Deformation and damage of cement sheath in gas storage wells under cyclic loading

Abstract

The mechanical damage and failure of cement sheaths in gas storage wells under cyclic loading have been studied extensively. However, because the test device cannot restore the wellbore condition, most studies have been theoretical or regular experimental. If the load‐bearing mode and stress environment in a test device differ from those in a wellbore, then the damage and failure modes will deviate from what occurs in the actual wellbore. Therefore, it is necessary to explore a method that restores the wellbore condition and design a wellbore simulation device that reveals the deformation and damage of the cement sheath. In this study, the development laws of microannulus and microcracks in the cement sheath were studied by triaxial cyclic loading, low‐field nuclear magnetic resonance imaging, and scanning electron microscopy. A device to evaluate cement sheath integrity was then developed. A stress equivalence method was proposed, based on the principle that the stresses at the first and second interfaces of the device are equal to those of the wellbore cement sheath. This method was used to design material, size, and experimental conditions to simulate the load‐deformation law of a cement sheath in a gas storage well. The damage failure mechanism was investigated using the simulation results and computed tomography. Micropores and microcracks in the cement sheath increased continuously under cyclic loading. Damage accumulated, causing strength failure as the period of the cyclic loading increased. Plastic deformation of the cement sheath occurred under cyclic loading, but interfacial peeling did not. The reason is that the cement sheath is under compressive stress during loading and unloading, and the interface between the cement sheath and the outer wall is not damaged. This study provides a new method for studying the mechanical failure of a cement sheath under complex wellbore conditions.

Experiment and failure mechanism of cement sheath integrity under development and production conditions based on a mechanical equivalent theory

“…Through previous research, a stress-equivalent method based on equalizing F I G U R E 2 Schematic of wellbore simulation test device the interface stresses of the first and second interfaces of the wellbore cement sheath and the cement sheath interface stress in the device was developed, as shown in Equation 1. 27 In particular, we ensured that the circumferential stress was equal on all sides by making the wellbore cement sheath proportional to the inner and outer diameters of the cement sheath in the device (1). Based on the equalization of the interface pressure (radial stress) between the wellbore cement sheath and the cement sheath in the device, the internal casing pressure and simulated formation confining pressure in the device were calculated (2).…”

“…25,26 Based on the principle that the first and second interface stresses of the cement sheath in the simulation device are equal to that of the wellbore cement sheath, Li et al tested the sealing ability of the cement sheath during the cyclic loading of gas storage wells and found that fatigue failure was the main cause of integrity failure. 27 Extensive research has been conducted to investigate the integrity of cement sheaths and their failure mechanisms; many shortcomings still exist. Prior simulations and tests focused mostly on the effect of high pressure or cyclic loading of the wellbore on the integrity of the cement sheath, ignoring the complex variants such as casing pressure test and the blow off that occur before acid fracturing and production.…”

Effects of the porous structure of cement sheaths on the deformation failure mechanism of wellbore cement sheaths

“…However, cement is a heterogeneous material with numerous micropores, which can considerably influence the deformation and failure of cement sheaths under conditions of in situ stress and hydraulic fracturing [35][36][37] . At present, most researches on the deformation and failure of cement sheaths do not consider the effects of pore structure, and mainly focuses on the mechanical characteristics of cement sheaths, failure crack geometry, and damage laws [38][39][40][41][42][43] . Few studies have been conducted on the distribution characteristics of porous structures in cement sheaths and their influence on the stress distribution and deformation of cement sheaths, especially the effects on the failure mode and failure path of cement sheaths.…”

Theoretical study on the hydraulic sealing integrity of cement sheath in horizontal shale oil wells under fracturing: Crack propagation mode and mechanism

“…[8][9][10][11] Mechanical failure and microannulus can significantly reduce the sealing ability of the cement sheath toward wellbore fluids, 10,12,13 resulting in failure of the hydraulic sealing. [14][15][16] In addition, a weak bonding strength at the interface and poor cementing quality can cause the cement sheath to lose its barrier function. 17,18 In recent years, some scholars further discovered that even if the cement sheath has good strength and deformation ability, the sealing failure of the cement sheath will also occur.…”

“…8–11 Mechanical failure and micro-annulus can significantly reduce the sealing ability of the cement sheath toward wellbore fluids, 10,12,13 resulting in failure of the hydraulic sealing. 14–16 In addition, a weak bonding strength at the interface and poor cementing quality can cause the cement sheath to lose its barrier function. 17,18…”