Page 279 - Introduction to Petroleum Engineering

266 PRODUCTION PERFORMANCE
when injected water arrives at a production well. It is observed as a significant
increase in produced WOR or produced WCT. Similarly, the breakthrough of injected
gas is recognized as a significant increase in production GOR.
The recovery factor for dry gas reservoir depletion can be as high as 80%–90% of
original gas in place (Ahmed, 2000). Gas recovery from water‐drive gas reservoir
depletion ranges from 50% to 70% of original gas in place (Ahmed, 2000). It is
typically lower than gas recovery from dry gas reservoir depletion because gas can
be trapped by encroaching water.

Example 13.7 Primary Depletion of an Undersaturated Oil Reservoir
An undersaturated oil reservoir is being produced by primary depletion until
reservoir pressure is just above bubble point pressure. The reservoir did not
have an initial gas cap and is not in communication with any mobile water.
There is no water production. What are the possible drive mechanisms?

Answer
Since there is no initial gas cap, we have I = 0. The reservoir is under primary
gc
depletion; therefore there is no mobile water and no fluids are being injected.
Consequently, we have I = 0 and I = 0. The remaining drive mechanisms are
w
i
solution gas drive and connate water and rock expansion drive. The resulting
drive indices equation is I sg I e 1. It can be used to establish a suitable
material balance equation.

13.7 INFLOW PERFORMANCE RELATIONSHIPS

Wellbore inflow represents fluid flow from the reservoir into the wellbore. Reservoir
fluid flow may be modeled using either analytical methods or numerical methods.
Analytical methods rely on models of inflow performance relationships (IPR). Inflow
refers to fluid entering the production tubing from the reservoir. By contrast, outflow
refers to fluid flowing through the production tubing to surface facilities. An IPR is
the functional relationship between reservoir production rate and bottomhole flowing
pressure. Darcy’s law is a simple example of an IPR for single‐phase liquid flow. The
gas well back pressure equation is an example of an IPR for single‐phase gas flow.
Figure 13.5 illustrates the relationship between an IPR curve and a tubing performance
curve (TPC). The IPR curve is solid and the TPC curve is dashed. TPC curves repre-
sent outflow and can be calculated using models of fluid flow in pipe.
The IPR versus TPC plot is a plot of fluid flow rate q fluid versus bottomhole flow-
ing pressure p . Reservoir pressure p is the pressure at q fluid = 0. The intersection
wf
res
of the IPR and TPC curves identifies the flow rate and bottomhole flowing pressure
that simultaneously satisfy inflow into the well from the reservoir and outflow from
the well. The subscript “op” in the figure designates the operating flow rate q fluid,op at
a particular bottomhole flowing pressure p .
wf,op