In this paper, a coupled mechanical model is proposed to analyze the influence of the thermal stress, the non-uniform geologic stress and the fracture pressure on the mechanical characteristics of the cement during fracturing of shale gas wells.
In this model, the casing-cement-formation assembly is described as a multilayer string system with elastic property. The system is subjected on a coupled effect of three kinds of load: the thermal stress generated by heat transfer from the wellbore to the formation, the hydraulic pressure from fracturing operation, and the non-uniform geologic stress. The analysis model is established and the governing equation is deduced based on the elastic mechanics. A computer program is developed to solve the equation. On this basis, the influence of cement thickness, elastic modulus, and Poisson's ratio on the mechanical characteristics of the cement sheath is discussed in detail.
Analysis results show that temperature variation generates circumferential compression stress on the system. While the fracturing pressure and the non-uniform geologic stress produce a periodical circumferential stress and radial stress. Under the combined loads, the cement is inclined to tension failure. The temperature has a positive effect on the safety of the cement sheath. The orientation of the tension failure is determined by the non-uniform stress field, which is paralleled to the major principal stress.
This study has a reference for the prediction and control of well integrity of shale gas wells during fracturing operation.
Due to the low porosity and low permeability of shale reservoirs, the multi-stage hydraulic fracturing is the important and irreplaceable techniques for shale gas development (Bai, 2014; Liu, 2014). During the operation, the fracturing fluid is pumped into the reservoir through the casing, which would induce well failure and cement sheath damage. The schematic diagram of multi-stage fracturing operation is shown in Fig. 1.
Almost all shale gas wells have well integrity problems, such as sustained casing pressure (Ma, 2017; Zhu, 2016), casing collapse and cement failure, etc. Many published literatures have discussed the problems. Some papers (Tao, 2017; Chu, 2015; Liu, 2017; Shen, 2017) have proposed that the gas channeling from the reservoir to the wellhead through the damaged cement sheath is a main factor generating the well failure. In the fracturing operation, the cement sheath is suffered tension stress under the external loads. Mostly, the tension stress is much higher than the tension strength, which is just 3.2MPa-4.3MPa (Tao, 2017). Moreover, the tension strength would decline under cyclic fracturing pressure (Shadravan, 2015). Some laboratory tests (Shadravan, 2015; Bois, 2012; Goodwin, 1992) show that the radial crack generated by the tension stress is the main failure mode. Besides, the non-uniform geologic stress acting on casing and cement is another important reason for well failure. The influence of non-uniform geologic stress on the collapse strength of casing has been detailed discussed (Fang, 1995, 2015; Yin, 2006; Li, 2009; Wang, 2015).
