This paper discusses two critical aspects of fracturing in cemented cased horizontal wells; the role of cement bond strength in fracture initiation, and the influence of spacing between perforation clusters on fracture width. It shows that opening of inclined and transverse fractures requires shear sliding between the formation and cemented casing. Strong cement bond or external casing attachments near the perforated interval hinder this shear sliding and hamper free opening of the fracture. If the initiated fracture is longitudinal, its opening causes tensile failure of the bond between casing and cement. This enhances the link between fracture and perforations and assists successful execution of the job. Close spacing between perforation clusters also results in fractures that are narrower than predicted by theories of fracture mechanics. Furthermore fracture width tapers quickly as it extend beyond the wellbore. Together these two mechanisms can explain why these types of fractures are known to occasionally encounter operational problems that include inability to inject at the planned rate during the early stage of each fracture and sudden screen-outs even with very low proppant concentration.
The paper offers perforation recommendations for enhancing borehole/fracture connection, as well as guidelines for successful execution of the treatment.