Investigation Of Stress And Channeling In Cased Holes Under Fluid Pressure

The hydraulic fracturing is aimed at increasing cracks in the formation, but it may also cause fracking fluid flowing along space in the cement sheath or its surroundings, which commonly known as channeling. The channeling due to hydraulic fracturing could cause cross-contamination between reservoirs, and make further hydraulic fracturing operation impossible. This paper aims at understanding the mechanism of the fracking induced channeling and gives some anti-channeling suggestions.The strength theory and fracture mechanics theory are used to investigate the forming mechanism of two channels, which cause cased holes losing sealing capability during hydraulic fracturing. The two channels are: channel formed by the brittle failure of cement sheath due to lack of strength in the well-bonded cased hole; and channel formed in hydraulic fracturing by propagation of the small fluid gap which exist before hydraulic fracturing between the cement and the casing(as so-called the first interface) or between the cement and the formation(as so-called the second interface).The three-dimensional finite element method is used to simulate the channeling behavior caused by the brittle failure of the cement sheath when cased hole is well bonded. Stress distribution around a perforation hole in a fluid-pressured cased hole is analyzed. The fluid pressures leading to failure of formation or cement sheath are respectively calculated. The anti-channeling ability of the cement sheath under different media parameters, perforating condition and in-situ stress condition is studied. It is shown that cement with lower Young’s modulus and higher Poisson’s ratio has a stronger ability to prevent channeling in the cement sheath. Channel is more likely to occur in formation with low Young’s modulus and high Poisson’s ratio. The perforating conditions and in-situ deviatoric stress have little influence on the cement sheath channeling, while the in-situ hydrostatic stress has some effect.The fluid gap in the interface is regard as a fluid-filled interfacial crack in this paper. Investigation is made on the propagation of interfacial crack in a cased borehole under the fracking pressure. Interface channeling occurs if the interfacial crack propagates. The cohesive element is used to simulate the behavior of interface when cracks exit in the first interface or second interface of the cased hole. The influences of cement properties, formation properties and in-situ stress on the interface channeling are analyzed. The results show that cement with high Young’s modulus and high Poisson’s ratio can improve the anti-channeling ability of interface. The interface channeling hardly occurs when hydraulic fracturing is performed in formation with low Young’s modulus and low Poisson’s ratio. The increase of horizontal stress can improve the anti-channeling ability of interface, but variation of vertically stress has little influence on the anti-channeling ability of interface.After taking the two channeling mechanisms into account, measures are given to prevent the channeling during the hydraulic fracturing. High Poisson’s ratio cement is preferred in cementing operation, and its Young’s modulus should be in a proper range. The fracking position is better selected as formation Poisson’s ratio low and formation Young’s modulus not too high or too low. Besides, the fracking is better performed in positions with low horizontal stress and high vertically stress.