2014-09-192014-09-191992-10-05SOARES, José Agnelo. Um estudo da estabilidade mecânica de poços a partir de perfis geofísicos em formações pouco consolidadas. 1992. 114 f. Dissertação (Mestrado) - Universidade Federal do Pará, Centro de Geociências, Belém, 1992. Curso de Pós-Graduação em Geofísica.https://repositorio.ufpa.br/handle/2011/5777The stability of boreholes can be evaluated from the rocks' mechanical properties, which are compared to the acting stress field using an appropriate failure criterion. The elastic parameters can be measured on rock samples in the laboratory (static parameters) or can be calculated from acoustic velocities and the formation bulk density (dynamic parameters). Dynamic elastic parameters can be obtained from wireline logs, but in poorly consolidated rocks the arrival of the shear wave is often masked by the Stoneley wave, which may travel faster and has a higher amplitude. The giant Campos basin offshore Brazil contains such rocks. We performed laboratory measurements of acoustic velocities on samples from this basin and we use them to correct the shear velocities obtained from waveform processing. Based on this we calculate elastic properties (e.g. Poisson's ratio, bulk modulus) of the rocks which we assume to be homogeneous and isotropic. Mechanical properties such as the shear, compressional and tensional strength are obtained from empirical relations with mineralogy and the elastic parameters. The stress field was evaluated using an uniaxial strain model and actual formation pressure data. From this, near-wellbore stresses were calculated using the modified Kirsch equations. The stress concentrations around the wellbore are illustrated for a number of representative cases. Wellbore failure occurs when these local stresses exceed the formation strength. Three different failure criteria were tested: The Mohr-Coulomb, the Griffith and the Fairhurst criterion. They respectively represent failure due to shear, tension and a combination of compression and tension. Actual wellbore instability was determined from the differential caliper measurement. Two sequences were analyzed, both consisting of intercalations of sands and shales. We found that the Fairhurst criterion offered the best match with the actually observed wellbore failure, and we propose it as a method to predict borehole instability for this basin.porAcesso AbertoPerfilagem geofísica de poçosMecânica de rochasDissertaçãoCNPQ::CIENCIAS EXATAS E DA TERRA::GEOCIENCIAS::GEOFISICA::PROPRIEDADES FISICAS DAS ROCHAS