Dielectric Scanner Service Reliably Identifies Heavy Oil Reservoir in Thin Beds

Published: 12/08/2011

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Low-resistivity effect of thin beds

Where thinly laminated reservoir layers are intercalated with conductive nonreservoir layers, the apparent formation resistivity is significantly reduced, which biases conventional petrophysical evaluation: The apparent clay volume is increased, and the calculated hydrocarbon volume and permeability are underestimated.

An operator was faced with this challenge in Venezuela for a thinly bedded interval overlying a thicker clean-sandstone reservoir. The induction log accurately measured formation resistivity in the thick sandstone reservoir, but exhibited a pronounced decrease in the section laminated with thin conductive clay beds. The resulting water saturation determined from conventional porosity-resistivity analysis was in turn overestimated.

Graphic: Dielectric Scanner Service Reliably Identifies Heavy Oil Reservoir in Thin Beds
The articulated pad of Dielectric Scanner service greatly improves contact with the formation in rugose boreholes, a condition that previous mandrel-type electromagnetic propagation tools were sensitive to.

High-resolution water-filled porosity, independent of salinity

Dielectric Scanner multifrequency dielectric dispersion service measures high-resolution permittivity and conductivity at four frequencies to provide dielectric dispersion at a 1-in [2.54-cm] vertical resolution. Because there is a large difference in the permittivity of water from that of rock matrix or hydrocarbons, the resulting determination of water-filled porosity is independent of formation resistivity, water salinity, and estimated Archie electrical parameters. The conductivity and permittivity measurements provide reconstructed shallow resistivity, water salinity, and waterfilled porosity at a 4-in depth of investigation.

Dielectric Scanner service also provides textural information for determining the Archie mn exponents for carbonates and cation exchange capacity (CEC) for siliciclastics instead of relying on potentially incorrect estimations from conventional log analysis or waiting for laboratory core analysis.

High-quality reservoir within thin beds

In this heavy oil reservoir, there is very shallow or no invasion. In these conditions, the shallow dielectric water-filled porosity measurement is made in the uninvaded zone, resulting in an accurate determination of fluid saturations regardless of bed thickness. Conventional porosity-resistivity evaluation significantly underestimated the hydrocarbon volume in the thin beds, but in the thick reservoir section below X,X17 ft, the conventional and Dielectric Scanner service's saturation determinations match (Track 2).

Graph: Dielectric Scanner Service Reliably Identifies Heavy Oil Reservoir in Thin Beds
The conventional resistivity-based determination in Track 2 significantly underestimates the residual heavy oil saturation because of the thin-bed effect, but the oil saturation determined from Dielectric Scanner service reveals that these thin beds are similar to the thick sand reservoir below X,X17 ft.

Dielectric Scanner service's water-filled porosity measurement also identified the potentially high reservoir quality of the thin sands. The location and thickness of each of the thin sands in Track 5 is confirmed by the resistivity image from the FMI fullbore formation microimager in Track 9. In consideration that the large hydrocarbon volume measured in each individual sand is identical to volume measured the thick sand section, the thin sands are probably clay-free, porous, and permeable and therefore constitute additional net pay thickness.

Resistivity anisotropy logs from Rt Scanner triaxial induction service and NMR logs from MR Scanner expert magnetic resonance service are also available for this well for confirmation of Dielectric Scanner service's thin-bed analysis. Resistivity anisotropy (Track 5) evaluates the bulk hydrocarbon volume (Track 2), and NMR identifies the volume of the high-viscosity oil as a short node (approximately 3 ms) of the longitudinal relaxation time (T1) distribution (Track 10). Any discrepancies with the dielectric measurements result from the much lower resolution of the resistivity and NMR measurements. The agreement of these three methods confirms the the accuracy of Dielectric Scanner service's determination of hydrocarbon volume in both the thin beds and the underlying thick reservoir section below X,X17 ft, whereas the conventional porosity-resistivity evaluation significantly underestimates the hydrocarbon volume in the thin beds.

Venezuela, South America, Onshore

Challenge: Evaluate the hydrocarbon volume and the reservoir quality potential of a thinly laminated clastic reservoir where intercalated clays and fresh formation water bias the resistivity measurements used in conventional saturation determination.

Solution: Obtain Dielectric Scanner multifrequency dielectric dispersion measurements to directly determine high-resolution water-filled porosity as the basis of accurate saturation determination in the inches-thick sand beds.

Results: Measured the water-filled porosity of the individual thin beds to calculate a large hydrocarbon volume that indicates that the sands are a high-quality reservoir.