The drill cutting removal has been investigated for decades in order to deal with the hole cleaning problems. The cuttings which are drilled out from the deep formation of the well are normally the most challenging one to transport. The inclination of the well makes it even
more challenging in order to clean out the wellbore and mitigate occurrence of stuck pipe situation. [37]
It is also proven the inefficient removal of small sized cutting are the main reason behind the excessive torque and drag. The process of cuttings transport is influenced by many factors such as forces which are acting on the cuttings. These forces determine the mechanism of cuttings to become transported, deposited or suspended. The hydrodynamic forces, static forces and colloidal forces are those which act on cuttings in the annulus. Annulus is the most critical section of the well due to limitation of pumping capacity and high pressure drops through the drill pipe and the drill bit.
The characterization of the drilling fluids is directly related to their ability for cleaning the wellbore. However the properties of the cutting and operational parameters are also factors which in addition of the drilling fluid properties play a key role to ensure a perfectly cuttings free wellbore. The cleaning process is explained by defining two definitions. The critical suspension velocity (CRV) is the minimum flow velocity for initiation of bed erosion and the critical deposition velocity (CDV) which is the minimum flow velocity to prevent bed formation. CRV and CDV are function of the necessary flow rate to clean the wellbore[37].
In this section we are going to investigate the cutting transport characterization of reference system and Nano-treated drilling fluids by holding the operational parameters and cutting size density constant. The cutting transport simulation is performed in a real well geometry having vertical section, bend and inclined section.
5.1.1 Simulation setup
The well depth was designed to be 11003 ft and the size of casing and the open borehole are 12,615 “and 12,250 “respectively. The drill pipe outer diameter (OD) is 5” and the bottom hole assembly (BHA) are also included into the design parameters. The detailed data regarding to the size of drill pipe, BHA and casing are presented in the appendix F. The
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76 simulation was performed by using the Well-Plan Software. The well schematic designed for simulation is shown at the figure 62. Operation parameters are presented in table 20.
Table 19 Operation parameters for the cuttings transport simulation
Cuttings
Figure 61 Schematic diagram of the designed well for cuttings transport simulation 5.1.2 Drilling fluids
The density of the drilling fluids was 1.02sg and the rheology data are given in table 21.
The reference mud system was formulated by mixing the 0.2gm low viscous CMC and 0.3gm XC in 25g Bentonite/500gm H2O with 2.5kcl. The test matrix is presented in table 20.
Table 20 Test matrix for the mud systems used for cuttings transport simulation Mud system Bentonite H2O LV CMC Xanthan KCl Nano silica
Ref #1 25 g 500 ml 0.2 g 0.3 g 2.5 g 0.0
#2 25 g 500 ml 0.2 g 0.3 g 2.5 g 0.2
#3 25 g 500 ml 0.2 g 0.3 g 2.5 g 0.25
#4 25 g 500 ml 0.2 g 0.3 g 2.5 g 0.3
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77 The rheological data in table 21 were used in order to perform the simulation for the cutting transport. The objective is to compare the system and determine whether the Nano treated drilling fluid provide better hole cleaning than the reference system or not.
Table 21 Rheological data for the mud systems used for cuttings transport simulation
5.1.3 Simulation result and discussion
The Cutting transport simulation was performed through the height of the cutting bed given a certain rate of penetration (ROP) and pump pressure. The removal of the drill cutting is critical at inclined section and the designed well has an inclination of almost 40 o at its deposition by 150 ft and prevention of cutting bed is not an alternative. However the system which had the best performance with no deposition of cutting down to 10500 ft. MD was the 0.25 Nano-treated systems. The most critical part is the section with 38 o degrees where 1 inch of cuttings bed deposition is expected.
For this simulation, we used a pump rate of 500GPM in order to compare the performance of the drilling fluid. Figure 47 shows the minimum flow rate required to completely transport cutting out of the well. The selected fluid with 0.25 g Nano silica provides the completely clean well with the minimum flow rate of 537 GPM. This is significantly lower than the flow rate required using three other mud systems. The reduction is 41% compared to the reference
RPM Ref #1 0.2 Nano #2 0.25 Nano #3 0.3 Nano #4
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78 system which will require a lower pumping capacity. One can notice that all the Nano-treated system have higher performance than the reference system.
Figure 62 Well inclination and bed height for simulated drilling fluids
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79 Figure 63 Comparison of minimum flow rate to transport all cuttings for simulated drilling
fluids 5.1.4 Minimum flow rate
Another comparison of the minimum flow rate was performed based on the different well inclination. The simulation result is presented at figure 65. As it can be observed the most challenging inclination for the samples containing Nano silica are the horizontal section where the inclination exceeds the 85o. However the reference system demands only a flow rate of 560 gmp to ensure cutting transport for section with inclination higher than 50o. According to Torbjørnsen et al. (1994) the inclination between 40o-60o is the most difficult section in order to transport the cuttings. The reference system is
The operational parameters are presented in table 22.
Table 22 Operation parameters for the cuttings transport simulation
Cuttings
Minimum flow ratre, gpm 915,6 741,6 536,8 659,7
0
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80 .
Figure 64 Comparison of minimum flow rate necessary to transport all cutting in different hole angles for simulated mud systems