Mechanism of casing deformation relieved under densified-multi-cluster fracturing based on an analytical model

Abstract

The densified-multi-cluster fracturing, a technology being used to enhance shale gas production, also shows a special advantage on casing deformation relieved in WeiRong oil field, southwest China, compared to the conventional multi-cluster fracturing. In view of this, an analytical model was established to illustrate the mechanism of casing deformation relieved under densified-multi-cluster fracturing from the viewpoint of casing stress decreased. Two steps of critical calculations were included in the analytical model, namely, calculation of fracture-induced stresses, calculation of casing total stresses. Based on the fracture mechanics, the fracture-induced stresses under the stress-shadow were respectively calculated in reservoir plane and vertical profile. Then, the fracture-induced stresses were transformed into the casing additional loads, under which the casing additional stresses were calculated. Adding the casing additional stresses to initial stresses, the casing total stresses were obtained under densified-multi-cluster fracturing. Furthermore, the analytical model was validated by comparison of analytical results to numerical results on fracture-induced stresses and casing stresses, with the maximum deviation less than around 7.4 %. By the analytical model, it can be found that the reservoir re-distributed stresses (i.e., casing loads) were decreased and furthermore the casing stresses were decreased with increasing of fracture cluster amounts, which validates the phenomenon of casing deformation relieved under densified-multi-cluster fracturing. Finally, the mechanism of casing deformation relieved was comprehensively revealed.