Summary
The debonding of cementing interfaces caused by perforation will increase the possibility of microannuli/microcrack propagation during hydraulic fracturing. This phenomenon will pose a threat to well integrity not only with stimulation operations but also subsequent long-term production. The objective of this work is to quantitatively investigate the debonding degree of cementing interfaces after perforation. A numerical model was used to simulate the process of the wellbore being penetrated by the perforating gun. Then, a real dimensional perforation experiment was conducted with a circular target specimen. The numerical debonding area of the casing/cement interface was verified by this experiment with the method of computerized tomography (CT).
The numerical-simulation and experiment results show that the cementing-interface debonding is mainly caused by the displacement difference between the casing, cement sheath, and formation during perforation. The debonding area and the microannuli of the casing/cement interface are larger than those of the cement/formation interface. Sensitivity analysis reveals that the debonding area of the cementing interface shows a positive relationship with the hexogen explosive (RDX) load and the elastic modulus of casing. Therefore, the casing with lower elastic modulus should be preferred to reduce the debonding area of the cementing interface caused by perforation if the principal casing-design criteria were met.
The combined technology of perforation and hydraulic fracturing has been the main development method for unconventional oil and gas to date. However, perforation can cause crack damage to cement sheath or de-bonding of the cement interfaces, resulting in fluid channeling during hydraulic fracturing. From an operational perspective, it is thus critical to be able to characterize the degree and range of cement sheath damage after perforation. To achieve the objective, a numerical method of perforation is set up based on the ALE (Arbitrary Lagrange-Euler) algorithm in LS-DYNA software. The numerical results are verified by the ring target simulation test. The cement-interface damage zone shapes like a saddle after perforation, and there is a narrow range of damage around the perforation tunnel. The damage zone is minimized with the enhancement of cement compressive strength but conversely with shear modulus. Cements with lower shear modulus and higher compressive strength should be selected during well cementing for reducing cement sheath perforation damage. Additionally, a shaped charge with smaller liner diameter is more effective in enhancing the seal integrity during hydraulic fracturing.
The cement sheath are damaged around the hole after perforation, and the micro-cracks in the cement sheath will cause fluid channeling during hydraulic fracturing. Focused on this problem, a numerical model was set up to calculate the crack propagation length in cement sheath during hydraulic fracturing by using the Cohesive Zone Method (CZM). Then the influence of different parameters on the unsealing length of cement sheath during hydraulic fracturing was analyzed. The results show that higher casing pressure can help to reduce the unsealing length of cement sheath. It is significant to perforate with lower density and phase to ensure the sealing integrity of the cement sheath. This research can evaluate the sealing performance of the cement sheath during hydraulic fracturing and give some guiding for the design of fracturing schemes.
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