Geophysical Prospecting doi: 10.1111/j.1365-2478.2011.01009.x Research note: laboratory study of the influence of changing the injection rate on the geometry of the fluid front and on P-wave ultrasonic velocities in sandstone Sofia Lopes 1,3∗ and Maxim Lebedev 2 1 Department of Exploration Geophysics, Curtin University, GPO Box U1987, 6845 Perth, Western Australia, Australia, 2 CO2CRC, Department of Exploration Geophysics, Curtin University, GPO Box U1987, 6845 Perth, Western Australia, Australia, and 3 CSIRO, Earth Science and Resource Engineering, 26 Dick Perry Avenue, 6151 Kensington, Western Australia, Australia Received March 2011, revision accepted July 2011 ABSTRACT Forced imbibition was performed in reservoir sandstone by injecting water into a dry sample. The injection was monitored with X-ray computed tomography and acoustic acquisition to simultaneously visualize the displacement of the fluid and quantify its presence by calculating saturation and P-wave velocities. We observed a strong influence when changing the injection rates on the acoustic response. Upon decreasing the injection rate from 5 mL/h to 0.1 mL/h, P-wave ve- locities decreased sharply: 100 m/s in 1 h. This behaviour is related to the partially saturated conditions of the sample (76% of saturation) before decreasing the injection rate. The air that is still trapped is free to move due to a decrease of pore pressure that is no longer forced by the higher injection rate. After 1 hour, P-wave velocities started increasing with small changes in saturation. Stopping injection for 16 hrs decreased saturation by 8% and P-wave velocities by 100 m/s. Restarting injection at 5 mL/h increased saturation to 76% while P-wave velocities fluctuated considerably for 2 hrs until they stabilized at 2253 m/s. Through the computed tomography scans we observed a water front advancing through the sample and how its shape changed from a plane to a curve after decreasing the injection rate. Key words: Injection rate, Imbibition. 1 INTRODUCTION Quantification of fluid flow through porous media is an essen- tial part of hydrocarbon recovery and reservoir characteriza- tion. In particular, the controlled replacement of one fluid by another is a common procedure in order to stimulate reservoir performance, for example, recovering oil by means of water flooding (Craft, Hawkins and Terry 1991). When injecting fluids into a rock, acoustic data can indicate the presence of a fluid. However, its interpretation requires a well-founded ∗ E-mail: s.correialopes@postgrad.curtin.edu.au knowledge of heterogeneous porous media, particularly of acoustic signatures in multi-phase reservoirs. It is well-known that the presence of fluids results in a change of the elastic properties of a rock, reflecting the mag- nitude of solid/fluid interaction (Gu´ eguen and Palciauskas 1994; Fortin, Schubnel and Gu´ eguen 2005). More particu- larly, seismic wave velocities and attenuation are affected by the degree of saturation and spatial distribution of fluids (Li, Zhong and Pyrak-Nolte 2001; M ¨ uller, Gurevich and Lebe- dev 2010). Wulff and Mjaaland (2002) used ultrasonic wave transmission and reflection to monitor the displacement of a water front within a sandstone block. The velocity, ampli- tude and central frequency of the transmitted waves (from the C  2011 European Association of Geoscientists & Engineers 1