Impact of Off-Balance Fracturing on Borehole Stability and Casing Failure

Abstract

Abstract Creation and presence of the hydraulic fracture can cause casing failure both during and after the fracturing operations. Although such failures are known to occur and have been a concern to many operators, the petroleum industry literature contains very little information on the subject. During the fracturing operations, casing can fail under tension across the fractured interval either by de-threading of the collars or by tensile failure at perforations. The three main factors contributing to this failure are thermal shrinkage of the casing, off-balance fracture growth, and pseudo-openhole environment. Other contributing factors are borehole inclination with respect to the fracture plane, and the quality of the cement bond. Factors contributing to casing failure during well production include changes in in-situ stresses due to reduction in reservoir pressure, off-balance fracture growth, formation layering and its inclination with respect to the wellbore, and shear strength of the interfaces. In particular, off-balance growth may create fractures that are not symmetrical with respect to the wellbore, and have different lengths in different layers. The resulting differential reservoir compaction induces shear stresses along the interfaces which can cause casing failure along the weakest interface. The low formation shear strength along the propped fracture plane increases the probability of shear movement along it. Presence of the fracture will direct this movement towards the casing and can cause its shear failure. Rapid production pressure declines increase the shear stresses acting on the fracture plane and can cause movement along the fracture plane and casing failure. Introduction Casing failure is a consequence of large formation deformations at the borehole. These deformations are caused by compressive, tensile, or shear stresses in the formation. Failure occurs when the magnitude of these stresses exceeds the strength of the formation. Most of the existing literature on casing failure is based on the assumption of formation shear failure. The analyses cover conditions under which the shear stresses in the formation exceed its strength as defined by the Mohr-Coulomb envelope. Failure is assumed to occur either due to slippage at the weak interfaces between overburden formations, at the top of the producing zone, or due to formation compaction. The main cause of most casing failures is viewed to be reduction in reservoir pressure that compacts the reservoir and induces shear stresses in the interface between overburden layers. If and when these stresses exceed the shear strength of the interface, slippage is assumed to occur, resulting in wellbore and casing failure in the same or in adjacent wells. The compaction pattern of the reservoir is assumed to be symmetrical with respect to the wellbore. This causes the highest compressive stress applied to the casing and resulting from compaction to be at the wellbore. Other contributors are fluid injection and changes in formation temperature during water flooding. A comprehensive review of these is provided by Dusseault et al[1]. De Rouffignac et al[2] report over 3% failures per year in the Diatomite reservoir of South Belridge field. Yudovich et al[3] report a very high failure frequency of about 2/3 of all casings in Ekofisk. Pattillo et al[4] report casing failures in the Valhall Field in both horizontal and vertical wells. While above authors do not discuss the contribution of fracturing, all these three fields use well stimulation as part of the completion and production processes. Streit et al[5] present analytical equations that correlate the occurrence of faulting to reduction in pore pressure. They apply their correlation to the Ekofisk reservoir with reasonable results.