Operational phase

Flowrate and RPM (rotations per minute)

The first action taken when ECD’s are an issue is usually to reduce the flowrate within the hole cleaning limitations of the drilling “system.” The minimum acceptable flowrate is a function of various parameters, like mud rheology, rpm, slide frequency, hole size, ROP [1].

ECD’s are increased for long wells when the pipe rotation exceeds a certain limit due to the increased distance that the fluid must travel to surface due to a spiraling flow path when the pipe rotation is too high. Reducing the pipe rpm may therefore help reducing ECD’s [1], [9].

In horizontal wells it is a good practice to slowly increase the flowrate from a low speed to the maximum, instead of breaking circulation at the planned drilling flowrate since some mud systems may gel up during static conditions [1]. When the mud is static and gelled, circulation will create large ECD spikes. Breaking circulation means getting the drilling fluid into

movement (circulation) after a period of static conditions (e.g. taking a survey or making a connection) by starting the pumps or the rotary [86]. Increasing the flowrate gradually will help to break down the mud’s gel strength and reduce the surge effects when the pumps are started. This will ensure a minimal effect on ECD and cuttings loading [1].

92 ROP

The amount of cuttings in the hole is directly associated with the downhole annular pressure [1]. The cuttings weight adds weight to the fluid, so the more cuttings located in the hole, the higher is the bottom-hole pressure. Controlling the ROP may therefore help reducing ECD’s.

Slide drilling practices

Slide drilling can result in the build-up of a cuttings dune right above the BHA, and lead to increased ECD’s if the cuttings dune is interrupted [1].

Back-reaming

According to [9] back-reaming and/or pumping out (no rotation) should be avoided when possible in high-angle wells because it is the single-most dangerous operation in an ERW.

The result may be plugging the wellbore around the drillstring (pack-off) or collapse of the wellbore wall around the drillstring. It also leads to a maximum risk of stuck pipe, BHA equipment failures, key seating, lost returns and of destabilizing the wellbore [9].

Back-reaming is to be avoided if possible since it increases the risk of cuttings dune to form and therefore increase the bottom-hole pressure/pack-off if the dune is suddenly interrupted (which again may lead to increased ECD’s).

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Figure 45: Standard tripping vs. back-reaming [9].

Down-reaming

Down-reaming means enlarging the wellbore because the hole was drilled to small or to move the pipe in order to clean the drilled hole (with both rotation and circulation) [52], [87].

Down-reaming may have a negative impact on ECD, may deteriorate the well and may

increase the risk of packing off [1], [9]. As for back-reaming, down-reaming increases the risk of cuttings dune to form and therefore increase the bottom-hole pressure/pack-off if the dune is suddenly interrupted (which again may lead to increased ECD’s).The best way to avoid down-reaming is to make sure that the well is as clean as possible before tripping [1].

94 2.5.7.6 ECD drivers

The various ECD drivers have been discussed above. Figure 46 below shows the different drivers and their contribution with varying hole sizes. Managing the ECD is basically all about give and take priorities; in order to improve a critical ECD situation one must either increase the hole size or decrease the section depth (either solution will reduce the ECD).

Figure 46: ECD drivers – what drives the ECD will be different for every single hole size [9].

95 2.6 Hole cleaning

One of the most essential challenges found in an ERD well is hole cleaning. Hole cleaning is the capability of a drilling fluid to suspend the cuttings of drilled rock and transport them from the wellbore to the surface [37]. In “the good old days,” when the drilling process was less scientific and technical, a driller’s main goal was to drill the hole as fast as possible [14].

Today there is hundreds of parameters and sensors continuously feeding us with important information that can be used to analyze the hole conditions [14]. Both the flowrate and the drilling rate must be kept within certain limits in order to ensure good hole cleaning [2]. Hole cleaning is often misunderstood and according to [9]; “We often don’t rotate fast enough, we rarely circulate long enough and there are a lot of misconceptions about sweeps, wiper trips and mud properties [9].”

In general, for high angle wells, cuttings will fall to the low side of the hole and away from the primary fluid flow at the top of the hole [1]. When the cuttings are transported to the surface, they also have a tendency to sink due to gravity forces [2]. This phenomenon makes the cutting removal process very difficult and requires special techniques for different well inclinations. The drilling parameters, BHA and bit design, mud rheology and the observed hole conditions will all contribute to the rig system’s capability to clean the hole in an efficient and safe manner. Mims and Krepp [1] state that high flowrates and pipe RPM throughout the entire drilling process will ensure a robust and efficient hole cleaning.

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Figure 47: An illustration of the wellbore cross section with cuttings bed showing the basic flow configuration for cutting transport modeling. The critical flow rate for cutting transport does not affect the cuttings bed [61]. In order to obtain an effective hole cleaning, the desired

flowrate must exceed this critical flowrate. Modified after [61].

It is ideal to transport the drilled cuttings out of the wellbore as fast as possible. If the cuttings accumulate, it may lead to an increase in the bottom-hole pressure, which again may lead to stuck pipe or circulation losses [2]. In order to obtain an effective drilling operation it is very important to ensure efficient cuttings transport and hole cleaning. Insufficient hole cleaning may cause costly drilling problems, like [37]:

• Mechanical pipe sticking

• Premature bit wear

• Slow drilling

• Formation fracturing

• Excessive torque and drag on drill string

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• Difficulties in logging and cementing

• Difficulties in casing landing

The most common problem regarding high-angle/extended-reach drilling is excessive torque and drag on drillstring, which often leads to the inability of reaching the desired/target depth [37].

Figure 48: Hole cleaning in a vertical well – drilling mud charging and cuttings upward transportation [11].

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The following is essential in order to achieve successful hole cleaning in ER wells according to [9]:

• High flowrate (say > 1000 gpm (3800 lpm) in a 12¼“ hole)

• Gauge hole

• Continuous rotation, and RSS (rotary steerable system) is a necessary requirement for optimal hole cleaning

• Slow ROP

• Ideal mud properties

• Sweeps compensate for less-than-ideal of the above

Hole cleaning efficiency is affected by the following factors [9], [14]:

• Rotary speed*

• Turbulent or laminar flow

• Cuttings size

• Mud solids (colloidal)

• Cuttings dispersion

*The bold ones are the most important factors – all of them can be controlled.

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