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Variations in Formation Water Salinity and Its Bearing from Oil API in a Shallow Heavy Oil Bearing Sandstone Reservoir in Ratqa Field, North Kuwait
(1) Exploration & Development, Ministry of Oil, Kuwait, Kuwait.
The extensively drilled, Lower Fars Formation of Miocene age, hosting heavy oil in the Ratqa Field of North Kuwait, comprises alternations of fluvial to estuarine channel sands and associated overbank shales, ranging in depth from 260 feet in the South to 550 feet in the North, capped by a shale, considered the regional top seal. The formation thickness ranges from 750 feet in the South to 900 feet in the North. The sands are disposed along a relatively gently sloping SW-NE structural monocline, without any observable structural or stratigraphic entrapment. Large variations in formation water salinities have been observed from well testing and from log derived estimations. Presence of very low salinity water at shallow depths of 200-300 feet and enormous increase in the salinity with depth, at places, indicates possible contamination of formation water having over 100,000 ppm as NaCl concentration, with a relatively fresh water.
This paper presents the results of an innovative study based upon an integrated analysis of formation water salinity and oil API gravity distribution across the field. Log derived salinity and porosity measurements from 84 wells used to map formation water salinity and porosity variations across the field, indicate presence of permeability compartments in the reservoir, attributable to the possible presence of faults and/or lithofacies variations. Mapped Oil API gravities ranging from 18 to 10 degrees, across the field, indicate a general deterioration, towards the South and SW. Observed trends in API and salinity distributions are indicative of a causal relationship. Geological sections constructed from log correlation profiles reveal that gravity driven fresh water incursions, downdip, from the South and SW to the North and NE, are responsible not only for a SW-NE regionally tilted oil water contact, but also for water-washing and further degradation of the heavy oil, in the South and Southwest. It is further postulated that updip migration of oil is arrested by the downdip counter movement of formation water, facilitating hydrodynamic entrapment of the oil. This is supported by large variations in the formation water temperatures in the fresh water swept, high porosity and permeability corridors of the reservoir, indicating breaching of the shales at places. In the absence of seismic control, formation water salinity variations yield useful insights into the hydrodynamic setting and reservoir character.