Changes

  | part    = Critical elements of the petroleum system

  | part    = Critical elements of the petroleum system

  | chapter = Formation fluid pressure and its application

  | chapter = Formation fluid pressure and its application

  | frompg  = 5-1

  | frompg  = 5-33

  | topg    = 5-64

  | topg    = 5-35

  | author  = Edward A. Beaumont, Forrest Fiedler

  | author  = Edward A. Beaumont, Forrest Fiedler

  | link    = http://archives.datapages.com/data/specpubs/beaumont/ch05/ch05.htm

  | link    = http://archives.datapages.com/data/specpubs/beaumont/ch05/ch05.htm

==RFT tool==

==RFT tool==

[[file:formation-fluid-pressure-and-its-application_fig5-16.png|thumb|{{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.]]

[[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 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.

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==

==Differences between RFTs and DSTs==

{| class = "wikitable"

{| class = "wikitable"

|-

|-

! Consideration

! Consideration || RFTs || DSTs

! RFTs

! DSTs

|-

|-

| Time to take one measurement

| Time to take one measurement || Less than 5 minutes for permeable formations || More than 90 minutes

| Less than 5 minutes for permeable formations

| More than 90 minutes

|-

|-

| Drilling delay to run test

| Drilling delay to run test || About one logging run (wireline conveyed) || About equal to two trips with drillstring

| About one logging run (wireline conveyed)

| About equal to two trips with drillstring

|-

|-

| Sampling interval

| Sampling interval || Small || Several feet or more; generally tests multiple flow units

| Small,

| Several feet or more; generally tests multiple flow units

|-

|-

| Samples per run

| Samples per run || Many || Few

| Many

| Few

|-

|-

| Expense per test

| Expense per test || Small || Large

| Small

| Large

|-

|-

| Purpose of tool

| Purpose of tool || Pressure measurement || Fluid recovery and pressure

| Pressure measurement

| Fluid recovery and pressure

|-

|-

| Survey problems

| Survey problems ||

| * Getting good seat to measure pressure  * Screen plugging with material in drilling mud

* Getting good seat to measure pressure   

 

* Screen plugging with material in drilling mud

| * Packer failure  * Depth determination

 

* Packer failure   

* Depth determination

|-

|-

| Fractured reservoir

| Fractured reservoir || May be unreliable || Good if fractures intersect wellbore

| May be unreliable

| Good if fractures intersect wellbore

|-

|-

| Layered reservoir

| Layered reservoir || Not representative || Good if many layers are included in tested interval

| Not representative

| Good if many layers are included in tested interval

|-

|-

| Skin damage

| Skin damage || Can be major error || Can be measured and corrected for

| Can be major error

| Can be measured and corrected for

|}

|}

==Example: comparing RFT to DST==

==Example: comparing RFT to DST==

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

[[: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==

==RFT pressure profile==

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

[[: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==

==Operating an RFT survey==

The table below explains how to operate an RFT survey (see<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>).

The list 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>).

# Use well logs to pick permeable zones for formation pressure measurements. Look for an invasion profile.

# Plot mud hydrostatic and formation pressure at the well site to recognize anomalies or tool errors and to optimize station coverage.

# Occasionally repeat formation pressure measurements at the same depth to check for consistency.

# Repeat at some of the same depths for multiple surveys to help normalize the different surveys.

# Sample both water- and hydrocarbon-bearing intervals to establish both the water and hydrocarbon pressure gradients.

# Plot pressures at the same scale as well logs to aid in interpretation.

| Use well logs to pick permeable zones for formation pressure measurements. Look for an invasion profile.

| Plot mud hydrostatic and formation pressure at the well site to recognize anomalies or tool errors and to optimize station coverage.

| Occasionally repeat formation pressure measurements at the same depth to check for consistency.

| Repeat at some of the same depths for multiple surveys to help normalize the different surveys.

| Sample both water- and hydrocarbon-bearing intervals to establish both the water and hydrocarbon pressure gradients.

| Plot pressures at the same scale as well logs to aid in interpretation.

==Controlling RFT quality==

==Controlling RFT quality==

The table below describes how to control RFT quality. For details, see<ref name=ch05r9 />.

The list below describes how to control RFT quality. For details, see Gunter & Moore.<ref name=ch05r9 />

# Inspect the tool and check calibration before going in the hole.

# Run quartz and strain gauges simultaneously. Record both readings independently. Normalize to one another after completing the survey.

# Maintain a slight overflow of mud to keep the level in the borehole constant during the survey and to prevent mud hydrostatic pressure errors.

# 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.

# 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.

| Inspect the tool and check calibration before going in the hole.

| Run quartz and strain gauges simultaneously. Record both readings independently. Normalize to one another after completing the survey.

| Maintain a slight overflow of mud to keep the level in the borehole constant during the survey and to prevent mud hydrostatic pressure errors.

| 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.

| 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==

==See also==

* [[Methods for obtaining formation fluid pressures]]

* [[Methods for obtaining formation fluid pressures]]

* [[Determining formation fluid pressure from DSTs]]

* [[Determining formation fluid pressure from DSTs]]

==References==

==References==

[[Category:Critical elements of the petroleum system]]  

[[Category:Critical elements of the petroleum system]]  

[[Category:Formation fluid pressure and its application]]

[[Category:Formation fluid pressure and its application]]