Time lapse ultrasonic measurements of laboratory hydraulic fracture growth: width profile and tip behaviour.

Abstract

Summary In scaled laboratory tests, we perform acoustic measurements in a time-lapse sequence to separate the fracture response from the background signal. Using both compressional waves and shear waves (that are very sensitive to fluid filled fractures) we cannot only detect the hydraulic fracture, but also characterize its shape and geometry during its growth. We show the application of the technique to propagation, flowback tests and re-opening of hydraulic fractures. During fracture growth the acoustic waves excite diffractions at the tip of the fracture. We use the direct compressional and shear diffractions to locate the position of the tip of the fracture. The ultrasonic data show that the diffractions are most strongly excited at the fluid front of the hydraulic fracture. In some cases, a much weaker precursor diffraction can be detected which is interpreted as the dry tip of the fracture. Depending on the acquisition geometry, we detect many events related to surface waves propagating along the fracture. Also, we observe that shear waves detect the migration of the fluid front during re-opening of a pre-existing hydraulic fracture, in contrast with the compressional waves which are insensitive to the fluid front during re-opening. Observation of transmission signals through the fracture yields a very accurate measurement of the fracture width. In this way we have obtained the full fracture width profile during propagation and closure. Flow back resulted in closing the fracture at the wellbore, while it remained open farther from the wellbore.