Formation fluid pressure from repeat formation testers

Exploring for Oil and Gas Traps
Series	Treatise in Petroleum Geology
Part	Critical elements of the petroleum system
Chapter	Formation fluid pressure and its application
Author	Edward A. Beaumont, Forrest Fiedler
Link	Web page
Store	AAPG Store

RFT tool

Figure 1 Formation pressure is measured by the formation sampler when it is extended from the tool to contact the formation. Copyright: Springer-Verlag.

The repeat formation tester (RFT) tool was designed to measure formation pressure quickly and accurately. It measures pressure at specific points on the borehole wall. The diagram below shows a typical RFT tool. Formation pressure is measured by the formation sampler (Figure 1) when it is extended from the tool to contact the formation. Fluid samples from the formation can also be taken with the tool.

Differences between RFTs and DSTs

An RFT has several important advantages for formation pressure measurement over a DST. The table below lists some considerations.

Consideration	RFTs	DSTs
Time to take one measurement	Less than 5 minutes for permeable formations	More than 90 minutes
Drilling delay to run test	About one logging run (wireline conveyed)	About equal to two trips with drillstring
Sampling interval	Small,	Several feet or more; generally tests multiple flow units
Samples per run	Many	Few
Expense per test	Small	Large
Purpose of tool	Pressure measurement	Fluid recovery and pressure
Survey problems	Getting good seat to measure pressure Screen plugging with material in drilling mud	Packer failure Depth determination
Fractured reservoir	May be unreliable	Good if fractures intersect wellbore
Layered reservoir	Not representative	Good if many layers are included in tested interval
Skin damage	Can be major error	Can be measured and corrected for

Example: comparing RFT to DST

Figure 2 Plot of reservoir pressure vs. depth from a low-permeability chalk reservoir. Courtesy JPT.

Figure 2 is a plot of reservoir pressure vs. depth from a low-permeability chalk reservoir. The RFT data clearly show the hydrostatic gradient, the gas gradient, and the gas-water contact. Making the same interpretation from the DST data in this example is very difficult because data are from a low-permeability chalk reservoir. Reliable pressures are difficult to obtain in low-permeability reservoirs with DSTs. Extrapolated DST shut-in pressures from a partial buildup may not reflect actual fluid pressures. As rock quality increases, extrapolated pore pressure from DST buildup falls more and more closely to actual fluid pressure.

RFT pressure profile

Figure 3 Typical RFT pressure-time profile..

Figure 3 is a typical RFT pressure-time profile. Points are similar to points on a DST pressure profile.

Operating an RFT survey

The table below explains how to operate an RFT survey (see Gunter & Moore).

Step	Action
1	Use well logs to pick permeable zones for formation pressure measurements. Look for an invasion profile.
2	Plot mud hydrostatic and formation pressure at the well site to recognize anomalies or tool errors and to optimize station coverage.
3	Occasionally repeat formation pressure measurements at the same depth to check for consistency.
4	Repeat at some of the same depths for multiple surveys to help normalize the different surveys.
5	Sample both water- and hydrocarbon-bearing intervals to establish both the water and hydrocarbon pressure gradients.
6	Plot pressures at the same scale as well logs to aid in interpretation.

Controlling RFT quality

The table below describes how to control RFT quality. For details, see Gunter & Moore.

Step	Action
1	Inspect the tool and check calibration before going in the hole.
2	Run quartz and strain gauges simultaneously. Record both readings independently. Normalize to one another after completing the survey.
3	Maintain a slight overflow of mud to keep the level in the borehole constant during the survey and to prevent mud hydrostatic pressure errors.
4	Take mud hydrostatic pressures while descending into the hole to give the instruments time to equilibrate to changing temperature and pressure and to provide a mud hydrostatic pressure profile.
5	Check for tool errors by calculating mud hydrostatic pressures at different depths from mud weight; check them against measured mud hydrostatic pressures at the same depths.

See also

• Methods for obtaining formation fluid pressures
• Determining formation fluid pressure from DSTs

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