Measurement and Implications of the Dynamic Fracture Width in Hydraulic Fracturing Using FBG Strain Sensors

Fracture width and its variation are of significance to hydraulic fracture model validation. And it is also important to determine how the fracture extends as well as the properties of fracturing fluid and proppant in hydraulic fracturing. Hydraulic fracture width is 3-4 orders of magnitude less than the length and the height. Because fracture width is very tiny and the reservoir is at great depth, fracture width can only be determined through model estimation or numerical simulations. At present, even in the laboratory conditions, no direct measurement of fracture width is possible. This article is based on Fiber Bragg Grating (FBG) strain sensor and strain transfer theory. The hydraulic fracturing is controlled by great stress difference and prefabricated fracture direction in the true-triaxial rock test equipment. FBG strain sensors are distributed in the fracture propagation direction to monitor fracture behavior. So dynamic monitoring hydraulic fracture width in concrete materials can be realized in laboratory. The entire process of hydraulic fracture initiation and propagation is recorded by FBG strain sensors during hydraulic fracturing, with various stages of micro-fractures, macro-fractures and width changes. By multiple FBG strain sensors, displacements of fracture process zone before hydraulic fracture reaches FBG and fracture widths when the hydraulic fracture is crossing FBG are obtained. Then we found the speed and the shape of fracture in the process of hydraulic fracture propagation. Data show that fracture width does not increase monotonously with hydraulic fracture extension in the process of hydraulic fracturing. It fluctuates within a certain range along with the fracture extension.