Abstract
This paper addresses evaluating the evolution of stress inside the casing-cement sheath-formation system during the cement injection, setting, completion, and production stages of hydrocarbon recovery. This full-life-cycle analysis of cement sheath integrity gives rise to assessment of potential failure mode (i.e., tensile mode, shear mode, and microannulus) in different stages, and the prevention measures can be proposed accordingly. Considering the loading history, two regimes should be distinguished. Before the accomplishment of cementation, as the cement slurry can merely withstand its hydraulic pressure, the in situ stress and the wellbore pressure are withstood by the rock and the casing, respectively. Once the cementation process is completed, the stress increment (e.g., hydraulic fracturing pressure) is withstood by the casing-cement sheath-formation system. The autogenous shrinkage of cement adversely affects the resistance of the system to all types of failure, whereas a moderate swelling of cement is favorable to the cement sheath integrity. In addition, the cement sheath integrity is strongly influenced by the depth: the failure is encountered more easily at the shallow layer. Both the hydraulic fracturing pressure in the completion stage and the increase in casing temperature in the production stage may lead to tensile circumferential stress, and the hydraulic fracturing is the most critical stage for the integrity of cement sheath.
Highlights
Maintaining well integrity in the whole life of hydrocarbon recovery is a key issue adversely impacting the effective and economic production [1,2,3]. is issue becomes more severe when exploiting deep resources in recent decades
Hydraulic fracturing has been employed more and more extensively for unconventional resources such as shale gas. e high casing pressure encountered in hydraulic fracturing tremendously challenges the well integrity [5,6,7]
To avoid the risk of microannulus, cementing technologies that strengthen contact force of cement sheath with casing and formation can be taken such as using the expansive cement slurry system, applying annular pressure during cement setting, and reducing fluid density during cement setting. e loss of well integrity is strongly governed by the stress
Summary
Maintaining well integrity in the whole life of hydrocarbon recovery is a key issue adversely impacting the effective and economic production [1,2,3]. is issue becomes more severe when exploiting deep resources in recent decades. Mathematical Problems in Engineering sheath due to cement hydration [9], the increase in internal casing pressure during hydraulic fracturing [7], and the increase in casing temperature when producing deep oil/gas [4]. A novel cement sheath mechanical model has been introduced to reflect the failure modes of radial cracking, shear failure, and microannulus considering thermal loads, wellbores fracture, and coupling loads [13, 14]. E comprehensive analytical and numerical models for stress distributions around an inclined, cased wellbore by considering all wellbore processes are developed to apply to cement sheath failure [16]. Ird, conventional cement shrinks during hydration [9, 17], which may result in redistribution of stress This factor has not been systematically investigated in the risk assessment of well integrity. Possible failure mode of cement sheath integrity at each state is analyzed providing reference to take effective measures for maintaining well integrity
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