--> Acoustic Characterization of Sub-Seismic Scaled Heterogeneities in Carbonate Reservoirs: Example of Barremian Microporous Limestone From SE France: An Analog of Middle East Hydrocarbon Reservoirs

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Acoustic Characterization of Sub-Seismic Scaled Heterogeneities in Carbonate Reservoirs: Example of Barremian Microporous Limestone From SE France: An Analog of Middle East Hydrocarbon Reservoirs

Abstract

Carbonate rock physics properties are difficult to assess due to their biogenetic diversity and the overprint of a complex diagenetic history which can alter porous carbonates porosity during the whole life of these reservoirs. Common scales to characterize the latter are seismic profiles and wells log data. Despite seismic methods fail, in some cases, to characterize reservoir units for example because of seismic detection limit (≈20 m) or low impedance contrast between two lithified carbonate units, good relationships have been established between elastic properties and reservoir properties of these reservoirs. Nevertheless, elastic properties often in-lab determined, are measured on small-scaled (≥ 2 inch.) samples which are not always representative of the heterogeneity of the sampled units, leading in some cases to wrong interpretations. In this study, we propose to assess acoustic properties at an intermediate scale in terms of both seismic frequency (SF) and investigated area (IA), between seismic scale (SF:101-102 Hz, IA: km) and laboratory scale (SF:104-106 Hz, IA: cm). We designed on purpose, two acoustic probes which enabled us to perform crosshole acoustic measurements up to ultrasonic frequencies (SF: 102-104 Hz), between two wells located in a gallery from an underground laboratory (LSBB†, France). Confining pressure within the wells is ≈4-to-6 MPa and studied formation are located in the un-saturated zone of the aquifer. The wells were 2 m spaced and 20 m deep. Here we present a HR acoustic log (Δz = 10 cm) covering an inter-wells area of 30 m2 composed of tight and microporous reservoir units of Urgonian platform limestones. The key results of this experiment are: (1) Sub-seismic scaled heterogeneities such as diagenetic horizons within a same sedimentary facies or layer and related to porosity variations up to 10% can be successfully characterized using our methodology; (2) Potential diagenesis-induced seismic reflectors can be identified between and within layers under conventional seismic detection limits; (3) Tight units, regardless to their composition, display different fractures density, they show the same average P-wave velocity but scattering of the values is a function of the underwent deformations. Finally, coupling in situ HR acoustic log to conventional lab measurements can fill the gap in terms variations of acoustic response observed between field seismic and synthetic profiles used for the inversion of reservoir properties.