Impact of Capillary Forces on Residual Oil Saturation and Flooding Experiments for Mixed to Oil-Wet Carbonate Reservoirs

Shehadeh K. Masalmeh, Shell Technology Oman

This paper was prepared for presentation at the International Symposium of the Society of Core
Analysts held in Aberdeen, Scotland, UK, 27-30 August, 2012

Abstract:
An accurate determination of residual oil saturation (S_orw) has significant importance for managing ongoing waterflooding and the evaluation of the potential for tertiary EOR options. Oil trapping during immiscible displacement in porous medium is caused by capillary forces acting within the pore network. The trapped oil can be mobilized once viscous or gravity forces acting on the trapped phase exceeds the capillary forces. Numerous experiments have been conducted and reported in the past to correlate capillary or Bond number with S_orw. Most of the measurements have been performed on strongly water-wet porous media i.e, sand packs, outcrop or reservoir sandstone samples. For waterwet rock, S_orw is very well defined and insensitive to flooding rate and the critical capillary or Bond number required for mobilization of S_orw is in the order of 10^-5.

Relatively little data is available in the literature for carbonate rock and non-water-wet porous media. For non-water-wet systems, experimental artefacts due to the interference of relative permeability and capillary pressure (capillary end effect) may lead to remaining oil saturation (ROS) that is more than 15 saturation units higher than the true S_orw. Consequently, the waterflood recovery factor can be underestimated while the target oil for tertiary EOR processes is overestimated. It also leads to underestimation of the critical capillary or Bond number required to mobilize S_orw and a poor design of the EOR process.

This study is focused on the measurement of S_orw in carbonate reservoir samples from different rock types. For carbonates, the combination of wettability, wide pore size distribution and heterogeneity, adds to the complexity of determining S_orw and magnifies the experimental artefacts. The main conclusions of the study are:
1- Capillary end effect can dominate the results of the experiments (especially on short plugs) performed at reservoir rates of around 1 ft/d which leads to significant overestimation of S_orw and suppression of water permeability end point;
2- The relative contribution of capillary end effect increases as the permeability increases especially for heterogeneous carbonate rocks;
3- The critical capillary or Bond number of non-water-wet carbonates is much higher than those reported in the literature for water-wet sandstone; thus experiments can be performed at higher rates than those expected in the field without de-saturation of S_orw.
4- Once equilibrium between capillary and viscous (or gravity) forces is established, the remaining oil saturation is independent of the number of pore volumes injected.

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The name of this organization is the Society of Core Analysts (SCA) and was founded in 1986. This organization is a Chapter-at-Large of The Society of Petrophysicists and Well Log Analysts (SPWLA) and is established with two objectives:

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