Changes

==Types of periods during a DST==

==Types of periods during a DST==

There are three major types of periods during a typical DST:

There are three major types of periods during a typical DST:

* During '''shut-in periods''', the packer(s) is still inflated and the tester valve is closed. Ideally, pressure in the closed tool gradually builds up until it reaches equilibrium with the pressure of the isolated formation.

* During '''shut-in periods''', the packer(s) is still inflated and the tester valve is closed. Ideally, pressure in the closed tool gradually builds up until it reaches equilibrium with the pressure of the isolated formation.

The figure below shows a typical DST tool, the configurations of the tool during a DST, and the periods of a DST.

[[:file:formation-fluid-pressure-and-its-application_fig5-13.png|figure 1]] shows a typical DST tool, the configurations of the tool during a DST, and the periods of a DST.

[[file:formation-fluid-pressure-and-its-application_fig5-14.png|thumb|{{figure number|2}}See text for explanation.]]

During a DST, pressure is continuously recorded against time. The record begins as the DST tool is lowered down the borehole and ends when the tool returns to the surface. The figure below is a DST plot, showing the various pressures recorded during the different DST periods.

[[file:formation-fluid-pressure-and-its-application_fig5-14.png|thumb|{{figure number|5-14}}See text for explanation.]]

During a DST, pressure is continuously recorded against time. The record begins as the DST tool is lowered down the borehole and ends when the tool returns to the surface. [[:file:formation-fluid-pressure-and-its-application_fig5-14.png|Figure 1]] is a DST plot, showing the various pressures recorded during the different DST periods.

'''Point A''' is the initial hydrostatic pressure (IHP), the pressure exerted by the mud column in the borehole at the depth where the recorder is located.

'''Point A''' is the initial hydrostatic pressure (IHP), the pressure exerted by the mud column in the borehole at the depth where the recorder is located.

==Extrapolating true pressures==

==Extrapolating true pressures==

DST pressures may not be reliable because the tool is not shut in long enough for pressure to stabilize at final reservoir pressure. A graphical procedure devised by Horner<ref name=ch05r10>Horner, D., R., 1951, Pressure build-up in wells: Proceedings of the Third World Petroleum Congress, The Hague, Section II, p. 503–521.</ref> infers the true reservoir pressure by extrapolating the shut-in periods to infinity.

DST pressures may not be reliable because the tool is not shut in long enough for pressure to stabilize at final reservoir pressure. A graphical procedure devised by Horner<ref name=ch05r10>Horner, D., R., 1951, Pressure build-up in wells: Proceedings of the Third World Petroleum Congress, The Hague, Section II, p. 503–521.</ref> infers the true reservoir pressure by extrapolating the shut-in periods to infinity.

Below is an example showing how pressure is extrapolated from ISIP and FSIP on a Horner plot (pressure vs. psuedo or Horner time, or (T + ΔT)/ΔT

[[:file:formation-fluid-pressure-and-its-application_fig5-15.png|Figure 3]] is an example showing how pressure is extrapolated from ISIP and FSIP on a Horner plot (pressure vs. psuedo or Horner time, or (T + ΔT)/ΔT

 

==DST pressures from scout tickets==

==DST pressures from scout tickets==