This article was written
"Crain's Petrophysical Handbook" by E. R. (Ross) Crain, P.Eng. 2016.
webpage version is the copyrighted intellectual
property of the author.
Do not copy or distribute in any form without explicit
FORMATION TESTING BASICS
Formation testing on wireline was developed in the mid to
late 1950's to provide a faster and less expensive method of
formation evaluation than conventional drill stem testing
(DST). The earliest formation tester (FT) used a hydraulic
pad to obtain good contact with the borehole wall,
then a perforation charge was fired to create a pathway for
fluids to flow from the reservoir into a chamber at the base
of the tool. This could be done in open or cased holes.
Hydrostatic, shut-in, and flowing pressures are measured and recorded
as the test proceeds. During the flow period, formation
fluids flow into a collection chamber. When the test is completed, the
chamber is sealed and the tool brought back to the surface.
The recovered sample is analyzed in the lab to determine
the fluid properties. Shut-in pressures are plotted versus
depth to determine pressure gradients, gas-oil gas-water,
and oil-water contacts, as well as the location of over- and
Over the years, the tools evolved through many improvements,
for example the formation interval tester (FIT), multiple
dynamic tester (MDT) to name only a couple. Each service
company devised their own tools and trade names. Modern
tools can take multiple samples and allow the operator to
pump fluid from the reservoir instead of relying on natural
flow rates. This permits the tool to bypass the sample
collection chamber until a representative sample is
obtained, reducing the impact of mud filtrate contamination
on the final collected sample.
Graph of Shut-In
Pressure versus Depth showing different pressure gradients
over the reservoir, indicating different fluid densities.
Gas-oil and oil-water Contacts are marked at the changes in
slope on the gradient graph. It is sometimes difficult to
see the change in slope - try placing the graph horizontally
at eye level and sighting along the line. This is alled the
"Ant's-eye View". The bends in the line are much more
graph shows partially depleted reservoir pressures, with some
pressure isolation between the upper and lower sands. A gamma ray or
image log on the graph would help to distinguish reservoir
boundaries and internal barriers.
MODERN FORMATION TESTER TOOLS
description of a modern formation tester is courtesy of
Crocker Research. .
The Formation Evaluation Tool (FET) was developed by Crocker
Research in Australia and is widely used by other service companies
under license. Its foremost feature is its ability to pump from the
formation until a representative sample is present, that is until
the characteristics of oil, gas, or water are exhibited in the
resistivity, conductivity, and density sensors of the FET. Once a
representative sample is flowing through the tool the FET has the
ability to capture a predefined volume of this sample. This
predefined volume is based on the multisampler configuration which
is set prior to down-hole operation.
In addition the FET contains two Quartz
Pressure Gauges which have an accuracy of 0.01 psi. This in
conjunction with the tools pumping ability allows for accurate
shut-in pressures (SIPs) to be obtained with controlled draw downs.
The FET pump can be manually controlled enabling any user defined
draw down volume to be acquired, lowest being 1cc. In addition the
FET has the ability to reverse pump, that is pump fluids from the
borehole into the formation.
The proven benefit of reverse pumping is
the tool’s ability to “pump off” the formation, beneficial for
situations where the tool has been set for long periods of time.
This feature is credited with the fact that the FET has never been
stuck down-hole. The FET has been designed such that if there is a
loss of tool power for whatever reason the tool will automatically
retract (unset itself from the formation) enabling it to be
retrieved via the wireline cable.
During the operation of the tool, the
operator is able to give the following information to the client:
Schematic diagram of a Schlumberger MDT
Draw Down Pressure (DDP) in PSIA,
Draw Down Volume in cc,
Shut In Pressure (SIP) in PSIA,
Fluid temperature in °C,
For Constant Flow Tests (for every litre pumped);
Resistivity in Wm, Conductivity
Density in g/cc,
Temperature in °C,
Reservoir pressure in PSIA,
Flow rate in L/min,
and Permeability in mDarcy.
For Each Sample taken;
Resistivity in Wm,
Conductivity in mho/m,
Density in g/cc,
Temperature in °C,
Reservoir pressure in PSIA,
Flow rate in L/min,
Permeability in mDarcy,
Pressure at surface in PSIA,
and Volume captured at surface in
The primary purpose of a constant flow
test is to ensure that an uncontaminated sample of the Reservoir
fluid or gas is flowing through the tool. During a constant flow
test, for every litre pumped the resistivity, conductivity and
density of the hydrocarbon or water is monitored in search for a
“breakthrough”. Meaning, when all the mud filtrate has been pumped
from the reservoir and the actual uncontaminated hydrocarbon or
water is present. When this occurs there is a noticeable difference
in the FET’s sensor readings which corresponds to the properties of
the hydrocarbon or water expected. It is at this stage that a sample
is taken upon the client’s request. Therefore, a constant flow test
must be performed before a sample is taken to ensure a
representative (uncontaminated) sample is taken.
Secondarily, a constant flow test may be performed to gather the
properties of the hydrocarbon or water present after breakthrough in
terms of resistivity, conductivity and density. This may be used to
confirm the depth pressure gradients as well as reservoir contact
In addition to this, a constant flow test also results in the flow
rate and permeability of the fluid to be determined.
Reservoir fluid samples are captured
within the multisampler component of the FET. The configuration of
the multisampler depicts the quantity and volume of samples
The FET has the capability of attaching a PVT sampling assembly to
the bottom of the tool to capture 2 x 524.4cc formation fluid
samples per run.
any pretests are performed the tool packer must be set at the
correct depth. This is achieved via a gamma ray plot. A gamma ray
plot is printed and correlated with an existing gamma ray plot and
the packer depth adjusted accordingly. The requirement for the gamma
ray correlation is that the FET Software must be connected to the
logging unit’s depth system. The FET Software can connect to the
logging unit’s depth system via an RS-232 serial port.
Once the down-hole job is
completed, the end result in the FET Software is a graphical log
illustrating all sensor measurements over time for pretests,
constant flow tests, and samples captured. The client receives a
hardcopy and a data file in LAS format. Examoles are shown in the
next section od this webpage.
Modern fprmation testers can be
configured in various ways to sujt the test requirements.
(illustration courtesy Schlumberger)
FORMATION TESTER EXAMPLES
examples of modern formation tester log presentations is courtesy of
EXAMPLE 1: High
permeability sand with light oil.
This is probably the
easiest test. Oil breakthrough occurred only six minutes
after steady state flow began. Multi rate testing was done
which allows a plot of flowrate Q versus pressure drawdown.
Four samples were taken, all contained light oil
EXAMPLE 2: Low permeability sand with
low viscosity oil
drawdown was required to obtain 630 ml/min flow rate. Oil
breakthrough occurred after thirty-seven minutes of steady
flow (from the commencement of pumping). One filtrate and
one oil sample were taken. Permeability can be calculated
from the pressure drawdown or buildup curves. Note the
marked difference between the pretest buildup and the
EXAMPLE 3: High permeability loose sand
with viscous oil.
This is most
interesting. All previous attempts in these unconsolidated
sands with other wireline test tools had failed because of
lost seals. We were asked if the FET could sample with a
minimum flowrate even if several hours of testing were
required. This test was done with only 14 psi drawdown and a
flow rate of only 33ml/min. After some seven hours we had
only taken 13 litres of formation fluids. Oil breakthrough
occurred after some 80 minutes of steady flow (from the
commencement of pumping). Slug flow occurred
resulting in the spiky resistivity, conductivity and density logs.
Despite the sanding problems the tool moved the formation fluids
steadily until after some five hours, sanding effects show steps on
the pump motion. Nine samples were taken and heavy oil and some
filtrate was recovered. Despite the long test the tool came free
with only a minimum overpull. Conventional large cylindrical sample
chambers present a large area for differential sticking. The FET
involves no such chambers and thus is unlikely to be differentially
stuck. Moreover, the tool is pumped off the wall once the tool is
EXAMPLE 4: Moderate permeability sand
with viscous oil
possibility. Oil breakthrough occurred after about
thirty-five minutes of steady flow. Pressure drawdown was
107 psi at a flowrate of 880 ml/min, which equates to a
drawdown permeability of 380 md. After oil breakthrough the
pressure drawdown increased to 486 psi at 720 ml/min.
Assuming no relative permeability change (which may not be
valid) and a water viscosity 0.5 centipoise then the
oil viscosity is 2.75 centipoise. Please note that the oil
density is shown as 0.96 g/cc which checks well with the
known density. • Four oil samples were taken.
EXAMPLE 5: Gas sampling
Care is taken to
pump as little gas into the borehole as possible. Although
we have often sampled gas; no problem has ever been had when
circulating after testing. Gas breakthrough occurred after
thirty minutes of steady flow. Thereafter gas increased with
time but, curiously not at low flow rates. Two gas samples
were taken. The down-stroke of the pump (0.3 l/min) was at a
higher rate than the upstroke (0.2 l/min). It is clear that
at 0.2 l/min flow rate the filtrate supply from vertical
flow in the formation is enough to meet the FET flow rate
and no gas enters the tool. At 0.3 l/min the vertical flow
rate of filtrate is not enough to meet the FET flow rate and
thus gas enters the tool. The final pump stroke is at 0.45
l/min and the highest gas flow rate occurs. Clearly the gas
fraction of flow is rate sensitive.
EXAMPLE 6: Gas/Oil contact definition
This test was taken
three metres below Example 5. Oil breakthrough occurred
after eighty minutes of steady flow.
• Oil density was 0.98 g/cc and contrasts
strongly with the gas of Example 5. • This oil is heavily
biodegraded, the light ends have been removed by bacteria. Thus the
oil is very under-saturated. This is curious since the gas is in
contact with the oil. It seems likely that two stages of hydrocarbon
migration have occurred, one of oil and a later one of gas.
Halliburton SFT with FET, Gas sampling
This test was the first commercial use of the SFT chambers
in conjunction with the FET. 25 litres pumped in 31 minutes
before diversion into SFT chamber. Flowing pressure ~200 psi
below shut in pressure. Chamber filled in ~12 minutes. Note
the slow pressure build-up when chamber fills. This is
consistent with a compressible fluid in the tool. Minimal
filtrate in sample at surface. High quality sample produced.
Normal operation of the SFT is to open the Chamber after
pretest. The advantage of the FET is
the ability to removed an unlimited amount of fluid before
EXAMPLE 8: Halliburton SFT with FET -
test was the second commercial use of the SFT chambers in
conjunction with the FET. 4 litres pumped in 12 minutes
before diversion into SFT chamber. Flowing pressure is
~400psi below shut in pressure. Chamber filled in ~12
minutes. Note the rapid pressure build-up when chamber
fills. This is consistent with a noncompressible fluid,
water, in the tool.