Effect of Thermal Cycling on Cement Sheath Integrity: Realistic Experimental Tests and Simulation of Resulting Leakages

Abstract

The cement sheath is one of the most important well barrier elements in the well, both during production and after abandonment. However, normal production operations which involve temperature variations in the well, such as steam injection, stimulations and shut-down periods, may damage the integrity of the cement sheath. Temperature increase and decrease, i.e. thermal cycling, cause the casing to expand and contract, which creates debonding and cracking of the cement sheath and thereby loss of zonal isolation. This paper presents novel results from an experimental study of cement sheath integrity during thermal cycling. The temperature was cycled repeatedly from 5°C to 125°C in a controlled manner from inside the casing, and Portland cement with silica additive was tested with both sandstone and shale as surrounding rock. Debonding and cracking of cement were quantified and visualized by X-ray computed tomography (CT), and it was found that cracking and debonding occurred for the sandstone sample, whereas the shale sample remained almost unaffected. There were some initial defects in the cement sheath in the sandstone sample, and these small and scattered defects grew together during thermal cycling into a continuous leak path; i.e. resulting in a loss of zonal isolation.The digitalized 3D geometry of this leak path was imported into Computational Fluid Dynamics (CFD) software, thereby enabling a unique visualization of fluid flow through an actual leak path in degraded cement and an estimation of leak rates for different pressure differences. It is seen that microannuli are not homogeneous or uniform, and that fluid flow through microannuli and cracks is complex and not easily predictable.

Influences of Fracturing Fluid Injection on Mechanical Integrity of Cement Sheath under Four Failure Modes

“…Submitting Equations (2) and (3) into Equation (1), the thermal radial displacement and thermal stresses for the combined system can be determined and the detailed derivative results are given below [12].…”

“…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.…”

Digital Cement Integrity: A Methodology for 3D Visualization of Cracks and Microannuli in Well Cement

“…In our group, we have recently developed an experimental characterization procedure where we use CT for 3D digital visualization of well cement in realistic annular geometries. With our dedicated and tailor-built cement sheath integrity experimental set-up [41,42], we have visualized microannuli and cracks in cement formed during thermal cycling [41,[43][44][45][46] and pressure cycling [42,[47][48][49], and we have also used the digital cracks and microannuli obtained from CT scans as imported leak path geometries in computational fluid flow simulations [45,[50][51][52]. Furthermore, we have also used CT to study cement-formation bonding [53][54][55][56], cement durability [57,58], and cement-mud interactions [59].…”

Investigation of Thermal Stress of Cement Sheath for Geothermal Wells during Fracturing

“…Consequently, to ensure the wellbore maintains mechanical and hydraulic integrity during fracturing and long-term production, the open hole for geothermal wells is usually cemented with a steel casing. However, laboratory investigations and field practice have both shown that the cement sheath is likely to fail at some stage in downhole operations due to additional stresses within the cement sheath from variations of wellbore temperature and pressure [16][17][18]. For fracturing wells, wellbore temperature may suffer a very significant decrease (up to −70 • C) [19] because of high displacement and pump pressure during fracturing fluid injection, and the casing and cement sheath very likely fail in this case.…”