Abstract
Hydraulic fracturing (HF) test has been widely used to determine in situ stress. The use of a conventional continuum method for this purpose has raised considerable controversies concerning field tests, particularly in the determination of the maximum horizontal principal stress under preexisting fractures. Fracture mechanics methods are very promising when considering preexisting cracks. However, most fracture mechanics methods do not include the effects of confinement on fracture parameters that depend on confining stress. In the present paper, we proposed a modified approach based on fracture mechanics for stress determination considering the relation between fracture toughness and confining stress based on the Rummel and Abou-Sayed methods. Then, we conducted true triaxial hydraulic fracturing tests under different stress ratios for granite and sandstone specimens to verify the proposed approach. The observed typical pressure-time curves indicate that in the conducted hydraulic fracturing tests, the steady fracture growth was attained. Moreover, we demonstrated that the stress ratios influence crack orientations. The horizontal maximum principal stresses determined using the modified Rummel method achieve the lowest relative error compared with other considered stress estimation approaches. This modified fracture mechanics method could be used as a potential alternative approach to obtain a considerably more precise estimation of the maximum horizontal stress in hydraulic fracturing stress determination.
Highlights
In situ stress measurement can be performed to address a wide range of geomechanical problems, including faulting, earthquakes, and shale gas exploitation, as well as geothermal and geotechnical problems [1,2,3,4,5,6,7,8,9]
The relative errors of Shmin are less than 10%, while the errors of SHmax are slightly higher, falling in the range of 5.88%–25.63%. These results indicate that the conventional theory of hydraulic fracturing (HF) based on the strength criterion can be used to approximately assess the stress values, concerning the minimum horizontal principal stress
The determination of in situ stress using the HF tests based on the conventional method has raised considerable controversies with regard to field tests, especially in the case of preexisting fractures
Summary
Concerning maximum horizontal principal stress SHmax, it can be evaluated indirectly based on elastic theory and Kirsch’s solution which is only valid for a homogenous defect-free continuum [12]. It requires an accurate determination of either fracture reopening pressure or the rock tensile strength suitable for in situ HF tests. The applicability of this continuum method to SHmax estimation has been discussed in many studies [14, 15]. Fracture initiation is the main concern for stress measurement that needs to be analyzed using the fracture mechanics method
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