Page 234 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

Formation Evaluation 203

Devices that measure water-holdup are also available. These logs can be run
singly or in combination on a production combination tool so that a number
of parameters may be recorded on the same log sheet.
Temperature Log. A thermometer is used to log temperature anomalies produced
by the flow or fluid inside the casing or in the casing annulus. It is used to
help determine flowrates and points of fluid entry or exit, and is, perhaps, most
useful for finding fluid movement behind the casing.
injection Wells. Figure 5-125 is the response of the temperature log when fluid
is being injected into a reservoir. The sloping portion defines the geothermal
gradient the vertical portion defines the zone taking the water and is a function
of the geothermal gradient as well as the injection fluid temperature. Below
the sloping position, the temperature/curve rapidly returns to normal formation
temperature and the geothermal gradient. The vertical portion of the log clearly
indicates where the fluid is leaving the casing.
Production Wells. Figure 5-126 is the response of the temperature log when
fluid is flowing into a well from perforations in the casing. Three curves are
presented. This figure shows that curve response depends on whether the fluid
produced is hotter, the same as, or cooler than the geothermal gradient. If the
fluid is hotter or cooler, then the entry point is obvious. If the fluid temperature
is the same as the geothermal gradient, the change is so subtle that recognition
of the entry point may be very difficult. In this case, a high resolution thermo-
meter may be necessary to pinpoint the fluid entry location.

Flow Behind Casing (Annular Flow). Figure 5-127 is a typical response to annular
flow down the outside of the casing in a shut-in well. The figure shows water
entering the annulus at about 6,500 ft. Perforations are at -8,500 ft.
In a producing well, the shape of the curve defines the top of the annular
space and its relationship to the perforations.
Manometers and Gradiomanometers. Manometers are pressure-sensitive devices
used to measure changes in pressure that result from:

1. Leaks in tubing or casing.
2. Fluid inflow through perforations.
3. Gradient measurements in a static mud column.
They are particularly useful for determining pressure opposite a gas-bearing
horizon. This value is vital for calculating open-flow potentials in gas wells.
Gradiomanometers are used to check the difference in pressure over a 2-in.
interval in a producing well. This is then related to water-holdup in polyphase
fluid flow within the casing.
The pressure difference is converted to density and is used to interpret two-
phase flow (usually consisting of water as the heavy component and oil as the
lighter component). At any given level, the gradiomanometer measures the
specific gravity (density) of any fluids entering the borehole. The log reading
is related to water holdup and specific gravity by:
(5-1 22)
where ~gradiom.nomeler = specific gravity reading of the gradiomanometer, g/cc
p, = specific gravity of the formation water, g/cc
p, = specific gravity of the oil being produced with the water,
idcc
Y, = water holdup (or holdup of the heavy phase)