Journal of Southwest Petroleum University(Science & Technology Edition) ›› 2021, Vol. 43 ›› Issue (2): 128-137.DOI: 10.11885/j.issn.1674-5086.2019.12.05.02

• OIL AND GAS ENGINEERING • Previous Articles     Next Articles

Analysis of Collapse Matrix and Weak Plane of Bedding Strata

LIU Hailong1, XIE Tao1, ZHANG Lei1, CAO Wenke2, GAO Jiajia3   

  1. 1. CNOOC China Limited, Tianjin Branch, Tanggu, Tianjin 300459, China;
    2. School of Petroleum Engineering, Changzhou University, Changzhou, Jiangsu 213164, China;
    3. Petroleum Engineering School, Southwest Petroleum University, Chengdu, Sichuan 610500, China
  • Received:2019-12-05 Online:2021-04-10 Published:2021-04-23

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Abstract: There are a lot of hard brittle shale in deep strata of Bohai Bay, which is prone to cause wellbore instability. Taking hard and brittle shale as a kind of transversely isotropic medium, the stress calculation model of well circumference matrix and bedding plane is established. On the basis, influence of elastic anisotropy parameter, wellbore anisotropy and bedding formation on wellbore collapse is obtained. The results show that anisotropy has a significant effect on the stress distribution of the weak surface. The maximum error is about 31% compared with the isotropic model. With the increase of elastic modulus anisotropy, the risk of wellbore collapse increases. The direction of wellbore collapse is not only affected by the horizontal stress orientation, but also by the inclination and squint angle of weak bedding surface and well trajectory when drilling in a vertical well. With the increase of inclination and squint angle of weak bedding plane, the collapse shapes of the wellbore are as follows:matrix collapse only along the minimum horizontal stress direction, single diagonal collapse of bedding plane and double diagonal collapse of bedding plane. The collapse orientation of the matrix corresponds to the minimum in-situ orientation, and the collapse orientation of the bedding plane is consistent with the dip direction azimuth. The optimum drilling azimuth should be combined with the squint angle of bedding plane. When matrix failure and weak surface failure occur simultaneously, the collapse risk increases with the increase of well inclination. The collapse is the most serious along well deviation 30°~75°, near the maximum in-situ stress azimuth when drilling in a directional well. An example of a deep well in Bozhong structure is analyzed. The analysis results of wall collapse pressure are in good agreement with the actual situation. The research results have certain guiding significance for drilling operation in bedding strata.

Key words: transversely isotropic, mechanical parameters, weak plane, matrix, collapse

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