Concurrent Active Acoustic and Deformation Monitoring of Hydraulic Fracture in Laboratory Experiments

The results of laboratory experiments on concurrent active acoustic and deformation monitoring of a hydraulic fracture are presented. The experiments were carried out in a model material based on gypsum. For comparison, reference experiments were carried out to study ultrasonic waves propagation through a liquid-filled gap of controlled width between two precision glass plates. The purpose of the experiments was to study the dependence of the amplitude of the ultrasonic wave passing through the crack on the magnitude of its opening. In these experiments, a circular hydraulic fracture was created, the plane of which was perpendicular to the axis of the cylindrical sample. A cased injection well was located along the same axis, ending at the middle of its height. The sample was placed between two aluminum alloy disks equipped with piezoceramic transducers built into them, operating both in the emitter and receiver modes. Through the channel in the lower disk, the working fluid was supplied to the fracture. The sample was saturated with pore fluid through the upper disk. The entire assembly was placed in a hydraulic press providing a constant compressive force. The crack opening value varied depending on the flow rate of the fluid supplied to the center of the crack and was measured by the relative change in the distance between the disks of the assembly using inductive displacement transducers. Experiments were also carried out simulating a fracture filled with proppant. In this case, the crack aperture varied depending on the applied vertical force on the sample. Based on the results of experiments carried out under various conditions, the dependences of the amplitude of the ultrasonic wave that passed through the crack were plotted. It has been experimentally established that sound attenuation in the hydraulic fracture, which has a natural roughness, is two times lower than in the gap between precision glass plates. The obtained results will make it possible to estimate the value of hydraulic fracture opening in laboratory experiments conducted on larger samples using active acoustic monitoring.