Study On Sealing Failure Mechanism Of Cement Sheath During Oil Well Production Phase

Cement sealing failure could result in underground fluid channeling and wellhead annulus with pressure, affecting the productivity and operational safety of oil and gas well. With the development of oil and gas well engineering, exploration and development direction tends to drill deep and complex wells, which is easier to trigger cement seal failure, and cause serious harm to the wells. Therefore, further study of cement ring seal integrity has vital importance for ensuring the safety of the well exploitation. In the oil production phase, because of the casing pressure test, fracturing, steam injection and fluid hydraulic pressure changes in wellbore, casing pressure and temperature changes, which could result in the sealing ability reduction or failure of cement sheath. This paper mainly paid attentions to the casing pressure and wellbore temperature changes, and established a "casing-cementformation" analytical model, derived formulas of micro annulus and stress distribution, and proposed three criteria for judging cement failure using three coefficients of cement failure including tensile failure, shear failure and equivalent stress failure. The results show the change of casing pressure has an obvious impact on micro annulus and Von Mises equivalent stress, circumferential and radial stresses, and the change of casing pressure is an important factor affecting the integrity of the cement sheath. Furthermore, the temperature change is one of the main factors affecting the thermal stress and the displacement of cement sheath. When the wellbore temperature increases, the mainly form of cement failure is effective stress failure, the interface of cement sheath is the weakest position. In order to reduce the affection of heat stress to cement sealing, it can be solved by reducing the cement sheath’s elastic modulus and using low coefficient of thermal expansion. The results would provide a theoretical basis for the design of cement or evaluation of wellbore sealing system.