Abstract:
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.
Introduction
To assess water-weakening effects in reservoir rocks, previous experimental studies have focused on changes in the failure envelopes derived from mechanical tests conducted on rocks fully saturated either with water or with inert fluids. So far, little attention has been paid to the mechanical behavior of reservoir rocks during fluid substitution under conditions similar to enhanced oil recovery operations. Water-based fluids are often used during development and production operations of oil and gas fields. These operations include: (i) well drilling with water-based muds; (ii) reservoir secondary oil recovery by water flooding; (iii) chemical or thermal enhanced oil recovery; or (iv) hydraulic fracturing.
Mechanisms generally associated with fluid/rock interactions include: (i) hydro-mechanics: pressure drawdown/effective stress increase; (ii) petrophysics: change in surface tension/wettability and capillary forces; (iii) hydro-dynamics: seepage forces resulting from water pressure gradients. These mechanisms have been, and still are, extensively studied and assessed for problematic fields around the world. However, hydrochemical weakening of the rock frame specifically associated with exposure to water-based fluids has also been recognized as an important driving mechanism.
