Evaluating the Volume of Porous Medium Investigated During Slug Tests

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Abstract
This paper presents a methodology for quantifying the volume of porous medium investigated during a slug test in an unbounded porous medium, in the presence of a linear constant‐head or no‐flow boundary, and in the presence of a radial no‐flow boundary. For the unbounded case, type curves are generated for different values of the wellbore storage coefficient, which relate the distance travelled by a given pressure perturbation (1, 5, and 10% of the initial drawdown in the well), to dimensionless time. This distance is found to increase linearly on a log‐log plot until it reaches a maximum which is a function of the wellbore storage coefficient. The appropriate choice of dimensionless groups allows the different curves for each level of perturbation to be collapsed into one curve. Functional relationships offer an alternative to using the type curves graphically. For bounded systems, type curves relate the distance to the boundary to the time of 1 and 5% deviation from the unbounded response. Although these curves cannot be collapsed, the presented range of wellbore storage coefficients covers most practical situations. Developed relationships allow the estimation of the maximum distance travelled by the pressure perturbations in the unbounded case, and the maximum distance at which a linear constant‐head or no‐flow boundary, or a radial no‐flow boundary, still produces a given deviation in the pressure response measured at the well. An application shows that substantial error may result if the distance to a boundary is evaluated while neglecting storage in the well. Finally, application of the methodology developed for a linear no‐flow boundary to a real data set yields a realistic distance to the boundary and a far better match between simulated and measured data than if an unbounded system is considered. The type curves and relationships presented here should be applicable to slug test design and analysis.

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