When does a longer shut-in lead to a larger radius of investigation?

While the concept of radius of investigation is better understood for drawdown tests, its applicability to buildup tests is less certain. For example, a rule of thumb is that “one cannot see a particular feature in a buildup unless the radius of investigation during the preceding flow period has seen that feature”. In this paper, we clearly illustrate that the radius of investigation of a buildup can be larger than that of its previous flow period.Another common contention is that the radius of investigation of a buildup is limited by noise dominating the late-time pressure behavior. Oliver [Oliver, D.S., 1990. The averaging process in permeability estimation from well-test data. SPEFE. pp. 319–324.] and later Thompson and Reynolds [Thompson, L.G. and Reynolds, A.C, 1997. Well testing for radially heterogeneous reservoirs under single and multiphase flow conditions. SPEFE. pp. 57–64.] defined the radius of investigation based on the distance from the well to the region of the reservoir which has the greatest impact on the pressure derivative. We have used this approach to calculate the derivative and show that the ratio of noise to the signal from the reservoir does not necessarily increase. While many situations exist when radius of investigation of a build-up test is limited by the preceding flow period and noise, we show that when data is sampled appropriately, the radius of investigation of a buildup can easily go beyond that of the preceding flow period, and demonstrate when this may remain unaffected by noise.