Out-Of-Sequence Fracturing has been field-tested in Western Siberia (2014) and Western Canada (2017/2018) with operational success and positive well production performance. It is conducted by fracturing Stage 1 (at the toe) and then fracturing Stage 3 (toward the heel), followed by tripping back to place Stage 2 (Centre Frac) between Stages 1/3 (Outside Fracs). During placing the Centre Frac, Out-Of-Sequence Fracturing can exploit the reduced local stress anisotropy to effectively activate planes of weakness (natural fractures/fissures/faults/joints) to create failure surfaces with different breakdown angles in all directions. This results in branch fractures that can connect hydraulic fractures to stress-relief fractures created during placing the Outside Fracs, ultimately creating a complex fracture network, and thus, enhanced fracture connectivity.
Despite the published fracture modeling works (calibrated by field tests data) by this author, comparative analyses of wellbore breakdown character and hydraulic fracture orientation during Out-Of-Sequence Fracturing are still lacking. Thus, solutions to 3-D Kirsch Equations are provided for both low and high stress anisotropies to analyze the differences in breakdown gradient, failure angle and hydraulic fracture orientation under various geomechanical and treatment design conditions. Consideration is given to a jointed and intact rock from an isotropic stress state to a reverse faulting condition.
Results indicate that reduced stress anisotropy during Out-Of-Sequence Fracturing leads to favourable treating conditions: With a net fracture extension pressure greater than the reduced stress anisotropy, fracture complexity can be created via allowing the fracture to grow with different failure angles. Also, a well can be drilled and fractured at any inclination or azimuth with favorable breakdown gradients of 0.55-0.85 psi/ft. The reduced stress anisotropy may also trigger some challenges. Near-well stress concentration effects may become more pronounced, promoting longitudinal fractures initiation. For treatments with tortuosity greater than stress anisotropy, longitudinal fractures can be initiated instead of transverse fractures, since the tortuosity is transmitted to the wellbore body and not into the fractures. In this case, to initiate transverse fractures, wellbore must intersect pre-existing transverse notches or near-well pore fluid pressure must exceed the axial stress and rock strength (before hoop stress reaching the tensile failure point). Additionally, fracture may lose directional control and follow any path of weakness. Hence, rock fabric effects become more dominant under a low stress anisotropy regime, which means that with no pre- existing transverse natural fractures or notches, a longitudinal fracture can be generated at the bottom and top of an intact horizontal wellbore.
This is the first attempt in identifying the circumstances that should be avoided for optimizing an Out-Of-Sequence Treatment by examining the differences in breakdown gradient, failure angle and fracture orientation under various geomechanical and treatment design conditions during the low stress anisotropy regime.