Fluid status and saturation assessment in low-resistivity-pay carbonate reservoir using core scale petrophysical and resistivity modelling. 9th Middle East Geosciences Conference, GEO 2010.

Abstract

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 3-D imaging, minipermeameter, porosity, permeability, capillary pressure); and (2) checking the accuracy of the model through forward modelling using a research 2-D resistivity modelling software that simulates 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 drill stem tests (DST) production watercut values. Core-scale model provides an average saturation of 51%, which allows reproducing field watercuts. The forward 2-D resistivity modelling based on this model reproduced the acquired wireline laterolog curves and derived true resistivity profile, after adjustment of water salinity, consistent with regional data. In the absence of any nuclear magnetic resonance (NMR) or SIGMA log acquisitions, low-resistivity pay intervals in this reservoir have been successfully characterized. This was possible because the complete coring and testing program permitted 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 thus leading to effective decisions for completion strategy.