Reservoir Fluid Flow and Natural Drive Mechanisms

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Gravity Drainage Mechanism

Conditions Needed for Segregation

Gravity drainage is one of the most efficient recovery mechanisms when conditions are favorable. Under the influence of gravity, water, oil, and gas separate according to their densities. Gravity drainage is a slow process. The rate of recovery from a reservoir influ enced solely by this mechanism is time-dependent, similar to the case of the water drive mechanism.

Gravity drainage is most effective in thick reservoirs with high vertical fluid communication and continuity. It is also effective in thin reservoirs with an appreciable angle of dip (at least 10° to 15°) and a favorable permeability to flow in the vertical direction. Reservoirs with shale stringers or laminations are not good candidates for gravity drainage.

Conditions and parameters needed for effective gravity drainage are indicated by considering the following equation. The rate of segregation of gas in an oil reservoir is

$\left | q_{s} \right |=\dfrac{0.5\times 10^{-3}A\ \Delta \gamma \ \sin \theta }{\dfrac{\mu _{g}}{k_{g}}+\dfrac{\mu _{o}}{k_{o}}}$ …………(52)

where:

$q_{s}$ = rate of gravity segregation in $\tfrac{RB}{D}$ .

$A$ = cross-sectional area of the linear bed in $ft^2$ .

$\Delta \gamma$ = oil-specific gravity minus gas-specific gravity.

$\theta$ = angle of dip in degrees.

$\mu$ = gas viscosity in cp.

$K_{g}$ = gas, effective vertical permeability evaluated at $S_{o} = 1 - S_{wc} - S_{gr}$ , in md.

$\mu _{o}$ = oil viscosity in cp.

$K_{o}$ = oil, effective vertical permeability evaluated at $S_{o} = 1 - S_{wc} - S_{gr}$ , in md.

Equation 52 shows that the factors favorable to gravity segregation are—bullet list

$\Delta \gamma$ , the difference in specific gravity between the oil and the gas. The higher it is, the faster the segregation.
High vertical $k_{o}$ and $k_{g}$ .
Low $\mu _{o}$ .
A high dip angle.
A large cross-sectional area available to segregation.

Recovery Factor

As we stated, gravity drainage is the most efficient drive mechanism. When complete segregation (i.e, full gravity drainage) occurs, recovery may approach

$\dfrac{S_{o}-S_{or}}{S_{o}}$

If the initial oil saturation is 80% and S_or=25%, recovery is 68% of the initial oil in place. In many reef reservoirs where vertical communication is good and the oil viscosity is low it is not uncommon to obtain recovery by gravity segregation in the range of 60%. The main disadvantage of gravity drainage is that it is a slow process. Therefore, one hardly ever takes full advantage of gravity drainage because the oil production rate is normally much higher than the segregation rate.