttings to surface. As Fig. 3 illustrates, when eccentricity increases, particle/fluid velocities decrease in the narrow gap, especially for high-viscosity fluid. However, because eccentricity is governed by the selected well trajectory, its adverse impact on hole cleaning may be unavoidable.
Fig. 3—Fluid velocity profile in eccentric annulus (after Hzouz et al.).
Rate of penetration
Under similar conditions, an increase in the drilling rate always results in an increase in the amount of cuttings in the annulus. To ensure good hole cleaning during high ROP drilling, the flow rate and/or pipe rotation have to be adjusted. If the limits of these two variables are exceeded, the only alternative is to reduce the ROP. Although a decrease in ROP may have a detrimental impact on drilling costs, the benefit of avoiding other drilling problems, such as mechanical pipe sticking or excessive torque and drag, can outweigh the loss in ROP.
The functions of drilling fluids are many, and can have unique competing influences. The two mud properties that have direct impact on hole cleaning are viscosity and density. The main functions of density are mechanical borehole stabilization and the prevention of formation-fluid intrusion into the annulus. Any unnecessary increase in mud density beyond fulfilling these functions will have an adverse effect on the ROP and, under the given in-situ stresses, may cause fracturing of the formation. Mud density should not be used as a criterion to enhance hole cleaning.
Viscosity, on the other hand, has the primary function of the suspension of added desired weighting materials, such as barite. Only in vertical-well drilling and high-viscosity pill sweep is viscosity used as a remedy in hole cleaning.
The size, distribution, shape, and specific gravity of cuttings affect their dynamic behavior in a flowing media. The specific gravity of most rocks is approximately 2.6, therefore, specific gravity can be considered a nonvarying factor in cuttings transport. The cuttings size and shape are functions of the bit types (roller cone, polycrystalline-diamond compact, diamond matrix), the regrinding that takes place after they are generated, and the breakage by their own bombardment and with the rotating drillstring. It is impossible to control their size and shape, even if a specific bit group has been selected to generate them. Smaller cuttings are more difficult to transport in directional-well drilling, however, with some viscosity increase and pipe rotation, fine particles seem to stay in suspension and are easier to transport.
- Cameron, C. 2001. Drilling Fluids Design and Management for Extended Reach Drilling. Presented at the SPE/IADC Middle East Drilling Technology Conference, Bahrain, 22-24 October. SPE-72290-MS. http://dx.doi.org/10.2118/72290-ms.
- Sewell, M. and Billingsley, J. 2002. An Effective Approach to Keeping the Hole Clean in High-Angle Wells. World Oil 223 (10): 35.
- Pilehvari, A.A., Azar, J.J., and Shirazi, S.A. 1999. State-of-the-Art Cuttings Transport in Horizontal Wellbores. SPE Drill & Compl 14 (3): 196–200. SPE-57716-PA. http://dx.doi.org/10.2118/57716-PA.
- Azouz, I., Shirazi, S.A., Pilehvari, A. et al. 1993. Numerical Simulation of Laminar Flow of Yield-Power-Law Fluids in Conduits of Arbitrary Cross-Section. Trans. of ASME 115 (4): 710-716.
Noteworthy papers in OnePetro
Guild, G.J., T.H. Hill Assocs.; Wallace, I.M., Phillips Petroleum Co. U.K.; Wassenborg, M.J., Amoco U.K.: Hole Cleaning Program for Extended Reach Wells, 29381-MS, http://dx.doi.org/10.2118/29381-MS
A. Saasen, G. Løklingholm, Statoil ASA: The Effect of Drilling Fluid Rheological Properties on Hole Cleaning, 74558-MS, http://dx.doi.org/10.2118/74558-MS