Analysis of Interference Test Data Influenced by Wellbore Storage and Skin at the Flowing Well

Abstract

This paper presents new solutions for analyzing interference test data affected by wellbore storage and skin effects at the flowing well. Three parameters of interest - wellbore storage phenomenon, skin effects, and distance between two wells - are considered simultaneously. The new type curves should improve significantly the ability to analyze such data. Introduction With the advent of sensitive pressure gauges and because of the need to obtain precise information on reservoir heterogeneity for implementation of tertiary recovery programs, interference tests have become popular. It is well-established that the results of a multiwell test are influenced more by reservoir heterogeneity than those of a single-well test. The line source solution presented first by Theis1 serves as a starting point for the analysis of interference test data. This solution assumes that the storage capacity of the flowing well and the skin region around the sand-face have a negligible effect on the observation-well response. This assumption is valid if the distance between the flowing well and the observation well is large. However, if the distance is small, then the effect of storage and skin on the observation-well response can be significant. The pressure response at any point in the reservoir will be damped and delayed as a result of the storage effect at the active well (with or without the skin effect), because the main effect of the storage capacity of the flowing well is to cause a lag in the time required for the surface rate to equal the sand-face rate. For given reservoir properties, the magnitude of the delay depends on the wellbore volume, the nature of the storage phenomenon, and the skin effect. To our knowledge, the effect of wellbore storage on interference data was examined first in the groundwater hydrology literature.2,3 In petroleum engineering literature, Prats and Scott4 were the first to consider the effect of wellbore storage on the response of a pulse test. They concluded that the porosity-compressibility product, fct, of the formation would be overestimated and the hydraulic diffusivity, k/fctµ, would be underestimated if the wellbore storage effect was significant and not considered in the data analysis. Also, they noted that care should be taken in measuring pressure data if the wellbore storage effect was significant, because the magnitude of the actual pressure response would be lower than expected. They did not quantify the effect of a skin region around the well but did discuss qualitatively the influence of the skin factor. However, Prats and Scott assumed that the storage effect existed at the observation well and not the flowing well. In a subsequent paper, we show that their results can be used to analyze pulse test data influenced by the storage effect at the active well.