Abstract

Abstract Understanding the formation mechanisms of complex fracture networks is vitally important for hydraulic fracturing operations in shale formation. For this purpose, a hydraulic fracturing experiment under a core-plunger scale is conducted to investigate the impact of the bedding plane angle, borehole size, and injection rate on fracture initiation behaviors of laminated shale rock. The results on rock properties demonstrate that the anisotropic characteristics of shale rock are reflected not only in elastic modulus but also in tensile strength. The results of fracturing experiments show that the bedding plane dip angle and borehole size have significant effects on fracture initiation behaviors, in that fracture initiation pressure (FIP) decreases with the increase of those two factors. The impact of injection rate, by contrast, has no obvious variety regulation. The above data is further used to validate our previously proposed fully anisotropic FIP model, which shows better agreement with experimental results than those using other models under various parameter combinations. Finally, a postfracturing analysis is performed to identify the fracture growth patterns and the microstructures on the fracture surfaces. The results show that the hydraulic fractures (HFs) always grow along mechanically favorable directions, and the potential interaction between HFs and bedding planes mainly manifests as fracture arrest. Meanwhile, the roughness of fracture surfaces is physically different from each other, which in turn results in the difficulties of fluid flow and proppant migration. The findings of this study can help for a better understanding of the fracture initiation behavior of laminated shale rock and the corresponding fracture morphology.

Highlights

  • Horizontal drilling in combination with hydraulic fracturing is an efficient method for extracting oil and gas from unconventional reservoirs due to its ultralow permeability

  • Taking shale reservoirs as an example, large engineering practices have proved that the complexity of 3D fracture networks (FNs) after fracturing plays a decisive role in ensuring initial productivity and improving ultimate recovery

  • The experiment needs to be stressed that the experiment numbers of each type of test are corresponding to each other and the required specimens are taken from the same core column

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Summary

Introduction

Horizontal drilling in combination with hydraulic fracturing is an efficient method for extracting oil and gas from unconventional reservoirs due to its ultralow permeability. Taking shale reservoirs as an example, large engineering practices have proved that the complexity of 3D fracture networks (FNs) after fracturing plays a decisive role in ensuring initial productivity and improving ultimate recovery. This implies that understanding the formation mechanisms of FNs is vitally important for hydraulic fracturing treatment design and operation. Recent studies have shown that all laminated shale rocks display an obvious anisotropy in mechanical properties, including elastic modulus, Poisson’s ratio, tensile strength, and fracture toughness We performed core-plunger scale hydraulic fracturing experiment to investigate the fracture initiation behaviors of laminated shale rock. A postfracturing analysis was performed for the fracture propagation patterns, so as to identify the micromorphologies on the fracture surfaces

Experimental Methodology
Results and Analysis
FIP Analysis
Factors Influencing FIP
Postfracturing Analysis
A A’ O O’
Conclusions
FIP Estimate for Formations with Anisotropic Rock Strengths
FIP Estimate for Transversely Isotropic Formations
FIP Estimate for Formations with Anisotropic Fracture Toughness
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