Although it is often used in pressure transient testing, radius of investigation still is an ambiguous concept, and there is no standard definition in the petroleum literature. Pressure diffusion corresponds to an instantaneous propagation of the pressure signal in the entire spatial domain when a flow-rate or pressure pulse is applied (beginning of a drawdown or injection) to the sandface of a well. However, the initial pressure propagation is not diffusive but it propagates like a wave with a finite speed. If we have a pressure gauge at a distance r in a formation, we will only start to detect a pressure change (drop or increase) after a few seconds or minutes even if we have a perfect pressure gauge with 0.0 psi resolution. After the initial propagation, pressure starts to diffuse or propagates as diffusion, and we start to observe pressure change at a given space and time above the pressure gauge resolution and natural background noise, which could be as high as 0.1 psi. One of the constant background noises is the effect of tidal forces.
In this work, we present new formulae for radius of investigation in radial-cylindrical reservoirs and a new technique for general systems. The new formulation takes into account the production rate from the system, formation thickness, and gauge resolution. It is shown that the conventional radius of investigation formula , see Earlougher (1977)) for radial-cylindrical systems yields very conservative estimates. Radius of investigation is fundamental for understanding the tested volume; i.e., how much reservoir volume is investigated for a given duration of a transient test? For exploration wells, the reservoir volume investigated is one of the main objectives of running drillstem (DST) or production tests. Therefore, how far pressure may diffuse (radius of investigation) during a transient test is very important for exploration pressure transient well testing.