Schlumberger

Technical Paper: Dielectric Dispersion Logging in Heavy Oil: A Case Study from The Orinoco Belt

Society: SPWLA
Paper Number: 2009fe
Presentation Date: 2009
 

Abstract

The Magna Reserva Project aims at certifying the entire heavy oil reserves of the Orinoco belt. Recent results show that, as the drilling progresses away from producing blocks, conditions become increasingly complex. Reservoirs can be thinly bedded, generating low­resistivity pay intervals. Water salinities become fresh and variable, making reservoir saturation difficult to assess. Since Orinoco oils are much more viscous than water, zones of water production must be identified. Previous experience shows that resistivity and water cut are not linked: layers of similar resistivity have been tested to produce either oil or water.

Faced with these interpretation challenges, PDVSA pioneered the use of a new dielectric tool while seeking more accurate petrophysical models in heavy oil reservoirs with changing water salinity.

The tool is pad­mounted with two transmitters and eight symmetrically located receivers working in longitudinal and transversal mode. It acquires radial information up to 4 inches into the formation. The tool measures dielectric properties at four frequencies between 20MHz and 1GHz, providing a measurement of the dielectric dispersion at a 1-­inch vertical resolution.

A radial inversion is performed using all measurements, each weighted with its uncertainty. The radial model is a pad in the borehole, facing a mudcake and an invaded zone ramping into a virgin zone. The outputs are conductivities and permittivities of each layer at each frequency, and layer thicknesses. A petrophysical model transforms these results into petrophysical parameters for each layer. Acquisition uncertainties are included in the inversion and propagated to provide uncertainties on the final results.

Invasion in the Orinoco belt is shallow and this dielectric measurement investigates the entire region from the invaded to the virgin zone. The inversions provide water-­filled porosity and salinity in the invaded and virgin zones, quantifying oil in place at high vertical resolution, independently of water salinity.

Changing water saturations between the two zones implies oil displacement which can only occur at irreducible water saturation in heavy oil reservoirs. Water salinities are estimated even in hydrocarbon bearing layers, giving information on the significant water movements that have occurred in the area under study. Results obtained in the Junin block are presented and compared with test data. These results confirm that a number of low­resistivity layers, previously overlooked, are in fact oil bearing and must be included in the reserves estimate.

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