Study on Failure Mechanism of Cement Sheath Sealing of Shale Gas Well Under Alternating Loading and the Controlling Method

The continuously annulus pressure phenomenon is prominent in shale gas wells in China, which brings great challenges to safe and efficient development of shale gas. The main reason for the phenomenon is the sealing failure of cement sheath and one of important factors causing sealing failure of cement sheath is the alternating loading induced by massive hydraulic fracturing and wellbore temperature variation. Therefore, it is urgent to study the sealling failure mechanism under alternating stress, establish judging criteria for sealling failure and propose sealing controlling method. Triaxial stress instrument was used to test the mechanical properties of cement for cyclic loads under 5 different stress levels, based on these the residual strain model was obtained by fitting. The sealing evaluation device of full size cement sheath was developed to test the sealling of cement sheath under alternating dynamic loadding and the failure rule of cement sheath sealing was formed. The results showed that there was residual strain in cement after unloading and the accumulated residual strain increased with the increasing of cycle loading times. The model of residual strain was formed by fitting based on the large number of test results. Sealing failure occurred in the cement sheath after repeatedly alternating stress, which occurred in the unloading stage. Plastic deformation occurred in cement sheath under high loading, and residual strain was formed after unloading because the plastic deformation could not recover completely. Therefore, the deformation on the cement sheath interfaces was incompatible. The residual strain increased with the increase times of alternating stress same as that of the cement under cycle loading. The sealing failed when the accumulated residual strain exceeded the ultimate interface strain of the cement sheath interface and the cementation failure occurred. The judging criteria for sealing failure of cement sheath was proposed based on the fitted residual strain model. By reducing elastic modulus and maintaining the high strength of cement, the residual strain can be reduced under alternating stress. The sealing evaluation result showed that the sealing was enhanced of high-strength elastic cement sheath. So the sealing controlling method for cement sheath seal was formed. Shale gas reservoirs belong to low-porosity and low-permeability gas reservoirs. Horizontal drilling and staged fracturing technologies are commonly used to achieve cost-effective development of such reservoirs (Fisher et al., 2004; Mayerhofer et al., 2010; Lin and Ma, 2015; Zhou et al., 2016). Although these wells had good cementing quality and no annulus pressure phenomenon occurred after cementing, sustained annulus pressure appeared after fracturing. The annulus pressure affects shale gas exploitation and brings about the problem of safety and environmental protection. Therefore, it is very necessary to study the sealing integrity of cement sheath under dynamic loading simulating staged fracturing. Many scholars have studied the sealing properties of cement sheath. Goodwin and Crook devised a test model to determine the effects of excessive pressure or temperature changes on cement sheaths. They considered that exposure of casing to excessive temperature increases or internal test pressures caused diametrical and circumferential casing expansion. This circumferential force created a shearing force on the cement/casing interface, causing failure on the interface or radial fracturing of the cement sheath from the inner casing surface to the outer casing (Goodwin & Crook,1992). Jackson and Murphey designed an experiment to test the gas channeling through a cement sheath under various inner casing pressures. After the cement paste hardened, the inner casing pressures were increased gradually to measure whether gas channeling occurred with the gas pressure differential of 0.69 MPa (Jackson & Murphey, 1993). Li et al. designed a physical experiment to simulate the casing program, down-hole temperature and pressure environments, continuous changes in the inner casing pressure and to create a bearing process equivalent to that of the actual cement sheath under changing working conditions in order to explore cement sheath failure mechanisms (Li et al., 2016). But there wasn’t good bonding in the interface between cement sheath and simulation borehole, which is very important to the study of the sealing integrity of cement sheath. All of the above-mentioned researchers studied the sealing performance under the one time loading and unloading or heating and cooling, and none of them considered complex subsurface conditions for example multiple loading and unloading to simulate the multi-stage fracturing. Albawi and Andrade et al. generated induced stresses on the cement sheath and observed a breakthrough flow channel in the cement sheath. But they failed to consider the influence of the confinement pressure (Albawi et al., 2014; De Andrade et al., 2014, 2015). Yuan et al. developed a simulation experiment that placed a casing inside an outer resin cylinder and then poured cement paste into the annular space to study the integrity of the cement sheath under low cycle fatigue loads. This experiment applied direct axial pressure on the casing, which induced radial deformation on the cement sheath, with radial crack appeared in the cement sheath and tensile failure occurred at last. But the confinement of the experiment was too weak to simulate the constraint condition from the hard shale formation (Yuan et al., 2013). As for the mechanical properties of cement, Teodoriu (2012) conducted systematic test and studies on Grade G cement, and the studies of some other specialists mainly focus on the mechanical properties of modified cement (Benge et al., 1982; Le Roy-Delage 2000; Morris 2003; Wehling 2008; Paula et al., 2014; Quercia, 2016) Therefore, the mechanical properties of cement was tested under cyclic loads, based on these the residual strain model was obtained by fitting. Full size evaluation device of cement sheath sealing was developed to test the cement sheath sealing under alternating dynamic loading. Combination the testing results of cement mechanical properties with sealing, the failure rule of cement sheath sealing was formed. By reducing elasticity modulus and maintaining the high strength of cement, the sealing controlling method was proposed for cement sheath.