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

Abstract

A B S T R A C TForced 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 velocities 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.

Forced imbibition into a limestone: measuring P‐wave velocity and water saturation dependence on injection rate

“…All these features have been observed in our previous work on sandstones where the magnitude of the effect is more pronounced but the complexities are less (Lopes and Lebedev 2012). Decreasing the injection rate decreases the P-wave velocity and water saturation, immediately and for a significant period of time.…”

“…We follow the experimental workflow described in Lopes and Lebedev (2012). We study the influence of the presence of water and of changing injection rates on the evolution of acoustic wave velocities and water saturation on a Savonnieres Limestone (a weakly layered oolitic limestone from France).…”

“…There is also a significant dependence of the acoustic response with the injection rate (Meleán, Broseta and Blossey 2003;Karimaie and Torsaeter 2007). Lopes and Lebedev (2012) performed laboratory experiments where the injection rate was decreased and increased in a single, continuous imbibition. Cadoret, Marion and Zinszner (1995), Rangel-German and Kovscek (2002), Meleán, Broseta and Blossey (2003) and Karimaie and Torsaeter (2007) performed fluid injection experiments where specific velocity-saturation relationships were established with a variety of injection rates.…”

“…The raw data provided by the X-ray CT scanner is converted into "CT values" (given in Hounsfield, HU, units) whose threshold is set by the computer system. This paper is a sequel to the work presented by Lopes and Lebedev (2012), where the influence of the presence of water and of changing the injection rate on fluid distribution and acoustic velocities on sandstone was studied. As the density changes with the intrusion of fluids, the CT value of each pixel of the map changes.…”

“…The CT values are mapped and the resulting images are presented in a grey-scale: shaded grey (or black) for low CT values and light grey (or white) for high CT values. The purpose is to test if the observations of Lopes and Lebedev (2012) regarding the flow rate dependence on velocity-saturation relationships in sandstone can be also made in limestone. So, X-ray CT scanning provides us with a visual display of fluid distribution in the rock and the possibility of tracking its displacement with time.…”