| Popis: |
Abstract Bottom-hole stresses in a well bore have been calculated using the results of photoelastic analyses. Boreholes in the earth were simulated in marblette cylinders. Three hole bottom contours were studied, including flat and semi-hemispherical bottoms. Two types of surface stresses were found at or near the hole bottom, meridional and circumferential. Stress concentration factors are presented graphically as a function of overburden (geostatic) and well bore (hydrostatic) pressures. For well bore and overburden pressures encountered in normal mud drilling, radial stresses on a flat bottom hole may change from 74 per cent of overburden at the center to 192 per cent near the hole wall. Knowledge of triaxial states of stress across the hole bottom will enable a better understanding of rock strength under borehole conditions. The purpose of this study is to determine bottom hole rock stresses in terms of overburden pressure and borehole pressure. The knowledge of such stresses will enable better future prediction of rock strength and drilling action. Since hole end geometry is complex, serious mathematical difficulties are encountered in solving this problem. Consequently, the use of three dimensional photoelastic techniques was chosen to achieve these objectives. Of primary concern in the photoelastic study was the determination of the two principal stresses in a plane tangent to the surface of pressurized boreholes. Principal stresses act on planes in which no shear exists. |
