Water weakening triggers mechanical instability in laboratory fluid substitution experiments on a weakly-consolidated sandstone

摘要

An original testing protocol was designed to study the substitution of in situ fluids in reservoir rocks with minimal impact on the in situ effective stress. The weakening effect of the substituting fluid can therefore be easily identified. Our results for the weakly-consolidated Sherwood sandstone show 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; (iii) the mechanical instability detected by ultrasonic and micro- seismic monitoring is mainly localized in the water-flooded zone; (iv) this instability corresponds to a loss of cohesion; (v) during oil substitution by water, the instability is delayed and the loss of cohesion is less pronounced. This loss of 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 which (vi) increases during oil injection and (vii) decreases during water injection. Therefore ultrasonic monitoring can be used to detect the advancement of the fluid front within the rock pore space.