--> Abstract: Fluid Status and Saturation Assessment in Low Resistivity Pay Carbonate Reservoir, Using Core Scale Petrophysical and Resistivity Modelling, by Francois Umbhauer, Jean Pierre Leduc, Eric Guyotte, and Abubaker ElSadawi; #90105 (2010)

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AAPG GEO 2010 Middle East
Geoscience Conference & Exhibition
Innovative Geoscience Solutions – Meeting Hydrocarbon Demand in Changing Times
March 7-10, 2010 – Manama, Bahrain

Fluid Status and Saturation Assessment in Low Resistivity Pay Carbonate Reservoir, Using Core Scale Petrophysical and Resistivity Modelling

Francois Umbhauer1; Jean Pierre Leduc1; Eric Guyotte2; Abubaker ElSadawi3

(1) Well characterisation and operations, TOTAL S.A, Pau, France.

(2) Geoscience, Mabruk Oil Company, Tripoli, Libyan Arab Jamahiriya.

(3) Ecole Nationale de Geologie, Nancy, France.

Fluid status definition for completion strategy, or reliable saturation evaluation for hydrocarbon in place and reserves estimation in heterogeneous carbonates reservoirs, based on wireline resistivity logs, is often uncertain as these reservoirs often deviate from Archie’s law.

This can be explained by small scale heterogeneities (patchy macroporous oil zones embedded in a microporous matrix) which can generate an excess of conductivity (electrical bypass through water saturated microporous connected path). Applying Archie’s law with the standard m and n =2 values in such cases may lead to erroneous water saturation computation, with serious consequences on fluid status, completion strategy and project economics.

A methodology has been developed to tackle this problem by: 1) modelling reservoir heterogeneity at core scale using commercial geomodelling software, providing a reference oil volume calibrated by core petrophysical data (CT scan 3D imaging, minipermeameter, porosity, permeability, capillary pressure) ; 2) checking the accuracy of the model through forward modelling, using a research 2D resistivity modelling software simulating the invasion process, by comparison with the acquired wireline resistivity logs response.

This methodology was applied to a vuggy interval (“leopard-skin texture”) of a carbonate reservoir of Paleocene age, in the Sirte Basin, onshore Libya. The water saturation derived from resistivity logs interpretation with standards Archie’s parameters reached 77%, not consistent with DST production watercut values. Core scale model provides an average saturation of 51%, which allows reproducing field watercuts.

The forward 2D resistivity modelling based on this model lead to reproduce acquired wireline laterolog curves and derived true resistivity profile, after adjustment of water salinity, consistent with regional data.

In the absence of any NMR or SIGMA log acquisitions, Low Resistivity Pay intervals in this reservoir have been successfully characterized thanks to a complete coring and testing program, allowing a good calibration of petrophysical and electrical properties, an accurate modelling of reservoir heterogeneities, and a successful core-log upscaling process. Providing equivalent zones could be diagnosed through well data (log imagery) and geological knowledge (correlations) along uncored well sections, resistivity derived saturation can be corrected and efficient decision for completion strategy taken.