T. Namba and T. Hiraoka, Zakum Development Company (ZADCO)
SPE Member
Abstract
This paper shows that the horizons with finite vertical permeability can act as barriers for the injected water and prevent water slumping due to the effect of capillary force in a carbonate reservoir. The proposed concept 'capillary force barriers' can explain the various monitoring results consistently including the data obtained from pulsed neutron logs and dynamic testing tools.
Introduction
Thamama II reservoir of Upper Zakum Field is a Lower Cretaceous carbonate reservoir developed by five-spot pattern water injection. The open hole logs and pulsed neutron logs conducted in the observation wells indicate that the injected water is preferentially advancing in relatively thin horizons. The water slumping appears to be prevented by the underlying horizons. However, the core data do not show the existence of horizons which can be barriers within the reservoir. Furthermore, dynamic testing tools show the continuous pressure profiles, indicating that the reservoir is hydraulically connected.
In this paper, the role of capillary force on water movement is highlighted. As studied numerically by Goddin et. al, capillary force accelerates crossflow in water-wet stratified reservoirs. However, in oil-wet reservoirs, capillary force can oppose crossflows of water as suggested by Willhite. It is shown in this paper that capillary force can prevent water slumping in the reservoir and the apparently inconsistent observations can be explained by introducing the concept of capillary force barriers.
CONCEPT OF CAPILLARY FORCE BARRIERS
The water-oil capillary pressure is defined as the pressure in oil phase minus the pressure in the water-phase, or
(1)
Thus, the capillary pressure can be either positive (water-wet) or negative (oil-wet) depending on the wettability preference.
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