Rock-physics modeling of stress sensitivity and 4D time shifts in patchy cemented sandstones — Application to the Visund Field, North Sea

Abstract

The patchy cement model predicts stress sensitivity in poorly to moderately consolidated sandstones using a multiply nested Hashin-Shtrikman approach combined with contact theory. This model can be used to quantify stress sensitivity in reservoir sandstones where core samples are lacking or unreliable. We demonstrate a strategy based on this model that can be used to quantify 4D seismic time shifts and time-shift derivatives associated with stress changes. The time shifts will be a function of the rock stiffness (i.e., volume of patchy cement); therefore, it is important to determine the local changes in rock properties before we can predict stress and fluid changes from time-shift attributes. We apply this approach to four exploration wells in the Visund Field area where we have good control on the rock properties. We first compare modeled time shifts with observed 4D time shifts in the Visund S1 segment where pressure data are available, and we obtain a very good match at two selected well locations. Next, we use the observed seismic time shifts as constraints to estimate the pore-pressure changes in the Pan and Pandora segments just south of the depleting S1 segment. Pressure changes are significantly lower than in the S1 segment but still indicate potential pressure communication between these segments and the main Visund, in spite of fault barriers and deeper fluid contacts.