In-Situ Stress Predictions and Measurements in an Unconsolidated Sandstone Formation, The Lower Frio, East Texas

ABSTRACT: Field tests at a depth of 5000 ft, in the Lower Frio Formation (Jasper County, East Texas), demonstrated that core testing, log interpretation, and micro-fracturing techniques for in situ stress determination need to be modified when investigating unconsolidated sandstones. Anelastic relaxation-strains, though<200 microstrains, can be measured within the first 7 hours. Laboratory differential strain analysis had high failure rates, but provided reliable strain ratios. Shear-wave velocity anisotropy was measured from reinforced whole-cores with a new system of polarized transducer-mounts that allowed simpler travel-time determination and sample rotation. The measured core strain anisotropy had a consistent preferential orientation with an azimuth of N00E-N40E. A small number of borehole elliptical cross sections were elongated NW-SE, suggesting a S₂ azimuth of NE-SW. The mapped East-West strike of nearby gravity faults suggested a far-field in situ maximum horiwntal stress azimuth of N60E-N90E. Downhole stress tests in cased holes were performed after packers got stuck in open-hole micro-fracture tests. Stress magnitudes in the sand derived from logs agreed with downhole micro-fracture tests if in situ stresses are assumed to be at the limit of mechanical stability. These demonstrate the difficulty in core testing, log interpretation and stress testing techniques when applied to unconsolidated formations.