Quantifying layered reservoir properties with a novel permeability test
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This paper presents a new test procedure designed to measure individual layer reservoir properties (permeability, skin factor, and reservoir pressure) in a single wellbore and in a reasonable amount of time for low permeability gas wells. The method involves (1) isolating a small interval with packers, either in an open- or cased-hole, (2) injecting nitrogen into the interval at a rate such that the bottomhole pressure builds up to a pre-determined value (below fracturing pressure), and (3) shutting in the well, preferably downhole, and monitoring the pressure falloff. The pressure falloff data is then analyzed to determine reservoir properties. Essentially, this method can be described as a nitrogen slug test and can be applied to any formation where single-phase gas is produced. We have found that average reservoir properties determined with conventional analyses of a single pressure buildup test conducted over a large interval can lead to optimistic predictions of the future performance of a well producing from a layered reservoir system. Therefore, being able to quantify individual layer properties can lead to better reservoir characterization, modified completions, and improved performance projections. Existing techniques, such as multiple pressure buildup tests, are inadequate for measuring reservoir permeability in several intervals in a single wellbore because, often in low productivity gas wells, either a pre-stimulation rate is not attained or the time required to reach the correct semilog straight line for a conventional well test analysis is too long. Knowledge of layered reservoir properties impacts completion decisions (where to perforate), fracture design considerations, and post-fracture analyses.
