Abstract
Distance of investigation equations are often used, explicitly or implicitly, in rate transient analysis. One important application is for estimation of distance to boundary and its use for well-spacing optimization. The concept of radius of investigation has been used successfully in conventional reservoirs, and has been extended to unconventional reservoirs. Here, we clearly demonstrate the inconsistencies that arise from such extension, and develop concepts and methodologies to remedy such inconsistencies. In particular, we confirm that in radial systems, infinite acting behavior as observed at the production well is consistent with infinite-acting behavior in the reservoir. In other words, i.e. deviation from straight-line behavior on specialized plots is observed shortly after a minimal amount of depletion has occurred at the far boundary. This makes use of the concept of radius of investigation straight forward; a concept that suggests there should be an insignificant pressure drop at the radius of investigation. However, this is not the case for linear flow. The transient behavior as observed in production data is NOT consistent with infinite-acting behavior in the reservoir. The deviation from straight-line behavior on specialized plots is observed much later than end of infinite acting in the reservoir, when there is significant depletion in the reservoir. We demonstrate how misleading it would be, if the concept of distance of investigation is extended to this situation. To remedy this, "time of detection" is introduced as a practical way of relating these theoretical concepts to the practical questions of determination of drainage area for well- and fracture-spacing applications. We demonstrate the use of these concepts through a couple of thought experiments.
