Laboratory study of the influence of changing the injection rate on P-wave velocities and water saturation in a Limestone

Abstract

Time-lapse seismic signals may be caused by changes in the fluid distribution. While the underlying two-phase flow problems have been thoroughly studied, the dependence of wave velocities on saturation and fluid distribution are not fully understood. The aim of this work is to better understand and quantify the dependence of velocity-saturation relationships on the injection rate in a limestone sample. For that purpose a forced imbibition experiment at laboratorial scale with variable injection rate is performed. X-ray Computed Tomography (CT) and ultrasonic acoustic waves are simultaneously monitored. It is observed that the P-wave velocity decreases well before the saturation front approaches the ultrasonically monitored volume. This decrease is followed by an increase as the saturation front crosses the monitored position. Decreasing and increasing the injection rate results in decreasing and increasing both P-wave velocity and water saturation, respectively. This pattern of the acoustic response and water saturation with the change of the injection rate has been also observed on sandstones (Lopes & Lebedev 2012). Our work confirms that changing injection rates promote considerable fluid distribution that is reflected in the evolution of acoustic velocities.