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Accueil du site > Productions scientifiques > Séminaires à PHENIX > 2012 > Séminaire 05.10.2012 à 11h00

Séminaire 05.10.2012 à 11h00

par Benjamin Rotenberg - 28 septembre 2012

Mei Han, du Schlumberger Riboud Product Center, présentera un séminaire le 5 octobre 2012 à 11h00 dans la bibliothèque du laboratoire PECSA (7e étage, bâtiment F, porte 754) intitulé :

Continuous estimate of cation exchange capacity from log data : a new approach based on dielectric dispersion analysis

Résumé

The effect of clay on resistivity has been for many years a major difficulty in the interpretation of shaly-sand reservoirs. Waxman and Smits first formalized the problem in a fundamental paper in 1968, and set the basis for many further works, building upon their initial approach. The common foundation remains the essential role of the cation exchange capacity (CEC) in the modelization of the clay effect on formation resistivity. Nonetheless, more than ten years after the introduction of the Waxman-Smits model, Istvan Juhasz (1981) pointed out that the CEC still could not be determined directly from logging data. This situation has largely remained the same up to now.

Until today, CEC measurements have only been performed in the laboratory. Recently, the analysis of the dielectric dispersion made possible by a new wireline tool holds promise for continuously estimating the CEC from its direct effects on conductivity and permittivity dispersion with frequency. A higher CEC leads to higher dispersion.

The new tool measures dielectric dispersion over four frequencies ranging from tens of megahertz up to one gigahertz. Many physical phenomena occur close to the clay interface, from double layer polarization to bound water freezing. They are characterized by the bulk clay parameter or CEC. These effects are particularly significant at low formation water salinity. A comprehensive description of these effects would require building a model that involves many microscopic parameters and assumptions. Our approach is an alternative to overcome this complexity by including a highly dispersive phase representing the CEC effects in the formation model and empirically adjusting the characteristics of this phase based on core data.

This approach has been validated on core measurements from several wells and subsequently applied on 2 wells drilled in Girasol field, Colombia, operated by Mansarovar in association with Ecopetrol. The well is representative of reservoirs drilled on the western flank of the Middle Valley of the Magdalena River. A continuous estimate of CEC is derived from measured dielectric dispersion and compared with laboratory measurements performed on rotary cores selected to cover a wide range of CEC values. Agreement between the measured values and values estimated from the log is very good, opening the long sought after possibility of providing a continuous CEC estimate directly from logs. We believe that the present formalism will be applicable to a wide range of shaly sand formations, after suitable local calibration on cores.

Beyond the obvious interest that a continuous CEC holds for petrophysicists seeking to apply a shaly sand saturation equation, we also believe that such an estimate holds considerable interest for all disciplines that need to account for the presence and effects of different clay types, including drilling and completion.

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