Impact of SCAL (special core analysis) on carbonate reservoirs: How capillary forces can affect field performance predictions

Accurate capillary pressure data in both drainage and imbibition cycles can be essential for understanding hydrocarbon reservoir performance. Drainage capillary pressure is usually used to initialize reservoir static models, i.e., to determine initial saturation as a function of height above free water level and to calculate hydrocarbon volumes in place. However, imbibition capillary pressure is often not used correctly to model fluid flow in displacement studies. This is often due to a general lack of reliable experimental data to cover the predominant rock types during reservoir studies or the perception that effect of capillary force might become insignificant once production commences. In this paper we examine the effect of capillary forces, in addition to viscous and gravity forces, on sweep efficiency of immiscible displacement in a heterogeneous porous medium at different wetting conditions. Capillary pressure curves have been measured using core materials from a heterogeneous Cretaceous carbonate reservoir in the Middle East. The core plugs were selected from different rock classes to cover a range of permeability from 0.1 to 1000 mD. Capillary pressure data were obtained in primary drainage and imbibition after aging the plugs at connate water saturation to restore wettability. The results show, for the case under study, that there is a subtle balance between viscous, gravity and capillary forces during oil displacement. Ignoring any of these forces, especially for non-water-wet heterogeneous reservoirs, could lead to erroneous prediction and sub-optimal field development planning. The study shows that for the subject carbonate reservoir, water-flood recovery is strongly dependent on the shape of the imbibition capillary pressure curves.