Particle-Size Distribution Improves Casing-While-Drilling Wellbore-Strengthening Results
Authors: Bybee Karen 
Published: February 2015

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 128913, ’Particle Size Distribution Improves Casing- While-Drilling Wellbore Strengthening Results,’ by R.D. Watts, SPE, M.R. Greener, SPE, S. McKeever, SPE, P.D. Scott, SPE and D. Beardmore, SPE, ConocoPhillips, originally prepared for the 2010 IADC/SPE Drilling Conference and Exhibition, New Orleans, 2-4 February. The paper has not been peer reviewed.
The full-length paper describes the application of particle-size-distribution principles for determining materials to be added to the mud system during casing-while-drilling (CWD) operations. CWD has been demonstrated to stop or significantly reduce lost circulation and improve well-bore strength. The mechanism by which this improvement occurs is not well understood.
Introduction
If wellbore strengthening can be achieved systematically, then wells can be drilled in known loss areas without contingency strings of casing. In addition, in mature fields where producing horizons have altered pressures from either depletion or pressure maintenance, wells can be drilled with fewer casing strings. Sidetracks become economical because hole size can be preserved for an effective completion, and well costs are lowered by not using additional liners to reach the objective.
By adding particles to the mud to fill in the particle-size distribution, losses to natural fractures were stopped while directional wells were drilled with casing in the Piceance basin of Colorado. Applying what was learned in a field trial of CWD in the Alaskan Tarn field, the openhole leakoff resistance was improved by 3.0 lbm/gal drilling with 7.0-in. casing in a 67° angled well. With this success, a four-well CWD campaign was executed with two wells drilled each in the Kuparuk and Tarn fields on Alaska’s North Slope. Results were positive for 7.0-. and 7.625-in. casings, but wellbore strengthening did not occur sufficiently in the 5.50-in.-casing trials. Annular clearance appears to be a critical component for success and is not yet fully understood.
The results demonstrate that a significant improvement in fracture gradient can be achieved with the right clearance between the hole and the casing and the proper-sized particles added to the mud system. In addition, the amount of material added has been demonstrated to be as low as 2 lbm/bbl. With confidence that strengthening can be achieved to the levels of improvement demonstrated, wells can be evaluated with significant cost savings by eliminating casing strings and preserving hole size for completions or further drilling.
Smear Effect
The smear effect has been considered to be either a smearing and plastering of cuttings onto the wellbore wall or the increase in hoop stress around the wellbore resulting when a crack is formed and particles are forced into the gap, creating a wedge and thereby raising the effective fracture gradient. At Piceance, imaging logs taken in the overburden confirmed natural fractures with a radially averaged aperture of approximately 1.0 mm, and these could be correlated back to losses while drilling. Because CWD had stopped losses in these intervals, wellbore strengthening was believed to be the more likely mechanism.