Abstract
The heterogeneity of unconventional reservoir rock tremendously affects its hydrofracturing behavior. A visual representation and accurate characterization of the three-dimensional (3D) growth and distribution of hydrofracturing cracks within heterogeneous rocks is of particular use to the design and implementation of hydrofracturing stimulation of unconventional reservoirs. However, because of the difficulties involved in visually representing and quantitatively characterizing a 3D hydrofracturing crack-network, this issue remains a challenge. In this paper, a novel method is proposed for physically visualizing and quantitatively characterizing the 3D hydrofracturing crack-network distributed through a heterogeneous structure based on a natural glutenite sample. This method incorporates X-ray microfocus computed tomography (μCT), 3D printing models and hydrofracturing triaxial tests to represent visually the heterogeneous structure, and the 3D crack growth and distribution within a transparent rock model during hydrofracturing. The coupled effects of material heterogeneity and confining geostress on the 3D crack initiation and propagation were analyzed. The results indicate that the breakdown pressure of a heterogeneous rock model is significantly affected by material heterogeneity and confining geostress. The measured breakdown pressures of heterogeneous models are apparently different from those predicted by traditional theories. This study helps to elucidate the quantitative visualization and characterization of the mechanism and influencing factors that determine the hydrofracturing crack initiation and propagation in heterogeneous reservoir rocks.
Highlights
Hydrofracturing is the primary method of hydrocarbon reservoir stimulation that enhances unconventional gas recovery
A visual representation and accurate characterization of the three-dimensional (3D) growth and distribution of hydrofracturing cracks within heterogeneous rocks is of particular use to the design and implementation of hydrofracturing stimulation of unconventional reservoirs
The results indicate that the breakdown pressure of a heterogeneous rock model is significantly affected by material heterogeneity and confining geostress
Summary
Hydrofracturing is the primary method of hydrocarbon reservoir stimulation that enhances unconventional gas recovery. There are other methods, such as unconventional fracture models (UFM) (Kresse et al 2011, 2013), BEM (Zhang et al 2007), XFEM (Wang et al 2015), DDM (Gu et al 2008) These numerical studies are useful for understanding and characterizing the initiation and propagation of hydrofracturing cracks. These numerical models are generally applied to simple media and become extremely complicated when random discontinuities or material heterogeneities are involved. Despite limitations of the 3D printing technique applied to rock mechanics (e.g., low strength and poor brittleness of printing material), this technique shows obvious advantages for rock mechanics studies It is precise and allows flexibility for controlling the geometry of the specimen.
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