Abstract

Fracturing stimulation in deep tight gas reservoir in Eastern Sichuan Basin China shows that natural fractures obviously affect the treatment pressure. Accurate determination of breakdown pressure in the presence of pre-existing fracture can assist engineers better manage expected fracture gradients. The available fracture models for breakdown pressure prediction did not consider possible failure mode of hydraulic fracture influenced by pre-existing fracture. In addition, Elsworth indicates that breakdown pressure is a strong function of fracturing fluid infiltration. Infiltration due to pre-existing fractures is non-negligible to breakdown pressure prediction.

To overcome the limitation of current models, a model considering pre-existing fracture is applied to predict breakdown pressure. The stress distribution of pre-existing fracture intersected with perforation hole considering fluid infiltration is described based on stress distribution model of ellipsoid. The breakdown pressure prediction model is built considering uniform stress distribution and 3 possible failure mode, which are tensile failure of matrix, tensile failure of pre-existing fracture and shear failure of pre-existing fracture, described by corresponding failure criterion.

Sensitivity studies are conducted to investigate the influence of pre-existing fracture dip angle, orientation and location on failure mode and breakdown pressure. It indicates that the distribution of pre-existing fracture has great effect on hydraulic fracture failure mode and corresponding breakdown pressure. The breakdown pressure (1) increases with increasing dip angle and decreases with increasing orientation when matrix fails, and (2) doesn't always show the same tendency when pre-existing fracture fails. The novel model is further verified against measured breakdown pressure from field fracturing treatment of M3 well in deep sandstone formation in eastern Sichuan Basin.

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