Well performance analysis using 4D seismic

Well integrity and reservoir seal integrity are critical parameters for a hazard-free and cost effective production from a hydrocarbon reservoir or injection of CO2 into a geologic formation. Still given the natural variability of geo-mechanic properties in the subsurface and limited measurements to constrain these, seal breaches and out-oftarget injection around an injector well can occur. In such an adverse situation it is very important to get an overview over the affected area in the overburden, in order to know what kind of corrective measures in the production strategy have to be taken and/or in order to adapt the remaining drilling program for the field to avoid possible overpressured zones.

In this paper we show an example where an injector well generated an over-pressured region in the overburden. Higher pore pressure in the overburden results in decreased seismic velocities and hence time-lapse seismic could be used as a potential tool to image the affected region. We demonstrate that by estimating the apparent displacement of the seismic events and inverting these to relative changes in the velocity, we were able to image the over-pressured zone in the overburden. In particular, velocity changes less than  v/v~1% could be detected. Such a high sensitivity can only be reached with very good repeatability of the time-lapse seismic acquisition and an efficient estimation scheme that provides good displacement estimates with adequate resolution. Hence, we look in this paper also at the relation between the signal to noise ratio, the seismic bandwidth and the uncertainty of the estimated displacements. It should be noted that all conclusions drawn on the case study are the authors opinion and do not reflect any viewpoint of the license team.