Formation fluid pressure from repeat formation testers

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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[edit]

Figure 1 Formation pressure is measured by the formation sampler when it is extended from the tool to contact the formation.[1] 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[edit]

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[edit]

Figure 2 Plot of reservoir pressure vs. depth from a low-permeability chalk reservoir.[2] 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[edit]

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[edit]

The list below explains how to operate an RFT survey (see Gunter & Moore[2]).

  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[edit]

The list below describes how to control RFT quality. For details, see Gunter & Moore.[2]

  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[edit]

References[edit]

  1. ↑ Dahlberg, E., C., 1994, Applied Hydrodynamics in Petroleum Exploration, 2nd ed.: New York, Springer-Verlag, 295 p.
  2. ↑ 2.0 2.1 2.2 Gunter, J., M., Moore, C., V., 1987, Improved use of wireline testers for reservoir evaluation: Journal of Petroleum Technology, p. 635–644.

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