Radius of Investigation in Pressure Transient Testing

Abstract

The spatial domain of interpretation in pressure transient and formation testing is constrained by its radius of investigation. A classical descriptor for this radius relies on the time derivative of pressure within a bounded domain becoming time independent. Another approach quantifies the radius of investigation based on the distance at which the pressure change is greater than the characteristic noise or resolution in the measurement system. In this paper, we postulate that the correct measure of radius of investigation is the distance at which the inversion of noisy test data is able to discern an anomaly within a given tolerance. This paper details the design requirements for a testing and measurement system suitable for a given investigation distance, measurement noise, and the confidence requirements for the inferred distance. The approach is demonstrated with two specific models. The first is a formation with a sealing fault in an infinite medium, and the second is a formation with a single-layer boundary vertically displaced from a production probe. With normally distributed random and correlated noise, computed results show that the measure of distance investigated is highly dependent upon the background over which the anomaly is imposed. Although the methodology is illustrated with particular models, the principles are applicable in general.