SPE Lecture- May 2016: Assessment of mechanical weakening induced by fluid injection in a poorly consolidated sandstone

This talk presents a novel rock mechanics testing method for remotely monitoring, detecting and assessing mechanical weakening and possible sand production in sandstone reservoirs during water flooding or during drilling with water-based muds. Rock mechanics triaxial tests are conducted on sandstone reservoir samples subjected to realistic in situ stresses, pore pressure, and temperature. During these tests, in addition to the conventional stress and strain monitoring techniques, the new approach involves a combination of ultrasonic and micro-seismic monitoring techniques allowing for a remote detection and spatial localization of the possible mechanical weakening that ultimately leads to sand production. The original testing protocol allows for the substitution of the in situ fluid by a water-based fluid with minimal impact on the in situ effective stress in the rock sample. The weakening effect of the substituting fluid can therefore be discriminated from any mechanical artefact associated with a possible reduction in the effective pressure during the substitution.

Thanks to the conventional stress-strain monitoring technique, it has been observed that (i) the strength and static elastic moduli decrease when water saturates the pore space; (ii) the injection of water in a critically loaded sample initially saturated either with air or inert oil induces a drastic increase in the creep rate and the development of a mechanical instability. Thanks to the ultrasonic and micro- seismic monitoring of the fluid substitution process, it has been observed that (iii) the mechanical instability is mainly localized in the water-flooded zone; (iv) this instability corresponds to a loss of cohesion between the sandstone grains; (v) during oil substitution by water, the instability is delayed in comparison with the water injection in the dry sandstone and the loss of cohesion is less pronounced. This loss of grain-to-grain cohesion is expected to lead to sand production in weakly consolidated siliciclastic reservoirs. In addition, the injected fluids significantly affect the ultrasonic P-wave velocity: (vi) increases during oil injection; (vii) decrease during water injection. This helps in detecting the advancement of the fluid front within the rock’s pore space.

Speaker: Dr.  Jeremie Dautriat

Jeremie Dautriat is a CSIRO Research Scientist involved in Rock Physics and Rock Mechanics characterization and supervises the Geomechanics and Geophysics laboratory based in Australian Resources Research Center, Perth. He obtained a Ph.D. in Rock Mechanics from IFP Energies nouvelles and Ecole Poytechnique (France) on the characterization of the Hydro-Mechanical behaviour of heterogeneous carbonate rocks in 2009. His research focuses on fluid/matrix interactions and their impacts on the elastic and mechanical properties of porous rocks. He is mainly interested in the development of multi-scale approach to predict petrophysical property evolution induced by mechanical damage and/or reactive fluid percolation.