Method for estimating either flowback or the reservoir fluid production rate from either one individual inlet or the contribution from several inlets separated by intervals in a wellbore located in an oil and/or gas reservoir (Ω), said wellbore (W) being extended along a path (P) at least from a point of the surface of the reservoir (Ω) until a point in the subsurface, the wellbore (W) comprising: - a casing (T) comprising a plurality of inlet ports (I) distributed according to a first arrangement along the path (P) allowing the entrance of fluid into the casing (T); and, - delivering at least one sensor element (S) in the wellbore (W) within the casing (T) during either flowback or the production of fluid at a location of the wellbore (W) located in the subsurface and where the fluid flows, being the sensor element (S) dragged by the flow and wherein the velocity of the sensor element is almost the velocity of the surrounding fluid; - collecting the recorded data from the at least one sensor (S) element; and - determining the estimation of the velocity of the fluid flow as the velocity of the sensor element being dragged by said flow of either flowback or the production of fluid; wherein the method is characterized in that : - the casing (T) comprises a plurality of magnetic irregularity elements (C) distributed according to a second arrangement along the path (P); - the at least one sensor element (S) is adapted to record in time the magnetic perturbation generated by a magnetic irregularity element (C) when the at least one sensor element (S) is moving within the casing (T) and passing through a section of the wellbore (W) where the said magnetic irregularity element (C) is located according to the path (P) direction; and, wherein the method further comprises: - recording with the at least one sensor (S) the magnetic perturbations caused when the sensor element (S) dragged by the flow passes near each magnetic irregularity element (C) with known spacing between them. - a casing (T) comprising a plurality of inlet ports (I) distributed according to a first arrangement along the path (P) allowing the entrance of fluid into the casing (T); and, - delivering at least one sensor element (S) in the wellbore (W) within the casing (T) during either flowback or the production of fluid at a location of the wellbore (W) located in the subsurface and where the fluid flows, being the sensor element (S) dragged by the flow and wherein the velocity of the sensor element is almost the velocity of the surrounding fluid; - collecting the recorded data from the at least one sensor (S) element; and - determining the estimation of the velocity of the fluid flow as the velocity of the sensor element being dragged by said flow of either flowback or the production of fluid; - the casing (T) comprises a plurality of magnetic irregularity elements (C) distributed according to a second arrangement along the path (P); - the at least one sensor element (S) is adapted to record in time the magnetic perturbation generated by a magnetic irregularity element (C) when the at least one sensor element (S) is moving within the casing (T) and passing through a section of the wellbore (W) where the said magnetic irregularity element (C) is located according to the path (P) direction; and, - recording with the at least one sensor (S) the magnetic perturbations caused when the sensor element (S) dragged by the flow passes near each magnetic irregularity element (C) with known spacing between them. <EOS>