Uncertainty – Wellbore Stability from Logging Information

Gemma M Keaney and Martin D. Matthews
Knowledge Systems Inc, Stafford, TX

Shales account for about 75% of the drilled formations and >70% of the borehole problems at an estimated cost of $8*10⁹/year. Maintenance of wellbore stability is particularly critical in deviated boreholes, sections with widely varying rock strengths, and regions of high stress anisotropy such as proximity to salt and active tectonics. The degree of certainty in wellbore stability estimates from log data is examined through a case study. The relevant data included resistivity, sonic travel time, gamma ray, density, and mud weight as a function of depth, as well as estimates of maximum horizontal stress, cohesive stress and friction angle. A shallow kick pressure, several deeper MDTs and LOT's were provided for calibration.

The degree of certainty in overburden, pore pressure, and fracture gradient is examined through a case study interpreted by several trained analysts. This is combined with estimates of the uncertainty in the magnitude and orientation of maximum horizontal stress, cohesive stress and friction angle to arrive at an overall uncertainty in well bore stability. This is used to obtain estimates of the magnitude of uncertainty in mudweight required to maintain wellbore stability as well as determining the major factors contributing to this uncertainty.

The effect of choosing Modified-Lade, Drucker-Prager, Stassi-d”Alia, or Mohr-Coulomb failure criteria is examined as is the effect of Eaton, Matthews and Kelly, and Daines tensile fracture models.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005