In this paper we present dynamic and static elastic properties obtained for three brine-saturated carbonate reservoir samples recovered from an oil field in the Campos basin, offshore Brazil. The static properties were determined through the performance of stress cycles at the early stage of drained triaxial tests, before yield commenced. Two methodologies were used: (A) averaging all the stress-strain data acquired throughout the cycles; (B) considering only data from the unloading sections. The dynamic data was acquired concomitant to the mechanical loading on each test. Our most accurate ratios, from the methodology B, indicate that Edyn is ∼60% higher than the Esta, and that υdyn is ∼20% higher than the υsta. The importance of acquiring static properties from stress cycles instead of from the virgin loading curve is also discussed.
It has been extensively reported in the literature that the static moduli of porous rocks determined from static stress-strain data can differ significantly from the dynamic moduli derived from ultrasonic wave velocity measurements. Several attempts to explain this contrast have been reported in the literature. Often, time-dependent viscoelastic or elastoplastic deformation is invoked in relation to the magnitude of applied strain. In the case of fluid-saturated reservoir rocks, frequency effects associated with the mobility of the fluid within the pore space can also be operative.
Additionally, the methodology chosen for the measurement of the static properties makes a significant difference in such a comparison. Previously published studies show that performing unloading-reloading cycles at a given stress state produces static measurements more representative of the rock elastic behaviour. Nevertheless, most of those studies have focused on the investigation of clastic rocks, essentially sandstone reservoirs, and little experimental data have been reported on carbonate rocks.
In this article, we will discuss the dynamic to static ratios of elastic properties, namely Young's modulus (E) and Poisson's ratio (υ), determined for post-salt carbonate reservoir samples recovered from an oil field in the Campos basin, Brazilian offshore. The samples lithofacies was described as peloidal packstone with bioclasts and microoncoliths. The elastic properties were calculated from standard drained triaxial tests in which unloading-reloading cycles were performed, at stress levels still far from the yield commencement, with the concomitant acquisition of the response to active ultrasound signals. The study was performed within the context of a research project, IGEM-4D, a collaboration between CSIRO and Petrobras, developed from 2011 to 2013. The project aimed at evaluating petrophysics, geophysics and geomechanics aspects of carbonate reservoirs from the Campos basin.
