| Popis: |
This paper describes the application of particle size distribution principles for determining materials to be added to the mud system during casing while drilling operations. Casing while drilling (CwD) has been demonstrated to stop or significantly reduce lost circulation and improve wellbore strength. The mechanism by which this improvement occurs is not understood, however the results from this work significantly advance what is needed to get repeatable results.If wellbore strengthening can be systematically achieved, then wells can be drilled in known loss areas without contingency strings of casing. In addition, wells drilled in mature fields, where producing horizons have altered pressures either from depletion or pressure maintenance, 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 casing while drilling in the Alaskan Tarn Field, the open hole leak off resistance was improved by 3.0 pound per gallon (ppg) drilling with 7.0 inch casing in a 67 degree angled well. With this success, a four well casing while drilling 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 inch and 7.625 inch casings but wellbore strengthening did not occur sufficiently in the 5.50 inch casing trials. Annular clearance appears to be a critical component to 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 two pounds per barrel. 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. |
