Practical considerations for conducting Perforation Inflow Tests (PITA)
PITA helps address conventional flow and buildup testing techniques shortcomings for tight gas reservoirs.
What is PITA?
Perforation inflow test is a type of a well test where the well is perforated underbalanced and is allowed to flow into a closed wellbore. The resulting pressure buildup is measured and analyzed. Provided reservoir dominated flow was reached during the test, the analysis provides initial reservoir pressure and an estimate of near wellbore permeability. The well can also be perforated overbalanced and the resulting falloff can be measured and analyzed.
History and Background
As interest in tight gas reservoirs increased, conventional flow and buildup testing techniques showed a number of shortcomings (i.e. impractically long buildup times, large error bars on pressure extrapolation, non-unique estimates of permeability and skin during fracture flow). PITA gained popularity as a test for tight gas wells conducted prior to the fracture stimulation to obtain initial reservoir pressure, skin and permeability.
Sequence of Operations
- Run in hole (RIH) with perforating guns, perforate the formation. Note and record the fluid level prior to perforating.
- Do not monitor the buildup. Pull out of hole (POOH) with guns. It is important to run pressure recorders as soon as possible.
- RIH a wireline retrievable bridge plug, combined with pressure recorders suspended below. Set the plug as close to the perforations as possible. Running a plug is optional.
- Alternatively, the buildup can be monitored at surface together with intermittent fluid levels (if applicable).
- Leave the well standing and monitor inflow as per test design (between a few hours and a few weeks).
- POOH bridge plug, c/w recorders.
An impulse derivative is calculated and plotted, which is used for flow regime identification. If radial flow was reached within the time constraints of the test, permeability, skin and initial reservoir pressure can be determined uniquely. If the data is transitioning to radial flow, reservoir characteristics cannot be determined uniquely. In this case, history matching can establish the minimum and maximum values of permeability, skin and initial reservoir pressure.
Example 1: Radial flow (identified by a horizontal trend on the pressure derivative) was not reached during the test. Unique estimates of permeability and reservoir pressure will not be possible.
Example 2: Radial flow was reached during the test. Permeability and reservoir pressure can be determined uniquely.
Test Design Considerations
Tests should be designed so that radial flow occurs as quickly as possible. The following variables affect the time to reach radial flow:
- Flow capacity – The operator has no control over this.
- Skin – Positive skin slows down the buildup. Therefore, perforations must be good and effective for pressure communication with the reservoir.
- Cushion Pressure – If cushion pressure is high, wellbore storage will have less of an effect. However, the underbalance has to be high enough to see an indication that we are in communication with the reservoir. The effect of cushion pressure is minimal compared to skin and wellbore volume.
- Wellbore Volume – Wellbore storage attenuates the buildup and masks reservoir response, especially in low permeability gas wells. Wellbore volume has the biggest effect and deserves the most attention when planning a test. When time factor is important, the duration of the test can be reduced by setting a bridge plug (c/w recorders below) above the perforations. Depending on the wellbore configuration, the wellbore volume can be reduced even further by setting a bridge plug below the perforations prior to the test. Also, PITA is not recommended on horizontal wells simply because of the large wellbore volume.
PITA vs. Mini-Frac
With time, resource development shifted to even lower permeability plays. Horizontal completions became the norm and PITA was no longer the recommended option. Mini-frac tests were proving more successful in very low permeability reservoirs.
In low permeability formations, the time to reach reservoir dominated flow (i.e. radial flow) can be impractical. This is especially the case in the presence of positive skin and poor communication with the reservoir. With a mini-frac, we are ‘seeing’ past the damaged near wellbore region and radial flow develops quicker in most cases. However, depending on the volume of fluid injected, mini-frac tests do not always achieve test objectives. Depending on the size of the fracture created, radial flow may not be reached within the time constraints of the test.
If a supercharged pressure region exists around the wellbore as a result of drilling overbalanced, PITA can point to an artificially high pressure. Mini-frac is more likely to ‘see’ past the supercharged region and provide a representative reservoir pressure.
Choosing between PITA and Mini-Frac
- The choice comes down to which test is more convenient.
- PITA is recommended for formation permeability of 100 µD and higher.
- PITA can be can be performed successfully in tight formations (k < 1 µD), although it is more difficult to execute and measures should be taken to significantly reduce the wellbore volume.
- PITA does not require fracking and may be the choice if fracking is not an option.
- PITA is not necessarily cheaper than a mini-frac.
- PITA is not recommended for horizontal wells.
Example 3: Shale Gas (Montney, k ≈ 100 nD). PITA was completed on a vertical Montney gas producer to obtain an initial reservoir pressure and permeability. A bridge plug was set below the perforations to reduce the wellbore volume. The Montney formation was perforated under a full water column. Following the perforations, a packer assembly, including a plug and pressure recorders in the tailpipe, was set above the perforations.
The test was cut short by a leaking bridge plug. However, because a bridge plug was set below and above the perforations, pressure data was transitioning out of wellbore storage and into radial flow. Two models were generated to determine the range of initial reservoir pressure and permeability.
PITA is suitable for tight formations as a means to determine initial reservoir pressure and permeability in vertical wells. It is recommended for formation permeability over 100 µD. It can be successfully carried out in lower permeability formations, although measures should be taken to significantly reduce the wellbore volume.
Vadim Milovanov, IHS Consulting (Engineering)
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