Abstract
The knowledge of hydraulic fracture morphology is significant for the analysis of fracture mechanisms. This paper utilizes passive Ultrasonic Travel-time Tomography (UTT) to characterize the hydraulic fracture. We constructed a velocity model based on X-ray computerized tomography (X-CT) images scanned on a real hydraulically fractured shale column. Then, ray-paths and travel times corresponding to the source-receiver configuration were calculated by curved ray-tracing schemes. Lastly, we performed tomographic inversions using total variation regularization (TVR). The simulation results showed that 3D passive UTT based on TVR is an accurate, efficient, and stable method to reconstruct the velocity structures with fractures, even in the case of sparse ray-coverage or high noise level. Meanwhile, we also verified that the passive UTT is a valid alternative to X-CT in depicting the hydraulic fracturing rock via a proper interpretation method.
Highlights
Hydraulic fracturing has been a well-known technique for shale gas development in the last decades [1]
To illustrate the superiority of total variation regularization (TVR) under sparse ray-coverage, we provided the pictures
Results show that total variation (TV) regularization can reasonably characterize and reveal the fractures geometry both in the case of dense and sparse ray-coverage, and it is stable and robust in the case of a high noise level
Summary
Hydraulic fracturing has been a well-known technique for shale gas development in the last decades [1]. As a method for studying its mechanisms, laboratory hydraulic fracture experiments have been extensively performed. The geometry of hydraulically induced fractures is especially significant in the analysis of the fracture mechanism. We can use the X-ray computerized tomography (X-CT) scanning technique to display and investigate the fracture morphology inside the rock [2,3,4,5]. One of the widely used techniques is the Ultrasonic Travel-time Tomography (UTT) technique. It has been used to investigate the hydraulically fractured process of granite [6], stress distribution of uniaxially loaded rock [7], and localized deformation in porous sandstones [8]
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