Changes

If a drill stem test is anticipated, low fluid loss mud will prevent excessive leakoff into the target zone and doping the mud with nitrates will distinguish filtrate from recovered formation water.

If a drill stem test is anticipated, low fluid loss mud will prevent excessive leakoff into the target zone and doping the mud with nitrates will distinguish filtrate from recovered formation water.

==Test tools==

==Test tools==

A drill stem test string consists of packers, a downhole shut-in valve, a safety joint, and pressure gauges (Figure 1). The bottom packer and blanked off gauge are shown as an “add on” to a straddle test.

A drill stem test string consists of packers, a downhole shut-in valve, a safety joint, and pressure gauges ([[:file:drill-stem-testing_fig1.png|Figure 1]]). The bottom packer and blanked off gauge are shown as an “add on” to a straddle test.

[[file:drill-stem-testing_fig1.png|thumb|{{figure number|1}}Drill stem test tool string.]]

[[file:drill-stem-testing_fig2.png|thumb|{{figure number|2}}Straddle drill stem test tool string. (a) Typical MFE inflate open hole string (straddle, off-bottom). (b) Typical MFE straddle string (open hole, conventional, off-bottom). (From <ref name=pt03r21>Flopetrol-Johnston, 1987, Downhole testing services brochure, p. DTS/M-28[5-87].</ref>.)]]

===Packers===

===Packers===

Compression set packers are generally more reliable than inflatable packers because they can withstand more differential pressure between the annulus and the drill string. The number of packers depend upon experience and test type (conventional, straddle, or hookwall.<ref name=pt03r20>Flopetrol-Johnston, 1980, Drill-stem testing manual: p 1#3–1#28.</ref> Figure 2 illustrates other types of test strings.

Compression set packers are generally more reliable than inflatable packers because they can withstand more differential pressure between the annulus and the drill string. The number of packers depend upon experience and test type (conventional, straddle, or hookwall.<ref name=pt03r20>Flopetrol-Johnston, 1980, Drill-stem testing manual: p 1#3–1#28.</ref> [[:file:drill-stem-testing_fig2.png|Figure 2]] illustrates other types of test strings.

 

[[file:drill-stem-testing_fig2.png|thumb|{{figure number|2}}Straddle drill stem test tool string. (a) Typical MFE inflate open hole string (straddle, off-bottom). (b) Typical MFE straddle string (open hole, conventional, off-bottom). (From <ref name=pt03r21>Flopetrol-Johnston, 1987, Downhole testing services brochure, p. DTS/M-28[5-87].</ref>.)]]

Packer selection is also determined by the need for a cushion. A ''cushion'' consists of water or gas and is run for the following reasons:

Packer selection is also determined by the need for a cushion. A ''cushion'' consists of water or gas and is run for the following reasons:

==Analyzing the Dst==

==Analyzing the Dst==

Once the drill stem test is concluded, mechanical validity and fluid recovery must be verified.

Once the drill stem test is concluded, mechanical validity and fluid recovery must be verified.

===DST chart analysis===

===DST chart analysis===

Descriptions of valid DST charts are shown in Figures 3, 4, 5, 6, 7, and 8. (See figure captions for discussion.)

Descriptions of valid DST charts are shown in Figures [[:file:drill-stem-testing_fig3.png|3]], [[:file:drill-stem-testing_fig4.png|4]], [[:file:drill-stem-testing_fig5.png|5]], [[:file:drill-stem-testing_fig6.png|6]], [[:file:drill-stem-testing_fig7.png|7]], and [[:file:drill-stem-testing_fig8.png|8]]. (See figure captions for discussion.)

 

[[file:drill-stem-testing_fig3.png|thumb|{{figure number|3}}Perfect chart. Gauge located inside and above the closing tool. (A) Add cushion/run in hole; (B) initial flow period; (C) initial shut-in period; (D) final flow period; (E) final shut-in period; and (F) pulling out of hole.]]

 

[[file:drill-stem-testing_fig4.png|thumb|{{figure number|4}}Collar leak. Gauge located inside and above the closing tool. Chart indicates increasing pressure during running in hole and shut-in periods]]

 

[[file:drill-stem-testing_fig5.png|thumb|{{figure number|5}}Fluid loss from drill pipe. Gauge located inside and above the closing tool. Bleeder valve on drill string left open during shut-in periods.]]

 

[[file:drill-stem-testing_fig6.png|thumb|{{figure number|6}}Perfect chart. Gauges inside above and outside below the closing tool. Pressure transient analysis done from these gauges. (A) Run in hole, gauge measuring hydrostatic pressure of mud column; (B) initial flow period; (C) initial buildup; (D) final flow period; (E) final buildup; and (F) release packer and pulling out of hole.]]

 

[[file:drill-stem-testing_fig7.png|thumb|{{figure number|7}}Perfect chart. Blanked off gauge below the bottom packer on a straddle test. (A) running in hole; (B) initial flow period; (C) initial buildup; (D) final flow period; (E) final buildup; and (F) pulling out of hole.]]

 

[[file:drill-stem-testing_fig8.png|thumb|{{figure number|8}}Bottom packer failure. Blanked off gauge below bottom packer on a straddle test. When the bottom packer fails, the pressure gauge will read some flow and buildup data but will not replicate gauges run above the bottom packer because of a restricted flow area around the packer elements. (Types of gauge failures are described in <ref name=pt03r20 />.)]]

===Fluid recovery (Nonflowing Test)===

===Fluid recovery (Nonflowing Test)===

===Calculating static reservoir pressure===

===Calculating static reservoir pressure===

Reservoir pressure (''P''*) is calculated by extrapolating the pressure data (from the first buildup on a DST) on the Horner (or superposition) plot to an infinite shut-in rime (Figure 9). This pressure provides a guide for selecting the slope of the second buildup Horner plot. If the second buildup slope extrapolates to a pressure significantly less than ''P''*, depletion might be suspected. To see true depletion, the reservoir would have to be very small. Although depletion is possible in rare cases, identifying it is usually a result of poor test design and analysis.

Reservoir pressure (''P''*) is calculated by extrapolating the pressure data (from the first buildup on a DST) on the Horner (or superposition) plot to an infinite shut-in rime (Figure 9). This pressure provides a guide for selecting the slope of the second buildup Horner plot. If the second buildup slope extrapolates to a pressure significantly less than ''P''*, depletion might be suspected. To see true depletion, the reservoir would have to be very small. Although depletion is possible in rare cases, identifying it is usually a result of poor test design and analysis.

A method for constructing the Horner plot is outlined as follows. After determining the effective producing time and producing rate, tabulate time and pressure for each buildup period. Make plots using Cartesian paper because this makes it easier to expand the plot (see Figure 9). Now plot ''P'' versus log[(''t'' + Δ''t'')/Δ''t''] for buildup #1 (the first buildup following the first flow period). Note that ''t'' in this equation is equal to the length of the initial flow period and that Δ''t'' is the time since the start of the buildup period.

A method for constructing the Horner plot is outlined as follows. After determining the effective producing time and producing rate, tabulate time and pressure for each buildup period. Make plots using Cartesian paper because this makes it easier to expand the plot (see [[:[file:drill-stem-testing_fig9.png|lFigure 9]]). Now plot ''P'' versus log[(''t'' + Δ''t'')/Δ''t''] for buildup #1 (the first buildup following the first flow period). Note that ''t'' in this equation is equal to the length of the initial flow period and that Δ''t'' is the time since the start of the buildup period.

 

Extrapolate this curve to ''P''* at log[(''t'' + Δ''t'')/Δ''t''] = 0. ''P''* provides the “guiding light” for determining the proper slope found using buildup #2. Now plot P versus log[(''t'' + Δ''t'')/Δ''t''] for buildup #2. Note here that this ''t'' is the effective producing time calculated from one of the methods outlined earlier.

Extrapolate this curve to ''P''* at log[(''t'' + Δ''t'')/Δ''t''] = 0. ''P''* provides the “guiding light” for determining the proper slope found using buildup #2. Now plot P versus log[(''t'' + Δ''t'')/Δ''t''] for buildup #2. Note here that this ''t'' is the effective producing time calculated from one of the methods outlined earlier.