Flow types of oil in reservoir

1. NUMBER OF FLOWING FLUIDS IN THE RESERVOIR
The mathematical expressions that are used to predict the
volumetric performance and pressure behavior of the reservoir
vary in forms and complexity depending upon the number of
mobile fluids in the reservoir.
There are generally three cases of flowing systems:
• Single-phase flow (oil, water, or gas)
• Two-phase flow (oil-water, oil-gas, or gas-water)
• Three-phase flow (oil, water, and gas)

2. RESERVOIR GEOMETRY
The shape of a reservoir has a significant effect on its flow behavior.
Most reservoirs have irregular boundaries and a rigorous mathematical
description of geometry is often possible only with the use of numerical
simulators.
For many engineering purposes, however, the actual flow geometry may be
represented by one of the following flow geometries:
• Radial flow
• Linear flow
• Spherical and hemispherical flow

3. Radial Flow
In the absence of severe reservoir heterogeneities, flow into or away from a
wellbore will follow radial flow lines from a substantial distance from the
wellbore.
Because fluids move toward the well from all directions and coverage at the
wellbore, the term radial flow is given to characterize the flow of fluid into the
wellbore.
Figure shows idealized
flow lines and iso-potential
lines for a radial flow system.

4. Linear Flow
Linear flow occurs when flow paths are parallel and the fluid flows in a single
direction.
In addition, the cross sectional area to flow must be constant.
Figure shows an idealized linear flow system.
A common application of linear flow equations is the fluid flow into vertical
hydraulic fractures as illustrated in Figure.

5. Spherical and Hemispherical Flow
Depending upon the type of wellbore completion configuration, it is possible
to have a spherical or hemispherical flow near the wellbore.
A well with a limited perforated interval could result in spherical flow in the
vicinity of the perforations as illustrated in Figure.
Figure Spherical flow due to limited entry.

6. A well that only partially penetrates the pay zone, as shown in Figure, could
result in hemispherical flow.
The condition could arise where coning of bottom water is important.
Figure. Hemispherical flow in a partially penetrating well

7. Types of Fluids
The isothermal compressibility coefficient is essentially the controlling factor in
identifying the type of the reservoir fluid. In general, reservoir fluids are
classified into three groups:
(1) incompressible fluids;
(2) slightly compressible fluids;
(3) compressible fluids.
The isothermal compressibility coefficient is described mathematically by the
following two equivalent expressions:
In terms of fluid volume:
Eq. (1)
Eq. (2)

8. Incompressible Fluids
An incompressible fluid is a fluid whose volume or density does not change
with pressure. That is:
Incompressible fluids do not exist; however, this behavior may be assumed in
some cases to simplify the derivation and the final form of many flow
equations.

9. Slightly Compressible Fluids
These “slightly” compressible fluids exhibit small changes in volume, or
density, with changes in pressure.
Knowing the volume (Vref) of a slightly compressible liquid at a reference
(initial) pressure (pref), the changes in the volumetric behavior of such fluids
as a function of pressure (P) can be mathematically described by integrating
Eq. (1), to give:
It should be pointed out that many crude oil and water systems fit into this
category.
Eq. (3)

10. Linear Flow of Slightly Compressible Fluids
Selecting the upstream
pressure P1 as the reference
pressure (Pref )and the flow
rate at Point 1 as:
Choosing the downstream
pressure P2 as the reference
pressure and the flow rate at
point2 as:
q1=Flow rate at point1, bbl/day
q2=Flow rate at point2, bbl/day
P1 = upstream pressure, psi
P2 = downstream pressure, psi
K = permeability, md
μ = viscosity, cp
c = average liquid compressibility, 1/psi

11. Example:
Consider the linear system given in the data and, assuming a
slightly compressible liquid, calculate the flow rate at both ends
of the linear system. The liquid has an average compressibility of
21X10^-5 (1/psi).

13. Compressible Fluids
Compressible fluids are defined as fluids that experience large changes in volume as
a function of pressure.
All gases and gas-liquid systems are considered compressible fluids.
The isothermal compressibility of any vapor phase fluid is described by the
following expression:
Figures 1 and Figure 2 show schematic illustrations of the volume and density
changes as a function of pressure for all three types of fluids.

14. FIGURE 1. Pressure volume relationship

15. FIGURE 2. Fluid density vs. pressure for different fluid types.

16. Linear Flow of Compressible Fluids (Gases)
For a viscous (laminar) gas flow in a homogeneous-linear system, the real-gas
equation-of-state can be applied to calculate the number of gas moles n at
pressure p, temperature T, and volume V:

17. It is essential to notice that those gas properties Compressibility Factor and Gas
Viscosity are a very strong function of pressure, but they have been removed
from the integral to simplify the final form of the gas flow equation.
The above equation is valid for applications when the pressure < 2000 psi.
The gas properties must be evaluated at the average pressure as defined below.

18. Example

19. Solution
Step 1. Calculate average pressure by using Equation
Step 2. Calculate the pseudo-
reduced pressure and temperature.

20. Step 4. Calculate the gas flow rate by applying Equation
Step 3. Determine the z-factor from the Standing-Katz chart to give:
Z= 0.78

21. Standing and Katz
compressibility factors
chart

22. Radial Flow of Incompressible Fluids
In a radial flow system, all fluids move toward the producing well from all
directions.
Before flow can take place, however, a pressure differential must exist.
Thus, if a well is to produce oil, which implies a flow of fluids through the
formation to the wellbore, the pressure in the formation at the wellbore
must be less than the pressure in the formation at some distance from the
well.
According to the Darcy’s equation used to determine the flow rate at any
radius r:
Eq. (1)

23. Consider Figure A, which schematically illustrates the radial flow of an
incompressible fluid toward a vertical well.
 The formation is considered to a uniform thickness (h) and a constant
permeability (k).
 Because the fluid is incompressible, the flow rate (q) must be constant
at all radii.
 Due to the steady-state flowing condition, the pressure profile around
the wellbore is maintained constant with time.
 Let (Pwf) represent the maintained bottom-hole flowing pressure at
the wellbore radius (rw) and (Pe) denote the external pressure at the
external or drainage radius.

24. Figure A. Radial flow model

25. Equation (1) can be written as:
Put (q) in Eq. (2)
OR
But
Eq. (2)
Eq. (3)

26. For incompressible system in a uniform formation, Equation (4) can be
simplified to:
Eq. (4)

27. Eq. (5)

28. The external (drainage) radius (re) is usually determined from the well
spacing by equating the area of the well spacing with that of a circle, i.e.,
where A is the well spacing in acres.
Equation (5) can be arranged to solve for the pressure P at any radius r to
give:

29. Example
An oil well in the Nameless Field is producing at a stabilized rate of 600
STB/day at a stabilized bottom-hole flowing pressure of 1800 psi.
Analysis of the pressure buildup test data indicates that the pay zone is
characterized by a permeability of 120 md and a uniform thickness of 25 ft.
The well drains an area of approximately 40 acres. The following additional
data is available:

30. Solution
Step 1. Solve for the pressure P at radius r.

31. Step 2. Calculate the pressure at the designated radii.

32. Use Linear Graph to generate the pressure profile on a function of radius
for the calculated data.

33. Results of the above example reveal that the pressure drop just
around the wellbore (i.e., 142 psi) is 7.5 times greater than at
the 4–5 ft interval, 36 times greater than at 19–20 ft, and 142
times greater than that at the 99–100 ft interval.
The reason for this large pressure drop around the wellbore is
that the fluid is flowing in from a large drainage of 40 acres.

34. Radial Flow of Slightly Compressible Fluids
The following equation can be used to calculate the flow rate for Radial Flow
of Slightly Compressible Fluids.

35. Example
The following data are available on a well in the Red River Field:
Assuming a slightly compressible fluid, calculate the oil flow
rate. Compare the result with that of incompressible fluid.

36. Solution
For a slightly compressible fluid, the oil flow rate can be calculated by
applying Equation
Assuming an incompressible fluid, the flow rate can be estimated by
applying Darcy’s equation, i.e., Equation

37. Radial Flow of Compressible Gases
For a radial gas flow, the Darcy’s equation takes the form:
The above approximation method is called the pressure-squared method and is limited
to flow calculations when the reservoir pressure is less that 2000 psi.

38. Example
The well is producing at a stabilized bottom-hole flowing
pressure of 3600 psi. The wellbore radius is 0.25ft. The
following additional data are available:
Calculate the gas flow rate in Mscf/day.

39. Solution
Apply Equation