ANÁLISIS ESTADÍSTICOS

The effects of production rate, oil viscosity, horizontal well length, vertical landing of well from the GOC, vertical permeability and anisotropy ratio on recovery, water cut and reservoir pressure are discussed below. The variations of water breakthrough time and gas breakthrough time with these parameters are also outlined.

o Production rate

The Figure 4.1, 4.2 and 4.3 below shows the effect of production rate on recovery, water cut and reservoir pressure in the oil rim reservoir.

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Figure 4.1: Effect of liquid flow rate on cumulative production (water cut limit = 0.935).

For the reservoir under consideration, the liquid production rate selected was

2250STB/D. This rate minimized water cut and maximized recovery from the system for the slated reservoir depletion period (see figure 4.1 and 4.2).

The total oil recovered after the 31.5year period when operating at 2250STB/D was about 9.2MMSTB which represents a recovery factor of about 34%.

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Figure 4.2: Effect of liquid flow rate on water cut progression

Figure 4.3: Effect of liquid flow rate on reservoir pressure

27 o Horizontal Well Length

Figures 4.4 through 4.6 show the results of varying the horizontal well length during the parametric study.

Figure 4.4: Effect of horizontal well length on cumulative recovery of oil (water cut limit of 0.935)

An increase in the horizontal well length results in an increase in cumulative production and a decrease in pressure drop for the same liquid rate in the system considered. This is due to reduced friction to flow as a result of reduced flow distance of fluids from reservoir to line sink (wellbore). The reduced pressure drop results in reduced water cut and hence increases recovery.

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Figure 4.5: Effect of horizontal well length on water cut progression

Figure 4.6: Effect of horizontal well length on reservoir pressure.

29 o Viscosity of Oil

Oil viscosity was also varied to evaluate its impact on oil recovery and coning tendencies.

Figures 4.7 through 4.9 show the results of the variation of oil viscosity during the parametric study.

Figure 4.7: Effect of oil viscosity on cumulative recovery of oil (water cut limit = 0.935).

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Figure 4.8: Effect of oil viscosity on water cut progression

Figure 4.9: Effect of oil viscosity on reservoir pressure.

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Oil viscosity (oil mobility) has a profound effect on oil recovery and water breakthrough as shown in figure (4.7-4.9). An increase in oil mobility (decrease in oil viscosity) resulted in an increase in recovery of oil and a decrease in water cut progression with time.

o Vertical offset of well from the GOC.

Figures 4.10, 4.11 and 4.12 show the effect of variation of vertical landing of well from the GOC on the recovery, water cut and reservoir pressure.

Figure 4.10: Variation of total oil recovered with time for oil rim reservoirs with varying well landing position.

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Figure 4.11: Variation of water cut with time for oil rim reservoirs with varying well landing position

Figure 4.12: Variation of reservoir pressure with time for an oil rim reservoir with varying well landing position

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An increase in the distance of the well from the GOC (i.e., the well is moving closer to the oil-water contact) results in an increase in water cut. Recovery however is maximum at the horizontal well’s landing of 44ft from the GOC. This position therefore represents the optimum position for production amongst well landing positions considered.

o Formation Vertical Permeability

The results showing the effect of varying the vertical permeability are displayed in Figures 4.13, 4.14 and 4.15.

Figure 4.13: Effect of vertical permeability on recovery of oil

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Figure 4.14: Effect of vertical permeability on water cut progression

Figure 4.15: Effect of vertical permeability on reservoir pressure

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Note, the results show that an increase in vertical permeability resulted in decrease in cumulative oil recovery. Increase in vertical permeability also resulted in reduced pressure drawdown for production at the same liquid rate. This is due to the relatively ease for fluid to flow in a system with higher vertical permeability. However, a more permeable system eases the pathway for water to cone. The severity of water coning increased with increasing vertical permeability at the initial stage of production. As production progressed, this trend reversed. This is due to the effect of pressure drawdown in the system which became dominant (see Figure 4.13-4.15).

o Vertical Anisotropy ratio

Figures 4.16, 4.17 and 4.18 show how variation in anisotropy ratio affects recovery, pressure drop and water cut in oil rim reservoirs. An increase in anisotropy ratio increases the tendency of gas and oil to cone and decreases oil recovery

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Figure 4.16: Effect of vertical anisotropy ratio on recovery of oil

Figure 4.17: Effect of vertical anisotropy ratio on water cut progression

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Figure 4.18: Effect of vertical anisotropy ratio on reservoir pressure

Water and Gas Breakthrough Time

Figures 4.19, 4.20 and 4.21 show the variation of breakthrough time (water and gas) with horizontal well length, well landing position from the GOC and oil viscosity respectively.

An increase in horizontal well length results in an increase in breakthrough time for gas and water as shown in Figure 4.19. Increasing the horizontal well length therefore increases breakthrough time which is desired.

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Figure 4.20 shows that an increase in horizontal well landing position from the GOC results to an increase in water breakthrough time and decrease in gas breakthrough time.

This means that the closer a horizontal well is positioned to the contacts, the shorter is the breakthrough time and vice versa.

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or/and gas breakthrough as the mobility ratios (water-to-oil and gas-to-oil) will favour preferential flow of water (water breakthrough) and flow of gas (gas breakthrough).