Case Studies Suggest a Better Approach to Analyzing Collapse of Inclined Boreholes

Abstract

SPE Members Abstract This paper addresses the comparison and selection of failure criteria for in-situ rocks in borehole stability analysis. Five commonly used criteria are compared with field cases for mud weight design. The result of case studies indicates that the Mohr-Coulomb criterion, Inner Drucker-Prager criterion and Middle Drucker-Prager criterion are overly conservative, and that the Outer Drucker-Prager criterion is underestimating for designing wellbore pressure to control borehole collapse. It is concluded that the von Mises criterion is more reliable than other failure criteria and should be used for borehole collapse analysis and mud weight design. Based on triaxial test data, this paper also provides failure diagrams for some reservoir rocks. These diagrams can be used as references for borehole stability analysis when similar rocks are encountered in petroleum drilling. Introduction Borehole collapse during drilling and production has long been a problem which results in substantial expenditure to the petroleum industry. Compressive failure analysis is frequently proposed to predict borehole collapse, and consequently, the wellbore pressure required to prevent hole collapse. However, it is unclear to petroleum engineers as which failure criterion should be used in the analysis. Mogi reported that the effect of confining pressure on the fracture strength of rocks follows closely the Coulomb criterion. However, from results of the triaxial compression tests in which all three principal stresses are different, the intermediate principal stress has an important influence, so that fracture criteria, such as Coulomb, Mohr or Griffith, are not entirely applicable. Mogi claimed that the observed behavior under combined stress is adequately described by a failure criterion, which has a form of generalized von Mises criterion. Bradley used the von Mises failure criterion for analysis of collapse of inclined boreholes. Aadoy and Chenevert found that bore hole collapse is caused mainly by shear but also, in some cases, by tensile failure. They utilized the von Mises yield condition for shear failure analysis, and they claim that when the wellbore pressure is equal to or less than the formation pressure, a possibility for radial tensile failure exists. McLean and Addis reviewed strength criteria used in many published models and showed that certain criteria do not stand up when compared with laboratory rock strength test data. Therefore, they highlighted the effect of strength criteria on mud weight selection showing some of the anomalies and contrary evidence which can make the choice of a strength criterion difficult and confusing when performing wellbore stability analysis. Based on the laboratory data, they recommended the Mohr-Coulomb criterion as the most realistic. P. 483