Page 112 - Introduction to Petroleum Engineering

ACTIVITIES 97

r = drainage radius (ft)
e
r = wellbore radius (ft)
w
S = dimensionless skin
Effective permeability in Equation 5.18 incorporates relative permeability into
the calculation of flow rate and PI. Skin S is an indicator of formation damage in the
vicinity of the well. The well is damaged if S > 0 and stimulated if S < 0. Skin is
discussed in more detail in Chapter 12.
Productivity index is positive for both an injection well and a production well. We
can verify our sign conventions for pressure differential and flow rate by noting that
a production well has a positive pressure differential (∆p > 0) and a positive flow rate
(q > 0); hence the productivity index in Equation 5.18 is positive. Similarly, an

injection well has a negative pressure differential (∆p < 0) and a negative flow rate
(q < 0), so once again the productivity index in Equation 5.18 is positive.


Example 5.5 Drainage Radius
Drainage radius can be estimated using well spacing, which is the area associ-
ated with a production well. For example, 16 equally spaced wells in a square
mile occupy an area of 40 acres. The well spacing is called 40‐acre spacing. We
can estimate the drainage radius by approximating the area as a circular area.
What is the drainage area of a well in a well pattern with 40‐acre spacing?

Answer
2
The drainage area A assigned to the well is 40 acres ≈ 1742400ft .
d
A 1742700
Drainage radius for a circular area is r ≈ π d = π ≈ 745 ft.
e

5.7 ACTIVITIES

5.7.1 Further Reading
For more information about rock properties, see Dandekar (2013), Mavko et al.
(2009), and Batzle (2006).

5.7.2 True/False
5.1 The oil–water contact is usually found below the gas–oil contact in a reservoir
with mobile gas, oil, and water.

5.2 Water cut is water production rate at the surface divided by the sum of oil
production rate and water production rate at the surface.
5.3 A positive skin represents stimulation.