PITA Model

The perforation inflow test analysis (PITA) model simulates the wellbore pressure response in a vertical well that is shut-in at the surface with fluid flow continuing at the sandface. The model assumes an infinite-acting reservoir with homogeneous characteristics. Due to the specific nature of this model, changing wellbore storage, dual porosity, and an observation well are not supported in this model.

A perforation inflow test is performed by shutting in the well at the surface and perforating wellbore casing in an under-balanced condition (i.e., the cushion pressure (pwo) is significantly less than the formation pressure). This creates a continuous pressure increase in the wellbore as fluid flows into the wellbore at the sandface. As the wellbore fills with fluid, this increase in pressure slows down and stabilizes near the initial pressure (pi) as shown in the plot below. This increase in pressure is measured over time, and the data obtained can by modeled to determine permeability (k), skin (s'), and initial pressure (pi) of the reservoir.

If a formation is perforated over-balanced (i.e., the cushion pressure (pwo) is greater than the formation pressure), the wellbore pressure decreases as the wellbore fluid flows into the formation. When the cushion pressure (pwo) exceeds the formation fracture pressure (during a fracture calibration test), the PITA model can provide meaningful results, if the falloff test is continued long enough to see a transition to reservoir (radial) flow after the fracture closes.

Pressure data during the test can be obtained at the wellhead when inflow is expected to be single-phase gas, and a fairly rapid pressure response is anticipated. The wellhead pressures can then be converted to sandface conditions for analysis. When single-phase liquid inflow is expected, sandface pressures must be measured directly.

The late time analysis is used to estimate the initial pressure (pi), permeability (k), and skin (s') for the model. These initial estimates can then be verified by the model. In cases where reservoir (radial) flow behaviour is not fully developed within the test period, better estimates of these key reservoir parameters are obtained directly from modeling.

The wellbore storage effect is accounted for by specifying the volume of the compressible fluid in the wellbore (Vw for liquid or gas) or by specifying a changing fluid (liquid) level condition with a wellbore volumetric capacity (Vu for liquid only). These values directly affect the results and must be estimated as accurately as possible.

Note:

Measured rates are not required for interpretation since the model calculates the fluid influx rates using the pressure data only.

Because only the allowable fluid inflow into the wellbore is needed, the PITA test can be run over a much shorter period of time than a conventional test. Thus, it is important to note that the reservoir information obtained may only represent a small portion of the reservoir near the wellbore, especially when the permeability (k) is low, or the skin (s') is high. Under these conditions, if a transition from wellbore storage to reservoir- dominated (radial) flow is sufficiently developed, meaningful reservoir parameters can still be obtained. If reservoir-dominated (radial) flow is not sufficiently developed, then a downhole shut-in is required to minimize wellbore storage and reduce the time required to achieve reservoir-dominated flow. In this case, a step-change in wellbore storage results when downhole shut-in occurs, and analysis / modeling is recommended to be undertaken using the Closed Chamber Test (CCT) analysis or CCT model.

A test design PITA model can be used to determine the appropriate test duration, and the need for down-hole shut-in. For a gas reservoir, down-hole shut-in can be simulated by simply reducing the wellbore volume (Vw). Similarly, for a liquid-filled system, the wellbore volumetric capacity (Vu) can be reduced to represent a small wellbore volume (Vw).

References

1. "Use of PITA for Estimating Key Reservoir Parameters", N. M. Anisur Rahman, Mehran Pooladi-Darvish, Martin S. Santo and Louis Mattar, Paper CIPC 2006 - 172, presented at 7th Canadian International Petroleum Conference, Calgary, AB, June 13 - 15, 2006.

2. "Development of Equations and Procedure for Perforation Inflow Test Analysis (PITA)", N. M. Anisur Rahman, Mehran Pooladi-Darvish and Louis Mattar, Paper SPE 95510, presented at 80th Annual Technical Conference and Exhibition of the SPE, Dallas, TX, October 9 - 12, 2005.

3. "Perforation Inflow Test Analysis (PITA)", N. M. Anisur Rahman, Mehran Pooladi-Darvish and Louis Mattar, Paper CIPC 2005 - 031, presented at 6th Canadian International Petroleum Conference, Calgary, AB, June 7 - 9, 2005.