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.
TX 75083-3836 U.S.A., fax 1.972.952.9435. AbstractThe West Sak viscous oilfield on the North Slope of Alaska is currently being developed with extended reach multilateral wells, with departure to depth ratios up to 5 to 1, in which horizontal slotted liners are utilized in conjunction with a TAML level 3 multilateral junction system. Centralizers are considered necessary on the slotted liners to avoid slot plugging, reduce drag, and limit differential sticking. Selection of proper centralizers to run through a casing exit, without a whipstock in place, has been key to ensuring a successful multilateral installation.Several failures of centralizers run on liners through casing exits have resulted in significant drilling lost time associated with fishing and milling pieces of centralizers in order to place the wells into proper service. After three such failures, the requirement to study the passage of a centralized liner through a casing exit became essential. A surface test fixture was utilized to simulate liners run through a casing exit to test several potential centralizer candidates using the loads estimated from modeling.Torque and drag modeling provided the side force estimates exerted on the liner and centralizer as they passed through a casing exit. This paper will the discuss the liner centralizer installation problems prior to the testing program, detail the modeling used to determine the loads exerted on the centralizer at the casing exit, show the results of the yard tests conducted on several commonly utilized industry centralizers, and make recommendations for proper liner centralization in multilaterals.
The use of artificial barriers placed at the top and bottom of the producing zones before fracture stimulating a formation can control excessive fracture height growth. The reason for such barrier placement is to contain the fracture treatment within the producing zone when the stress barriers above or below the zone cannot contain height growth. Two case studies are included in this paper on wells fractured in the Richfield formation in the Northern Michigan Basin and in the Codell and Sussex formations in the Denver-Julesburg Basin.
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