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 under-pressured reservoirs.

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 obvious.

This 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.

(Illustrations courtesy Crocker Research)

MODERN FORMATION TESTER TOOLS
The following 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 formation tetser

For Pretests;

• 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 in mho/m,

• 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 cc.

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 depths.

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 captured.

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.
 

Before 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
The following examples of modern formation tester log presentations is courtesy of Crocker Research.

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

Large pressure 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 drawdown curves.

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

EXAMPLE 4: Moderate permeability sand with viscous oil

The fourth 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.

EXAMPLE 7: 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 sampling commences.
 

EXAMPLE 8: Halliburton SFT with FET -  Water sampling

This 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.