ABSTRACT:

Theoretical predictions of borehole stability based on linear, isotropic, poroelastic theory and actual field data for the earth stresses and rock mechanical properties are consistent with observed ellipticity within shale and stable hole for sand at the AWP (Olmos) field, south-central Texas, provided that: (1) extended von Mises failure analysis is used rather than Mohr-Coulomb, and (2) rock failure envelopes are determined from core material saturated in synthetic formation brine rather than plugs saturated in drilling fluids. The inferred orientation of maximum compressive principal stress (N29E) is consistent with other techniques used to determine stress orientation at AWP (Olmos) and with the regional stress field, except where deviations occur near previously active normal faults.

1 INTRODUCTION

Several wellbores in the AWP (Olmos) field, McMullen County, Texas, exhibit borehole elongation suspected of being caused by stress-induced shear failure of the borehole wall. The amount and severity of borehole instability, as determined by oriented four-arm caliper logs, appears to be relatively unaffected by whether the holes were drilled with water-based or oil-based muds. The field observations of gauge holes in the pay sand and elongated holes in the overlying shale are consistent with theoretical calculations of borehole stability using actual field data for the earth stresses and rock mechanical properties. This analysis suggests compressive shear failure of the shale and resulting hole enlargement in the direction of minimum compressive horizontal principal stress, Sh. The direction of minimum hole diameter indicated by the four-arm calipers is therefore assumed to be the azimuth of maximum horizontal principal stress, SH.

2 BOREHOLE ELLIPTICITY DATA

These wells are located in three different fault blocks and therefore give a good indication of the stress orientation over a wide area of the field (Figure 2). The most consistent stress orientations are within the depth range of nominally 7500-9000 feet, which is predominantly shale. The azimuth of SH determined for this depth interval, averaged separately for each well, is plotted in Figure 2 and ranges from Í25¸ to Í38¸ in five of the six wells examined. The sixth well (B-3), which is intercepted by a fault just above the level of the Olmos Sand, yields an azimuth of N87°W. At depths shallower than 7500 feet, orientation of the ellipticity is highly variable, while for depths greater than 9000 feet there is very little ellipticity (see Figure 1). In compiling the above data, out-of-round intervals were generally considered to be those in which the two recorded diameters differed from one another by more than 1/4 inch on average over a continuous depth interval of at least 50 feet. The "best" and "good" data (Figure 1) were additionally restricted to intervals where the minimum hole diameter was to gauge and the hole deviation from vertical was less than 5°. The remaining data should be considered suspect and used with caution because of the possibility of key seating of the drill stem, washouts, or other borehole instabilities unrelated to stress-induced failure, but they have been included here for completeness.

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