Abstract

Hydraulic fracturing has been applied to enhance oil and gas production in hard rocks and in soft rocks such as unconsolidated sands. However, many uncertainties remain in the fracturing behavior of soft rocks. In this study, we present laboratory experiments in which hydraulic fracturing was conducted in unconsolidated sand specimens, and fracturing behavior was observed using X-ray computed tomography (CT). For the experiments, a test vessel was developed to hold a soft specimen in a cylindrical shape and to apply confining stress to the specimen. Part of the vessel was constructed from carbon fiber reinforced plastic to allow interrupted passage of X-rays. The test vessel is sufficiently compact for placement on the sample bed of the X-ray CT scanner. As a simulated borehole, a casing pipe was buried inside the specimen in the direction of the diameter. Pressurized fluid passed through a slit in the casing pipe and flowed into the matrix of the specimen. The pressurization initiated a planer fracture that grew axially within the specimen. X-rays were then transmitted continuously across the specimen in the axial direction, which allowed us to obtain a real-time CT image of a longitudinal slice of the fracture in the growth direction. The results clearly show characteristic behaviors of hydraulic fracture. The fracturing fluid reached the fracture tip and invaded farther into the rock matrix; therefore, a non-wetted zone did not develop at the fracture tip, in contrast to that observed in hard rocks. Such fluid invasion should affect the fracture formation mechanism in soft rocks.

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