Date: I2/ 11 / 2o H - UTPedia - Universiti Teknologi PETRONAS (UTP)

It is found that the ratio of the diameter of the coiled pipe to the cased bore has the most significant effect on the annulus filler concentration and pressure drop. One of the most common bulk removal processes is tubing runoff (CT) and circulation of solids with transfer fluids. Coiled tubing is considered one of the most efficient and cost-effective methods of filling removal in the industry.

Downhole packing removal is one of the most frequently used and yet challenging problems in coiled tubing units for downhole cleanup. In this situation, there is a tendency to deposit a solid bed in the lower part of the annulus. Presented in Figure 1.5 is a common equipment configuration for use in foam deburring.

Increase in the sticking potential of the spiral tube due to the sedimentation of the solid particles in the annulus.

Figure 1.1: Production rate after fill removal from vertical well in USA [1]

Numerical Analysis of foam-solid flow using ANSYS-CFX®

Quality offoam

They described the composition of the foam at each temperature and pressure using the Liquid Volume Fraction (LVF). The distribution of the bubbles and their size in the foam are described by its texture. The continuous liquid phase turns into a discontinuous situation when the foam quality exceeds a certain threshold level; and results in the formation of fog.

10] showed that the foam became unstable at liquid volume fractions of 0.02 to 0.03 and that when the quality exceeded 98%, the foam flowed as occasional foam particles and. The conclusion was that the apparent viscosity of the foam increases with increasing foam quality.

Effects oftemperature and pressure

The upper limit for stable foam remains to be determined, but is certainly a function of the rate of shear. Russell [13] noted that good bubble stability was generated by surfactant solutions with a quality of 99.1% foam without polymer, which is known to be stable. 2.6) m is the mass, M is the molecular weight of the gas, P is the absolute pressure, R is the gas constant, SVL is the specific volume of the liquid, svs is the specific volume of the solid, T is the absolute temperature , wG is the mass fraction of gas, WL is the mass fraction of liquid, ws is the mass fraction of solid and Z is the gas compressibility factor.

This model requires numerical solution if flow pressures are to be predicted and the friction factor is to be chosen carefully. Alexander and Ali [15] mentioned an example of the change in density caused by temperature for a given well condition as shown in Figure 2.2.

Figure 2.1: Effect of Bottom hole temperature on critical foam velocity [8]

Effect ofshear rate

For the prediction of friction losses due to the solid phase in the slurry flow of solid foam, a semi-empirical model has been presented by Okpobiri and Ikoku [19]. Furthermore, at low flow rates there was a more pronounced increase in clippings transport efficiency. It was found that a change in curvature ratio up to on the order of 0.002 has no significant effect on the pressure drop.

Many studies have been conducted in the past into cutting transport with foam. The volume of cuttings collected in the annulus showed a strong sensitivity to the fluid flow rate. This study also developed a mechanistic model for aerated fluid transport of the cuttings under EPET conditions; it was developed for predicting frictional pressure loss and the concentration of the cuttings in the annulus.

They noted the highest cutting transport when inner pipe was fully concentric as shown in Figure 2.10 which cannot be practically achieved. They concluded that the particle carrier transport efficiency increased with an increase in the quality of the foam. Improvement in the cutting transport was noted due to increase in the effective viscosity of the foam.

In calculating all the forces, many fluid variables are needed, such as density, viscosity, and velocity at the particle's position. The drag force, Fd, acts in the direction of the slip velocity (vs =vf —vp) between the liquid and solid particle. In the present study, the torsional force is included to validate the numerical model with experimental data.

The rotational force causes the rotation of the fluid domain, which helps in the suspension of the solid particles. In the present study, the velocity of the foam at the wall boundary and the wall shear stress are set to zero, therefore the boundary condition at .

Figure 2.3: Simulated frictional pressure gradient of water in \y2 tubing for different curvature rations [29]

Percentage reduction in fill concentration

There is a significant decrease in filler concentration when the foam quality is less than 80%, however, this decrease becomes almost negligible above 80%> foam quality. For example, at a constant diameter ratio of 0.50, there is a 30% decrease in filler concentration as foam quality increases from 70 to 80%. This change in density has a greater effect on the sliding speed of the cuttings than on the viscosity.

An increase in cuttings slip velocity will therefore slow the removal of filler concentration as foam quality increases; an occurrence that can be observed as foam quality increases from 80 to 90% in Figures 4.9 (a) to 4.9 (d).

Slope reduction in fill concentration

The fill gradient becomes twice the gradient at low quality when the particle size is 2 mm. The average fill gradient is 0.47 at low foam quality and increases to 0.18 when foam quality increases from 80 to 90%. Similarly, Figure 4.9 (d) shows the decrease in fill percentage for 3 mm particle size when foam quality varies from 70 to 90%.

The average fill gradient is 0.51 at low foam quality and it reduces to 0.24 when foam quality increases from 80 to 90%. 17] also found in their study that low foam quality is more effective in removing solid particles compared to high foam quality. There is insignificant change in the filling gradient for the small particle size particles ranging from 0.5 to 1 mm.

As the particle size increases from 1 mm, then there is a noticeable change in the gradient. It can be observed that as the particle size increases to 3 mm, the filling gradient in low foam quality becomes twice as compared to high foam quality. In all studies, a decreasing trend is observed for packing concentration with increasing fluid velocity for all CT/Annulus diameter ratios and packing sizes.

The relationship between the concentration of the filling with a size from 0.5 to 3 mm and the velocity of the foam at each diameter ratio is presented in this section. The effect of foam rate on fill concentration is discussed in two different ways as shown below;

Figure 4.9 (c) shows the percentage decrease in fill concentration at increasing foam quality from 70 to 90% at constant velocity of 6-ft/sec

Percentage reduction in fill concentration

For larger particle sizes, such as 2 mm and 3 mm, there is an almost constant percentage decrease in filler concentration as the foam velocity increases from 3 ft/sec. to 6 ft/sec. for all diameter ratios as shown in Figures 4.11 (c) and 4.11 (d).

Slope Reduction in Jill concentration

Figure 4.11 (b) shows the 1 mm infill concentration in the annulus for different foam velocities at 90% quality foam and CT/annulus diameter ratios. It can be seen from the graph that the filler concentration decreases significantly as the foam velocity increases from 3 to 5 ft/s. As the velocity increases from 5 to 6 ft/s, there is also no significant change in filler concentration.

Fill gradient around 2 is calculated at low foam rate and it drops to 0.2 at high foam rate when the fill size is 1 mm. For filler sizes of 2 mm, as the foam velocity is increased from 3 to 5 ft/sec, the filler concentration drops significantly to 9%; but when the foam velocity is further increased from 5 to 6 ft/sec, the filler concentration in the annulus is reduced to 7%. There is a higher fill gradient around 3 at low foam rate and it drops to 1.6 at high foam rate when the fill size is 2 mm.

Similarly, Figure 4.11 (d) shows a filler concentration of 3 mm particle size in the annulus for varying foam rates at 90% quality foam and CT/Rindring diameter ratio. Fill gradient around 4.7 is observed at low foam velocity and it decreases to 2.7 as the foam velocity increases from 5 to 6 ft/sec. It can be noted that as the foam velocity is increased from 5 to 6 ft/sec, the drop in fill gradient is nearly half that of foam flowing at 3 to 5 ft/sec.

It can be seen that there is a greater reduction in filler concentration at low foam velocities of about 3 to 5 ft/s. For small particles of about 1 mm and less, there is an insignificant change in the charge gradient at high foam velocity and a marked change in concentration is observed as the particle size increases from 1 mm. In all cases, there is a significant increase in pressure drop as foam quality increases for a constant ratio of diameter to foam velocity.

Figure 4.11 (c) shows the percentage decrease in fill concentration of 2 mm particle size at increasing foam velocity from 3 to 6-ft/sec with 90% quality foam.

Percentage increment inpressure drop

Slope increment inpressure drop

Figure 4.13 (c) mentioned above also shows the pressure drop as a function of quality at 5-ft/sec foam velocity and various diameter ratios. In all cases, an increase in foam velocity from 3-ft/sec to 6-ft/sec results in a dramatic increase in pressure drop at any constant diameter ratio. In the present study, a linear trend for pressure drop with increasing particle size is observed for all CT/Annulus diameter ratios.

The relationship between the annular pressure drop and the filling size is shown in Figure 4.16. Current study shows that CT/Annulus diameter ratio has the most significant effect on the pressure drop. It can also be observed that foam velocity has significantly more effect on pressure drop compared to foam velocity.

As accepted, the diameter ratio is found to have a significant effect on the pressure drop. The results show that there is a linear increase in pressure drop with increasing foam velocity. The effect of particle size on pressure drop was also calculated, and it was found that particle size had no effect on pressure drop.

The pressure drop in the annulus increases with the increase in foam rate, quality and diameter ratio. It is also observed that the filling particle size has no effect on the annular pressure drop. The present study is carried out by taking the effect of different diameter ratios of CT/Annuls on the filling concentration and pressure drop.