Abstract:

Cement sheath is mainly used to support casing, seal off annulus, and protect the casing from corrosive formation fluids in the casing well. During the production, the integrity of the casing–cement–formation system is generally damaged by perforation and fracturing operation, leading to failure of fracturing operation. Experiments indicates that Mohr-Coulomb failure criterion may fail in a HPHT condition when doing simulation applied to hydraulic fracturing. In this paper, triaxial tests of cement samples are carried out under HPHT condition. Cement samples are scanned by SEM to analyze the micro damage mechanism. Finite element simulation of fracturing process in casing well is done to analyze the integrity of the casing– cement–formation system. Experiments show cement sheath strength decreases in an extreme HPHT condition because the micro structure has already been destroyed by the long-term high pressure. The simulation shows that cement with low Young's modulus and high Poisson's ratio is crucial to retain the integrity of the casing–cement–formation system. Fracturing operations should be initiated on sections of cements with lower Young's modulus and higher Poisson's ratio. The experiment results and simulation model proposed can not only gain insight into interaction of casing metal, cement and rock, but also optimize the design of perforation operation for reservoir stimulation in casing wells.

Introduction

During well cementing operation, cement hardens to form cement sheath, and then form the casing-cement-formation system. Cementing is an important procedure for well completion. The ultimate purpose is to provide effective zonal isolation of the formation which have been penetrated by the wellbore. Once a well is put into production, the wellbore conditions will change under various processes, such as casing perforation, fracturing stimulation, acid treatment, etc. Pressure increases when perforation accompanied with the firing of the guns can lead to cement damage. If fracturing stimulation or acid treatment is adopted, the increase of the wellbore pressure is more harmful to the casing-cement-formation system because the fluid pressure is high and the fluid injection lasts from minutes to hours. Moreover, the change of external loading can be applied to the cement sheath. For example, the pressure on the external surface of the cement sheath will increase because of formation creep. In this case, it may lead to fracture propagation or annulus cracks around the cement sheath, eventually lead to the failure of fracturing operation. Scholars have done a lot of experiments to study the properties [1-3] of cement sheath. Many scholars [4] also studied the effect of wellbore pressure on the cement sheath, used the analytical model to predict well life and using a finite element model to simulate the actual working conditions. Experiments indicates that Mohr-Coulomb failure criterion may fail in a HPHT (high pressure and high temperature) condition [5-7], so the actual failure criterion in HTHP condition must be reconsidered when doing simulation.

In this study, triaxial tests are performed under various confining pressures and temperatures for cement samples. Cement samples under different confining pressure are scanned by SEM (Scanning Electron Microscope) to analyze the micro damage mechanism. We also present a 3D finite element model with perforation in the particular section to simulate the hydraulic fracturing in HPHT cased well, mainly analyzing the influence of formation and cement parameters on the integrity of the casing-cement-formation system. The coupled model not only can provide insight on interaction of casing, cement and formation, but also can optimize the design of perforating operation for reservoir stimulation in HPHT cased well.

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