Journal
BULLETIN OF ENGINEERING GEOLOGY AND THE ENVIRONMENT
Volume 81, Issue 1, Pages -Publisher
SPRINGER HEIDELBERG
DOI: 10.1007/s10064-021-02531-9
Keywords
Elliptical borehole; Hydraulic fracturing; Maximum circumferential stress; Fracture initiation
Funding
- National Natural Science Foundation of China [51874053, 52108386, 51734009]
- Fundamental Research Funds for the Central Universities [2021QN1027]
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This study investigated the hydraulic fracturing characteristics of elliptical boreholes. It found that the horizontal stress difference and borehole shape together affect the breakdown pressure. The experiments showed that the breakdown pressure decreases with the increase of the short-axis of elliptical boreholes when there is a horizontal stress difference. However, when the horizontal stress difference is zero, the situation is opposite.
Subsurface rocks are in the in situ stress state prior to drilling. The original and intact borehole subjected to stress redistribution can be deformed during field operations, and elliptical boreholes will be generated. Most laboratory scale hydraulic fracturing experimental specimens were predrilled in the center without considering lateral and axial pressure. In order to investigate the hydraulic fracturing characteristics of boreholes with non-circular appearance commonly seen in the field, hydraulic fracturing experiments were performed on boreholes with elliptical morphology. These experiments provide a comparison of elliptical and circular boreholes, and indicated that the horizontal in situ stress difference and borehole shape affect the breakdown pressure together. When the horizontal stress difference exists, the breakdown pressure reduces with the increase of the short-axis of elliptical boreholes. At zero horizontal stress difference, the situation is just the opposite. The analytic solutions of the stress fields near the elliptical borehole were obtained by employing the complex variable function. The fracture initiation conforms to the maximum tensile stress criterion. Whether the borehole is elliptical or circular, the damage process during hydraulic fracturing will cause fractures to eventually curve to the major stress direction.
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