Page 242 - Introduction to Petroleum Engineering

OIL WELL PRESSURE TRANSIENT TESTING 229
TAbLE 12.1 Types of Pressure Transient Test
Well Type Change in Flow Rate Pressure Transient Test

Production well Decrease Pressure buildup
Increase Pressure drawdown
Injection well Decrease Pressure falloff
Increase Injectivity

associated with rate changes in production and injection wells are summarized in
Table 12.1. Wellbore pressure is lower than formation pressure in production wells.
If the production well is shut in, the pressure in the wellbore will increase.
The corresponding PTT is called a pressure buildup (PBU) test. If the production
flow rate is increased, the wellbore pressure will decrease. In this case, the
corresponding PTT is called a pressure drawdown test.
In the case of injection wells, the wellbore pressure is higher than formation
pressure. If the injection well is shut in, the pressure in the wellbore will decrease,
and the corresponding PTT is called a pressure falloff test. By contrast, an injectivity
test is run by increasing the injection rate and monitoring the associated increase in
wellbore pressure. Injectivity tests can be run to confirm that an injection well can
achieve a desired injection rate.
Many transient well tests are distinguished by the way flow rate is changed.
Flow rate can be changed incrementally or in abrupt steps. A decrease from the exist-
ing flow rate to shut‐in is an abrupt change in rate and occurs in PBU and pressure
falloff tests. On the other hand, incremental changes from one rate to another are
used in gas well deliverability tests. Deliverability tests in gas wells provide
information about the flow capacity of gas wells and are discussed later.
Tests designed to measure the change in pressure at one well as a result of
an incremental change in rate at another well are called pressure pulse tests or inter-
ference tests. In this case, a change in flow rate at one well can induce a change in
pressure at another well if the wells are in pressure communication. Pressure
communication between wells means that a pressure change at one well can be
detected at another well.

12.2 OIL WELL PRESSURE TRANSIENT TESTING

The behavior of a pressure transient induced in the formation by a change in flow rate
is modeled using the diffusivity equation for a single‐phase liquid in a homogeneous
medium. The diffusivity equation in radial coordinates is the partial differential
equation

2 p 1 p p (12.1)
r D 2 r D r D t D