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. The 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 to the sandface (beginning of a drawdown or injection) 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, 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 new techniques 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 (Earlougher, 1977) for radial-cylindrical systems, which is given as (Equation), yields very conservative estimates, and it could be as high as 30 to 50% lower. Radius of investigation if fundamental for understating of 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 test (DST) or production tests. Therefore, how far pressure may diffuse (radius of investigation) during a transient test is very important for exploration well testing.

The challenge in estimating reserves from pressure transient well test data very often arises in oil and gas explorations as well as in other oil industry applications. Thus, determining radius of investigation during a pressure transient test becomes critically important. It may also be called transient drainage radius. 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. For instance, it is defined at http://www.glossary.oilfield.slb.com/ as the calculated maximum radius in a formation in which pressure has been affected during the flow period of a transient well test. This definition is not completely accurate when we apply an instantaneous source during which pressure may diffuse to a long distance. Therefore, to understand the radius of investigation, first we look at the pressure distributions in a 1D radial-cylindrical homogeneous reservoir produced by a fully completed vertical well, in which after the wellbore storage effect the flow regime is predominantly radial before the effect of any outer boundary. Note that this may not be true for wells in nonhomogeneous and heterogeneous formations and reservoirs. Nevertheless, understanding the fundamental radial flow regime is essential to interpreting pressure transient testing and its radius of investigation; *i.e.,* how much reservoir volume if investigated for a given duration of a transient test? For exploration wells, the reservoir volume investigated is one of the main objectives of running DST or production tests. Therefore, how far pressure may diffuse (radius of investigation) during a transient test is very important for exploration well testing where very important decisions are made based on total volume are seen by DST or other production tests.