View source for Formation fluid pressure from repeat formation testers

{{publication 
 | image   = exploring-for-oil-and-gas-traps.png
 | width   = 120px
 | series  = Treatise in Petroleum Geology
 | title   = Exploring for Oil and Gas Traps
 | part    = Critical elements of the petroleum system
 | chapter = Formation fluid pressure and its application
 | frompg  = 5-1
 | topg    = 5-64
 | author  = Edward A. Beaumont, Forrest Fiedler
 | link    = http://archives.datapages.com/data/specpubs/beaumont/ch05/ch05.htm
 | pdf     = 
 | store   = http://store.aapg.org/detail.aspx?id=545
 | isbn    = 0-89181-602-X
}}
==RFT tool==

[[file:formation-fluid-pressure-and-its-application_fig5-16.png|thumb|350px|{{figure number|1}}Formation pressure is measured by the formation sampler when it is extended from the tool to contact the formation.<ref name=ch05r5>Dahlberg, E., C., 1994, Applied Hydrodynamics in Petroleum Exploration, 2nd ed.: New York, Springer-Verlag, 295 p.</ref> Copyright: Springer-Verlag.]]

The [[Wireline formation testers|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 ([[:file:formation-fluid-pressure-and-its-application_fig5-16.png|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.

{| class = "wikitable"
|-
! 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==

[[file:formation-fluid-pressure-and-its-application_fig5-17.png|300px|thumb|{{figure number|2}}Plot of reservoir pressure vs. depth from a low-[[permeability]] chalk reservoir.<ref name=ch05r9 /> Courtesy JPT.]]

[[:file:formation-fluid-pressure-and-its-application_fig5-17.png|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==

[[file:formation-fluid-pressure-and-its-application_fig5-18.png|thumb|300px|{{figure number|3}}Typical RFT pressure-time profile..]]

[[:file:formation-fluid-pressure-and-its-application_fig5-18.png|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<ref name=ch05r9>Gunter, J., M., Moore, C., V., 1987, Improved use of wireline testers for reservoir evaluation: Journal of Petroleum Technology, p. 635–644.</ref>).

{| class = "wikitable"
|-
! 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.<ref name=ch05r9 />

{| class = "wikitable"
|-
! 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]]
* [[Wireline formation testers]]

==References==
{{reflist}}

==External links==
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* [http://archives.datapages.com/data/specpubs/beaumont/ch05/ch05.htm Original content in Datapages]
* [http://store.aapg.org/detail.aspx?id=545 Find the book in the AAPG Store]

[[Category:Critical elements of the petroleum system]] 
[[Category:Formation fluid pressure and its application]]