Influence of Tough Materials on Cement Sheath Interface Integrity for Shale Gas Fracturing Wells

To improve the cement sheath integrity during multi-fracturing for shale gas wells, four kinds of cement slurry systems were tested to evaluate cement interface integrity based on the cement sheath integrity device under the condition of cycle load. High-precision flowmeter was used to quantitatively detect the annulus gas flowing rate to evaluate the annulus sealing ability. Electron microscope scanning and nuclear magnetic resonance were adopted to measure the micro-structural changes of the cement sheath. The plastic finite element model of wellbore was established to simulate the damage of cement sheath interface during the cycle load. The results show that the annular gas flow rates of conventional cement slurry, 18wt.% latex, 36wt.% latex, and 18wt.% latex+1wt.% toughening were respectively 722 mL/min, 300-677 mL/min, 20-45 mL/min, 10-25 mL/min, indicating that the addition of latex can greatly reduce the gas flow rate in the annular interface and enhance the interface sealing ability. At the same time, the toughener can enhance the effect of the latex and improve the sealing integrity of cement sheath interface. After cyclic load, there were radial crack and microscopic crack at the cement interface for the conventional cement slurry, with the maximum width of 17 μm and length of 32 mm for radial crack. For the cement slurry with 36wt.% latex, there was only small micro annulus at the cement sheath interface. The hydration product was denser for the cement slurry with 36wt.% latex. Under cycle load, the pore size of the cement sheath body did not change much, but the pore size at the interface increases significantly. The finite element results show that plastic deformation occurred at the cement sheath interface under cyclic loading. With the increase of the number of cycles, the plastic strain increases continuously, and micro-annulus is easily formed. It could be concluded that the latex and ductile materials can effectively fill the gaps between the cement particles and reduce the pore size of the cement matrix. When the tough cement slurry is subjected to cyclic load, the latex can improve the microscopic morphological structure of the cement sheath and avoid cracks in the body, thereby greatly reducing the gas flow rate and improving the interface seal integrity. The toughening agent can have a strong bonding effect on the cement particles. When used in conjunction with the latex, the sealing effect of the cement sheath interface can be further enhanced. The combined action of these two agents can significantly improve the annular sealing ability of the cement slurry under cyclic loading. Under the action of cyclic load, plastic deformation will occur at the interface of cement sheath and accumulate continuously, and it is easy to form micro-annulus, which provides channeling channels for annular gas. For a shale gas well, a tough cement slurry system with 18wt.% latex agent and 1wt.% toughening agent was used during cementing construction. The mechanical properties of cement stone can meet the performance requirements of cement stone strength greater than 30 MPa and elastic modulus less than 7.0 GPa. At the same time, when using tough cement slurry, the cementing quality is also good, and there is no sustained casing pressure problem during the subsequent fracturing construction. By rationally optimizing the content of the toughness cement slurry additives, a good sealing effect can be provided for the annular space and the sealing integrity of the cement ring can be improved. © 2022, Chongqing Wujiu Periodicals Press. All rights reserved.