Within the analysis and search of an efficient secondary recovery it is essential to be able to express in the monitoring indicators of the company, the injection rate in terms of poral volumes contacted by injected water, and with that being capable of defining optimal sweep flows. In this regard, a debate is opened about how these contactable volumes must be estimated, through which method, in which production conditions: is it necessary to carry out the numerical simulation to obtain the reservoir model? Or can a capacitive-resistive model be accepted, or a Neuronal Training Network, or using a 2D model alongside the theorical patterns definition. Such methods generally consider neither pressure effects nor mass balance and matter conservation. In addition, not all of these are considered by practice communities as highly as numerical simulation. In the way of simplification, these methods are usually used to calculate static scenarios, which do not consider the interaction between injection and production flows in different pressure conditions both on the surface and underground. This could be a poor and i ncomplete analysis. It is main objective of this job to focus on obtaining a contactable volumes calculation methodology with numerical simulation, having the capacity to show the obstacles that have been overcame to achieve their estimation, report and interpretation. Among the process, considerations to reach a representative scenario of sweep zones by injection are described, such that they were reliable in obtaining of these volumes, as well as the optimal injection flow. The case in study is "Chachahuén Sur" Field, the development of the field began in 2013, after the exploratory and extension drilling campaign targeted at Rayoso Formation, a succession of early Cretaceous fluvial cycles. As of July 2021, 395 wells have been drilled. 76 of them are injectors wells and 47 more are expected to be converted. It must be overstated that all information obtained from the calculation was extremely important for planning the field development and the required investment level in surface installations to manage the production and injection flows.