Abstract
The significant decreased wellbore temperature and increased casing pressure during fracturing fluid injection present a big challenge for the mechanical integrity of cement sheath in fracturing wells. Based on the theories of elastic mechanics, thermodynamics, and a multi-layer composed thick-wall cylinder, this paper proposed a new mechanical model of cement sheath for fracturing wells, coupling pressure, and thermal loads, which consider the failure modes of de-bonding, radial cracking, disking, and shear failure. The radial nonuniform temperature change and the continuous radial stress and radial displacement at two interfaces have been considered. With the proposed model, the radial distributions of failure stress and the corresponding safety factor for cement sheath during fracturing fluid injection have been analyzed and compared under four failure modes. Results show that the decreased wellbore temperature will produce significant tri-axial tensile stress and induce cement failure of de-bonding, radial cracking, and disking. The increased casing pressure will significantly lower the risk of de-bonding but also aggravate radial cracking and shear failure. For integrity protection of cement sheath, increasing the injected fluid temperature, maintaining higher circulation pumping pressures, and adopting cement sheath with a low elasticity modulus have been suggested for fracturing wells.
Highlights
For maintaining gas well’s long-term and safe production, mechanical integrity of cement sheath has been given more emphasis in recent years
The results indicated that the decrease in wellbore temperature could induce radial tensile stress in the cement sheath
We further proposed a model of thermal stress for cement sheath during hydraulic fracturing [12], where radial non-uniform temperature change was involved, and the main influencing factors were investigated
Summary
For maintaining gas well’s long-term and safe production, mechanical integrity of cement sheath has been given more emphasis in recent years. Field experiences and laboratory studies have both proven that cement sheath is very likely to fail after downhole operations, such as pressure testing, hydraulic fracturing, acidizing, steam injection, and more [1,2,3]. The cement sheath fails mainly because of the loads from variations of wellbore temperature and pressure. Because of the high displacement and pump pressure fracturing fluid injection into wellbore, the wellbore temperature decrease (by −70 ◦ C) and casing pressure increase (by 70 MPa) are both very serious. This leads to cement sheath failure and continuous annular casing pressure. An analysis of the Energies 2018, 11, 3534; doi:10.3390/en11123534 www.mdpi.com/journal/energies
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