The importance of nuclear magnetic resonance (NMR) to characterize the porosity, permeability and facies properties of carbonates geothermal reservoir
Résumé
In this study, Nuclear Magnetic Resonance (NMR) measurements were combined to air permeability measurements and petrographic in order to better constrain the relationship between NMR relaxation time T, porosity and permeability in marine carbonates rock. 45 Bathonian limestones samples from six quarries located in the Aquitaine Basin (southwest of France, near the city of Angoulême), have been chosen for their petrographical and petrophysical characteristics analogue to the geothermal reservoir of the Oolithe Blanche in the Paris Basin. Thin-section have been observed under microscope to provide information about the facies and the diagenetic processes that affected the carbonates during burial. Six facies have been recognized, typical of depositional environments distributed on a prograding oolitic platform. From these samples, 45 air permeability measurements show a permeability (k ) range from 0,014 mD to 1505 mD. Water porosity experiment display a range of porosity (Φ) values from 3,3% to 21,7%. To complete this database, NMR relaxation time T (ms) were measured on each sample to: i) analyze the pore structure of these carbonates, ii) identify the dominant porosity type between microporosity and macroporosity, and, iii) better constrain the relationship between T distribution, porosity and permeability. A cut-off value was settled at T 120 ms to separate microporosity from macroporosity. The NMR analysis highlighted that: i) microporosity is the dominant type of porosity for each facies, ii) it exists different trends for microporosity within each facies: a) trend to low T values, b) trend to high T values, and c) no trend; and, iii) some samples do not follow the general trend within the same facies. Comparison between T distribution, porosity and permeability shows that: i) permeability-porosity relationship follows trends but there are some exceptions, these trends are: a) most of the highest permeability values are related to high porosity values and, b) the lower is the permeability, the lower are the porosity values; and, ii) microporosity, isolated by the cut-off value of T relaxation time, shows too much variability in the results obtained. It is well known that diagenesis have an impact on petrophysical and petrographical properties of carbonates. Consequently, it is not easy to estimate, for a same facies, the relationships between permeability and NMR in these rocks, without considering diagenetic processes (dissolution, dolomitization, dedolomitization, calcite cementation). By paying more attention to macroporosity, which seems to be a solid basis for further studies, it would be possible to overcome these difficulties in order to better constrain the various relationships that may exist within carbonates properties and establish predictive models of permeability. This study provides important information to guide the use of NMR to characterize reservoir quality.
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