Wellbore cleanout in drilling phase

The smaller hole sections are often drilled at highest inclination. Improved hole cleaning is required at high inclination since inefficient hole cleaning increases the risk of lost circulation.

Accumulated cuttings in the wellbore increase ECD. Also pack off situation is likely to occur while tripping and connections time.

Master Thesis, 2019 79

Figure 4.8. Effect of pack off on ECD (ASA)

Poor hole cleaning is usually detected by following the trend data.

1. Drilling parameters: Cuttings accumulated in the annulus affect the toque and drilling fluid pressure in the following ways:

 Fluctuating and increasing torque.

 ECD and ESD variations(Bekkeheien).

Note: The fluctuation in downhole torque can be recognized immediately by EMS present within the wired BHA. EMS could provide the real-time value of bit torque ((EMS), March 8, 2017). This might give an immediate indication of formation change or alert while drilling.

Similarly, ASM could give instant indication of pressure build at defined depths(Measurement, March 8, 2017.)

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Figure 4.9. Parameters monitored for effective hole cleaning(ASA)

2. Drilling fluid properties: Insufficient removal of solids/cuttings can be observed when there is an increase in viscocity, fluid density in/ out, sand content and presence of low gravity solids.(ASA)

Note: Any change in the fluid density, solids or cuttings in the fluid could easily be identified by the ASMs, but no technology has been developed to integrate WDP with tools that could measure the viscosity/ downhole mud rheology. Further research must be carried out in this area. Developing mud sensors that could analyze real time downhole mud properties at different depths, then fingerprinting of gas with mud in use will help in detecting the presence of reservoir fluids in annulus.

3. Cuttings Return: One of the significant function of drilling mud is to transport the cuttings bed to the surface. Sometimes, observing the cuttings in the shale shakers and understanding the hole clean or wellbore situation might be late/ ineffective. The

Master Thesis, 2019 81 typical warning signs are insufficient or no cutting returns for a particular rate of penetration along with the variation in cutting returns (ASA, 22.10.2004, 2006-09-21).

Note: It might be late when warning signs such as cuttings return indicate the downhole well conditions. WDP bring into future an automated well control management along with AOS (Automated Operating System) where the well could be controlled by taking the insight provided by the data (downhole pressure) from ASM and thereby mitigating any risks in the event of well control (Israel et al., 2018a). This could be the same with estimation of cuttings bed in the annulus.

As a safe practice in the industry, the following parameters as shown in the figure 4.10 are to be adjusted to get the optimum hole cleaning.

Figure 4.10. Parameters required for optimum hole cleaning(ASA)

a) Flow rate – The hole cleaning effect at higher flow rates will not necessarily be improved. There is also an increased risk of equipment failure due to excessive wear/erosion at higher rates. This it is important to ensure these parameters does not conflict with limitations for top side or downhole equipment and that the flow rate does not cause the ECD to exceed the fracture gradient. (ASA, 22.10.2004, 2006-09-21)

Master Thesis, 2019 82 WDP and ASM: Effect of ECD

Cuttings bed could accumulate in the wellbore that might lead to increased wellbore problems such as pack-offs or stuck pipe during lower completion run in. While drilling, if cuttings accumulate in the annulus, EMW increases which indicates the hole clean must be improved.

WDP along with pressure sensors gives real time EMW. This would signal if the flow rate should be increased for better hole cleaning.

b) Drilling string rotation: As a standard practice, drilling industry use continuous and steady rotation at the maximum available RPM. It is crucial to ensure that the rotation speed does not result in excessive surface torque or downhole vibrations.

WDP and EMS: Effect of torque and load

Any increase in the downhole torque and vibrations could be identified by the EMS present in the downhole tool. Hence any possibility of drill string twist could be easily avoided in highly deviated wells where rotation of the string is mandatory during the operation.

c) Drilling rate: A reduction in drilling rate might be required to achieve efficient hole cleaning. A steady drilling rate is recommended to enable early detection of downhole problems with changing trends in the parameters. This also helps in avoiding slugs of cuttings in the annulus. The figure 4.11 explains the standard drilling rate recommended for good hole cleaning in industry.

WDP and effect of ROP

WDP enabling the new technology of seismic-while-drilling which could forecast the formation ahead of the bit. This will provide an understanding of the optimum ROP that could be applied.

Increased ROP without proper real time formation evaluation might result in tool failure or stuck pipe scenarios. Hence, controlled optimum ROP with WDP network could decrease the unexpected tripping operations and save time. (Grymalyuk et al., 2016)

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Figure 4.11. Drilling rate practice in a section (ASA)

Problem 1:

NPT during WBCO:

In general, wellbore cleaning typically requires a higher time since it is difficult to estimate when the well is completely clean. Due to this, there is a practice of off-bottom circulation for a longer period without completely understanding the bottom hole condition. (ASA, 2006-09-21)The standard practice of CBU is shown below in figure 4.12.

Figure 4.12. General CBU recommendations (ASA)

Master Thesis, 2019 84 Solution 1:

WDP can optimize CBU

The requirement for minimum 3 to 4 times bottoms up is due to the large difference in the way cuttings are carried out of a vertical hole compared to a high angle hole. Cuttings from high angle holes might not appear as continuous flow at the shaker screen but can arrive in batches. Note: Placement of ASM throughout the drillstring at defined lengths will provide the annular pressure at those depths. This helps in understanding the pressure profile regimes along the wellbore annulus i.e. ECD. Based on this ECD estimation, CBU can be determined and hence wellbore cleanout time can be evaluated more precisely.