Slug Analysis

Subtopics:

During a slug test, a small volume of fluid is extracted from the reservoir. During a slug injection, a small volume of fluid is added to the reservoir. Although the tests and solution methods were developed independently, the analytic solutions for analyzing slug tests are identical to PITA Analysis.

In WellTest, the PITA analysis has been developed further than slug. PITA facilitates the estimation of p*, reservoir permeability and skin whereas Slug's Impulse Radial Analysi only provides an estimation of p*. Therefore, PITA is recommended for analyzing data from slug tests. Both PITA and Slug tests have (identical) modeling capabilities in WellTest.

Impulse Radial Analysis

Impulse radial analysis is used to analyze data from the transition period in between wellbore storage dominated flow and radial flow. The purpose of analyzing this transition period data is to determine the initial pressure of the reservoir (pi).

Transition period flow data will form a straight line when placed on a Cartesian plot of pressure versus reciprocal time (p vs. 1 / Dt) as shown below.

The basis of this approach is illustrated with the following mathematical development.

The wellbore flowing pressure (pwf) (during fluid influx) can be derived as:

For large times, the above equation can be approximated as:

Thus, a plot of p versus 1 / Dt will result in a straight line, and the intercept of which on the pressure axis provides the initial pressure (pi).

In order to select or confirm the range of data that can be analyzed by the impulse radial analysis, a log-log plot of the semi-log derivative versus delta time (Dt) can be used as shown below.

Note:

Any data that falls within the range of the -1 slope line can be used for analysis.

References

"A New Method for Determination of Average Reservoir Pressure", F.J. Kuchuk, SPE Paper 56418 Presented at 1999 ATCE, Houston, TX (October 3 - 6).