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''Formation testers'' are a class of wireline tools used to measure the downhole pressure of formations. Stationary measurements of formation pressure in an open hole are made at any number of depths during a single trip into the hole. These pressure measurements are useful in determining

''Formation testers'' are a class of wireline tools used to measure the downhole pressure of formations. Stationary measurements of formation pressure in an open hole are made at any number of depths during a single [[trip]] into the hole. These pressure measurements are useful in determining

# variations in pressure among various formations,

# variations in pressure among various formations,

Formation permeability has a significant effect on the drawdown response during a pretest. The pretest pressure recordings shown in [[:file:wireline-formation-testers_fig5.png|Figure 5]] illustrate typical records for sandstones at 0.1, 1, 10, and 100 mD (millidarcys). While these figures are qualitative, quantitative techniques exist for estimating permeability using both the pressure drawdown and buildup characteristics of the pretest or sample test.

Formation permeability has a significant effect on the drawdown response during a pretest. The pretest pressure recordings shown in [[:file:wireline-formation-testers_fig5.png|Figure 5]] illustrate typical records for sandstones at 0.1, 1, 10, and 100 mD (millidarcys). While these figures are qualitative, quantitative techniques exist for estimating permeability using both the pressure drawdown and buildup characteristics of the pretest or sample test.

==Pressure applications==

==Pressure applications==

Although formation testers can take samples, they are often run solely for the pressure information available from the pretest. [[:file:wireline-formation-testers_fig6.png|Figure 6]] shows how such pretest data can be useful. In this figure, the well is shown penetrating a reservoir that has gas, oil, and water intervals. The formation tester is set at numerous depth intervals across this reservoir. The formation pressure recorded by the tool is indicated by an “x,” while the hydrostatic mud column pressure is indicated by an “o.” The degree of overbalance (that is, the difference between the mud and formation pressures) is clearly visible from the schematic. The fluid gradients are also detectable, and the gas column is readily distinguished from the oil, which is also distinguishable from the water. The location of the gas-oil and water-oil contacts can also be determined from the formation pressure profile.

Although formation testers can take samples, they are often run solely for the pressure information available from the pretest. [[:file:wireline-formation-testers_fig6.png|Figure 6]] shows how such pretest data can be useful. In this figure, the well is shown penetrating a reservoir that has gas, oil, and water intervals. The formation tester is set at numerous depth intervals across this reservoir. The formation pressure recorded by the tool is indicated by an “x,” while the hydrostatic mud column pressure is indicated by an “o.” The degree of overbalance (that is, the difference between the mud and formation pressures) is clearly visible from the schematic. The fluid gradients are also detectable, and the gas column is readily distinguished from the oil, which is also distinguishable from the water. The location of the gas-oil and water-oil contacts can also be determined from the formation pressure profile.

After a reservoir has been produced, some pressure decline can be expected. Formation testers are frequently run in development or in-fill wells. When compared to initial reservoir pressures, pressure profiles in these wells often show that certain zones may have produced more than neighboring zones, thereby indicating the presence of permeability barriers.

After a reservoir has been produced, some pressure decline can be expected. Formation testers are frequently run in development or in-fill wells. When compared to initial reservoir pressures, pressure profiles in these wells often show that certain zones may have produced more than neighboring zones, thereby indicating the presence of permeability barriers.

Pressures in zones of injection have also been monitored using the Formation tester. Such a case is shown in [[:file:wireline-formation-testers_fig7.png|Figure 7]]. In this example, 22 wells were drilled some years after water flooding was begun in a reservoir. Formation tester pressure data from these 22 wells were used to plot a contour map of the formation pressure. This map clearly shows high pressure ridges associated with the banks of injection wells and troughs associated with the producing wells.

Pressures in zones of injection have also been monitored using the Formation tester. Such a case is shown in [[:file:wireline-formation-testers_fig7.png|Figure 7]]. In this example, 22 wells were drilled some years after water flooding was begun in a reservoir. Formation tester pressure data from these 22 wells were used to plot a [[contour]] map of the formation pressure. This map clearly shows high pressure ridges associated with the banks of injection wells and troughs associated with the producing wells.

==Fluid SamplinG==

==Fluid sampling==

Based on the ability of the fluid to freely fill the pretest chamber, a larger sample of formation fluid can be taken for analysis on the surface. The larger samples can range from 1 to 10 gal or more. Due to mud filtrate invasion, a large fraction (if not all) of the retrieved fluid may be mud filtrate. Proper analysis of the sample involves discriminating the filtrate from the native formation fluids.

Based on the ability of the fluid to freely fill the pretest chamber, a larger sample of formation fluid can be taken for analysis on the surface. The larger samples can range from 1 to 10 gal or more. Due to mud filtrate invasion, a large fraction (if not all) of the retrieved fluid may be mud filtrate. Proper analysis of the sample involves discriminating the filtrate from the native formation fluids.

Measurements of water resistivity, API gravity, gas to oil ratio, and water chemistry can be performed at the wellsite with prior planning with the vendor. Formation fluid samples can also be maintained at formation pressures and shipped to laboratories for detailed analysis. However, shipment of pressurized samples may require use of special vessels approved by the Department of Transportation, and prior arrangements should be made with the involved vendors.

Measurements of water resistivity, [[API gravity]], gas to oil ratio, and water chemistry can be performed at the wellsite with prior planning with the vendor. Formation fluid samples can also be maintained at formation pressures and shipped to laboratories for detailed analysis. However, shipment of pressurized samples may require use of special vessels approved by the Department of Transportation, and prior arrangements should be made with the involved vendors.

==See also==

==See also==

* [[Basic open hole tools]]

* [[Basic open hole tools]]

* [[Basic tool table]]

* [[Basic tool table]]

* [[Formation evaluation of naturally fractured reservoirs]]

* [[Formation evaluation of naturally fractured reservoirs]]

* [[Quick-look lithology from logs]]

* [[Quick-look lithology from logs]]

==References==

==References==

[[Category:Wireline methods]]

[[Category:Wireline methods]]