Impact of hydraulic fracturing on cement sheath integrity; A modelling approach

- Well integrity after fracturing raised concerns in some areas.
- Cohesive zone methods is developed to simulation microannulus formation during fracturing.
- Formation of limited microannulus is unavoidable in many practical situations.
- Formation of microannulus by itself doesn't endanger environment as far as microannulus crack is contained.

Recent arguments about the possibility of well leakage and underground water pollution in particular regions have raised significant concerns regarding wellbore integrity during hydraulic fracturing in shallow formations. In this paper, we take a look at the containment of annulus cracks that might develop during hydraulic fracturing treatments. Wellbore integrity is highly dependent on the integrity of the bonding between the cement and the formation as well as the bonding between casing and cement. Cement heterogeneity results from unsmooth borehole surfaces, complex geological conditions, mud cakes, and cement contamination. Excessive fluid pressure during hydraulic fracturing not only provides the driving force for the initiation and propagation of fractures in the reservoir, but also in special cases, it may lead to fracture propagation around the casing, i.e. annulus cracks. A coupled three-dimensional poroelastic model with embedded cohesive zones is introduced here to simulate different fracture propagation scenarios that may occur in vertical and horizontal wells during hydraulic fracturing stimulations in cased-hole zones. The cohesive layer theory is utilized to model the initiation and propagation of transverse, longitudinal and delamination fractures. Using the numerical analysis provided in this paper, a few hydraulic fracturing cases were simulated by taking advantage of the treatment pressure data and petrophysical logs, and the results were compared with the post-treatment radioactive tracer logs available for these wells.