Cement Sheath Integrity and Decay Law of Interface Mechanical Properties Under Ultra-high Temperature Thermal Cycling

The failure of cement sheath integrity caused by temperature fluctuations and continuous changes in the wellbore and cyclic loading and unloading in periodic steam stimulation of heavy oil thermal recovery wells will threaten the integrity of the wellbore, lead to frequent safety issues such as wellhead movement. A method for evaluating the bonding strength of cement sheath under high-temperature thermal cycling environment was proposed in this paper to clarify the failure mechanism of cement sheath integrity and the degradation law of interface bonding strength under high-temperature thermal cycling, and to avoid the failure of the interface bonding between the casing and the cement sheath during steam stimulation. A proprietary testing device for the bonding strength of cement sheath that integrates pouring, curing, heating, and testing was independently developed. The interface integrity and mechanical integrity of the "production casing-cement sheath-technical casing" system under thermal cycling were tested and evaluated. Experimental results on the integrity and interfacial bonding performance of cement sheath under thermal cycles at three different temperatures (30 ℃↗150 ℃↘30 ℃, 30 ℃↗200 ℃↘30 ℃, 30 ℃↗250 ℃↘30 ℃) obtained. The research results indicate: the ultra-high temperature thermal cycling has a significant negative impact on the mechanical and interfacial integrity of the cement sheath. With the increase of temperature and thermal cycle times, the shear strength of cement sheath interface decreases sharply, and the integrity failure degree of cement sheath body also increases. The cement sheath interface debonds after 7^th thermal cycle of 150 ℃, 5^th thermal cycle of 200 ℃, 1st thermal cycle of 250 ℃, respectively. The interaction between casing and cement sheath completely disappear (the shearing force, chemical bonding force, friction, shearing strength is all equal to 0) after 13^th thermal cycle of 150 ℃, 9^th thermal cycle of 200 ℃, 7^th thermal cycle of 250 ℃, respectively. The body failure includes radial cracking and circumferential cracking. Radial cracking, circumferential cracking, and interface debond cause the failure of the cement sheath integrity, sealing integrity, and interface integrity. Micro cracks and micro gaps will generate which cause deformation, damage, and leakage of cement sheath. The casing becomes a free section, causing safety issues such as annular pressure and wellhead movement.