Distribution laws and effects analysis of casing external pressure taking elastic parameters matching into account

Abstract

Distribution laws of casing external pressure, which are obtained by different scholars in different conditions, are not the same. Thus, a model to calculate the external pressure of casing is established with finite element method. In the model a contact is built between the outer wall of the casing and the inner wall of the cement sheath. The casing external pressure can be got through extracting contact force of results. The numerical results and analysis show that: The largest casing external pressure exists in the direction of minimum horizontal stress of formation rather than in the direction of the maximum horizontal stress. There exists such matching between the cement sheath elastic modulus and formation elastic modulus: When the elastic modulus of the cement sheath is small, reducing its value is conducive to reduce the casing external pressure and to prolong casing service life. When the elastic modulus of the cement sheath is large, increasing its value is conducive to reduce the casing external pressure and to prolong casing service life. The casing external pressure will become large with the increase of Poisson's ratio of cement sheath. With the increase of the degree of the formation stress unevenness, the maximum casing external pressure increases and the smallest external decreases. The increasing of the non-uniformity degree of formation stress makes the non-uniformity degree of casing external pressure and the risk of casing failure increase. The study can provide certain reference for drilling workers to take measures to prolong service life of casing.

Keywords

External pressure / Casing failure / Service life / Elastic modulus / Poisson's ratio

Acknowledgments

The Authors would like to thank the National “863” Plan Project of China (No. 2013AA09A215) and CNPC's Science and Technology Research Project (No. 2014B-4315) for providing financial support to this research work.

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