Schlumberger

Technical Paper: Application of a New Array Dielectric Tool to the Characterization of Orinoco Belt Heavy Oil Reservoirs

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Presentation Date: 2008
 

Abstract

The Magna Reserva Project aims at the certification of an estimated 236 billion barrels of reserves found in 27 blocks of the Orinoco Belt, covering a surface area of 18,220 km2. The project contemplates volumetric quantification of oil in place and mapping of the type and quality of crude oil to optimize recovery.

Preliminary log and core results show that, as the drilling of stratigraphic wells progresses away from blocks already in production, reservoir conditions become increasingly complex. Thinly bedded reservoirs and lower resistivity pay zones have been encountered. Fresh water, varying water salinities, biodegraded oil, tar and oil reservoirs that are not irreducible saturation have all been seen.

In an environment where oil can be 10,000 times more viscous than water, it is indispensable to predict zones of potential water production. Previous experience in the Orinoco belt has shown that resistivity and water cut are not directly linked and that medium resistivity layers can produce either oil or water.

For these reasons, PDVSA elected to participate in the field test of a new Array Dielectric device that represents a radical departure from earlier tools of the 1970’s and 1980’s. It is a pad-mounted tool consisting of two transmitters one inch apart and eight symmetrically located receivers transmitting simultaneously in longitudinal and transversal mode. Each transducer consists of two co-located magnetic dipoles, highly isolated and perpendicular to each other. Two electric dipoles measure the dielectric properties of the mudcake. The symmetric design allows full borehole correction and compensation for pad tilt. The tools measures dielectric properties at three frequencies between 100 MHz and 1 GHz.

By inverting the array data, the effect of the mudcake can be quantified and accounted for, giving the tool a depth of investigation that penetrates from 1 inch to 4 inches into the formation. As the heavy oil sandstone formations found in the Orinoco Belt rarely experience an invasion greater than two to three inches, the measurement spans the entire range from invaded to virgin zone, in spite of the inherent shallow penetration of dielectric measurements.

Using a suitable invasion model, the data can be inverted to give water-filled porosity and water salinity at 1 inch and 4 inches into the formation, with a vertical resolution of 1 inch. This allows quantification of oil in place at a previously unattainable vertical resolution, independently of water salinity, resolving the thin bed analysis and low water salinity problems simultaneously. Comparison of water saturations at 4 and 1 inch into the formation confirms that a heavy oil bearing sand is at irreducible water saturation or not, an essential information when steam injection projects are planned.

Knowledge of formation water salinity can be obtained even in layers that contain hydrocarbons, giving precious reservoir information regarding the significant water movements that have occurred in the vast area under study.

Results obtained on several wells logged with the tool in recent months in the Junin and Carabobo blocks are presented and discussed.

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