Slug Model

This 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.

The slug test is performed by shutting in the well at surface and introducing an instantaneous pressure drop across the sandface either by perforating the wellbore casing, or opening a downhole valve. This creates a continuous pressure increase in the wellbore as fluid flows into the wellbore at sandface. As the wellbore fills with fluid, this increase in pressure slows down and stabilizes near the initial pressure (pi) as shown in the figure below. This increase in pressure is measured over time, and the data obtained can be modeled to determine permeability (k), skin (s'), and initial pressure (pi) of the reservoir.

Because only the allowable fluid inflow into the wellbore is needed, the slug 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 be obtained.

The impulse radial analysis is used to estimate the initial pressure (pi) for the model. Permeability (k) and skin (s') cannot be determined uniquely from the impulse radial analysis; therefore an initial guess for these values is required.

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).

Note:

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

References

1. "Analysis of Slug Test or DST Flow Period Data", H.J. Ramey, Jr., R.G. Agarwal and I. Martin, JCPT (July - September 1975) 37 - 47.

2. "Annulus Unloading Rates as Influenced by Wellbore Storage and Skin Effect", H.J. Ramey, Jr. and R.G. Agarwal, SPEJ (October 1972) 453 - 462.

3. "Analysis of Slug and Drillstem Tests", A.M.M. Peres, Ph.D. Thesis, The University of Tulsa (1989).